Current schedule (overview)

Eileen Meyer ( STScI ) in Pratt at 12:00 on 22 May 2013

Probing the Nature of kpc-scale Relativistic Jets with Hubble and Fermi

I will first present recent work utilizing Hubble archival imaging data of the famous optical jet in the nearby radio galaxy M87. Using state-of-the-art astrometry techniques and over 12 years of archival data, we reach unprecedented accuracies in measuring the velocity field of the optical jet to less than 0.1c accuracy. We find large-scale deceleration over the length of the jet, as well as an surprising array of behavior in individual knots, visible to the naked eye. We find clear evidence of apparently relativistic material moving through standing shocks, as well as non-radial motions and decelerations. In the second half of the talk I will present a recent study on another famous large-scale jet, 3C 273, conducted with the Fermi gamma-ray telescope. With 4 years of data from Fermi, we put new limits on the gamma-ray emission from inverse-Compton upscattering of CMB photons by the relativistic plasma in the large-scale jet. This limit is well below that expected from a continuation of the UV-Xray spectrum, all but ruling out IC/CMB as the source of the X-rays in this source, a long-standing source of debate. This result has strong implications for our understanding of the energetics of jets and the scale of their impact on their environments.

James D. Phillips ( CfA ) in Pratt at 12:30 on 29 May 2013

Testing the equivalence principle 10,000 times better on a sounding rocket

The equivalence principle is at the heart of gravitation theory. It has been tested with increasing accuracy for many centuries; the present upper limit on difference of acceleration for tested pairs of substances is 2 times 10^-13 g. Theories that unify gravity with the other forces tend to predict a violation, but few predict the magnitude. We propose to test the acceleration difference to 2 times 10^-17 g in an experiment launched into free fall by a sounding rocket. The test masses are dropped (released) eight times, for 120 s each time. Between drops, the test masses are held electrostatically and the entire payload is inverted, which reverses the position of the Earth and leaves most systematic error effects unchanged. The high sensitivity is possible in a short time for several reasons: 1) the SAO laser distance gauges measure to 0.1 pm in 1 s; 2) the position of the structure around the test masses follows that of the test masses by virtue of a servo (but not a drag-free satellite); 3) the test masses are unconstrained during drops, avoiding constraint force imperfections; 4) the position measurement is to a plate that is almost stationary with respect to the test masses, by virtue of the position servo; and 5) there are two cascaded thermal low-pass filters with time constants 1000 times longer than the 120 s drops.

Nimisha Kantharia (National Centre for Radio Astrophysics, Pune, India) in Pratt at 12:30 on 5 June 2013

TIFR GMRT Sky Survey - status and results

I will talk about The TIFR GMRT Sky Survey (TGSS) which is an all sky survey at 150 MHz being made with a 20" resolution and rms noise of 5-7 mJy/beam using the Giant Metrewave Radio Telescope in India. The survey covers the sky north of declination -55 degrees and aims to be complete to 40 mJy. Till date, five data releases which cover about 4000 square degrees of the sky south of the celestial equator have been made. I will summarize the survey, present its current status and show some promising results from the releases so far.

Eleni Kalfountzou ( Univ. of Hertfordshire ) in Pratt at 01:30 on 6 June 2013 (Thursday)

Susanna Kohler (University of Colorado (Boulder)) in Pratt at 12:30 on 12 June 2013

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Kenji Hamaguchi (NASA/GSFC) in Pratt at 01:30 on 20 June 2013 (Thursday)

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Jack Steiner ( CfA ) in Pratt at 12:30 on 26 June 2013

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Julie Hlavacek-Larrondo ( Stanford Univ. ) in Pratt at 12:30 on 3 July 2013

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Michael McDonald ( MIT ) in Pratt at 12:30 on 31 July 2013

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Eric Miller ( MIT ) in Pratt at 12:30 on 18 September 2013

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Chris Done ( Durham University ) in Pratt at 12:30 on 25 September 2013

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Daniel Castro ( MIT ) in Pratt at 12:30 on 2 October 2013

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Previous talks (overview)

HEAD (CfA) in Pratt at 12:30 on 1 September 2004

HEAD Dry Run

Cathy Clemens (CfA) in Pratt at 12:30 on 15 September 2004

NESSIE - the New England Space Science Initiative in Education - a NASA Education and Public Outreach (E/PO) program at CfA in collaboration with the Museum of Science and Tufts

NESSIE is one of seven regional broker/facilitators operating out of the NASA Science Mission Directorate (SMD; formerly the Office of Space Science). It is jointly run by the Museum of Science, CfA and Tufts. NESSIE's overall mission is to form and foster partnerships between NASA space scientists and educators throughout New England. In particular, NESSIE acts as a clearinghouse and point of contact for educators and scientists, assists educator/scientist partnerships to plan and implement E/PO projects, assists scientists in doing E/PO programs, distributes NASA materials, informs educators of E/PO opportunities with scientists, and helps E/PO product developers create and disseminate appropriate materials.

David Ballantyne (University of Toronto (CITA)) in Classroom at 12:30 on 20 September 2004 (Monday)

Neutron Star Superbursts as Probes of Accretion Disk Physics

The bright X-ray emission from a superburst on the surface of a neutron star can act as a spotlight to illuminate the disk surface. The X-rays cause iron atoms in the disk to fluoresce, allowing a determination of the ionization state, covering factor and inner radius of the disk over the course of the burst. Here, we review the results of time-resolved spectral fitting of the superburst from 4U 1820-30, in which we found strong evidence that the inner region of the accretion disk was disrupted by the burst. We will also discuss different physical processes that may explain the results.

Katrien Steenbrugge (SRON) in Pratt at 12:30 on 22 September 2004

Ionization structure of the warm wind in the AGN NGC 5548: discrete or continuous?

I will present result from the 540 ks Chandra HETG/LETGS campaign on NGC 5548. In particular I will concentrate on the study of the warm absorber, and its ionization structure. The ionization structure is important in determining the physical condition in the warm absorber, as well as its possible geometry. A discrete ionization structure is expected in a cloud model, while a continuous ionization structure could arise in narrow outflows.

Silvia Piranomonte (ASI Science Data Center, Italy) in Pratt at 12:30 on 29 September 2004

Sedentary Survey of BL Lacs

The ``Sedentary Multi-Frequency Survey of High Energy Peaked BL Lacs (HBLs)'' is a large, 100\% identified, and statistically well-defined sample of highly X-ray dominated (very high fx/fr) BL Lacertae objects. The survey, the largest flux limited and complete sample of BL Lacs existing today, is based on a very efficient multi-frequency selection technique that exploits the unique broad-band spectral properties of extreme BL Lacs. I present the results of our spectroscopic identification campaign which led to the identification of all candidates in the sample. I show optical spectra for these sources and I discuss the properties of the sample, including the luminosity function and cosmological evolution of HBLs. This large survey allows us to study in detail the role of many parameters in blazar classification criteria and in blazar physical models. In particular, following new results on the cosmological evolution also in other recent samples of BL Lacs, I investigate the controversial issue of the correlation between the synchrotron peak and radio luminosity and as a consequence, I discuss about the need to review the ``blazar unified scenario model'' first proposed by Fossati et al 1998.

Sergio Colafrancesco (Osservatorio Astronomico di Roma) in Pratt at 12:30 on 6 October 2004

Non-thermal phenomena in galaxy clusters - Radio halos, hard X-rays and gamma rays

- ( - ) in Phillips at 12:30 on 13 October 2004

Chandra Fellows Symposium

Ryan Hickox ( CfA ) in Classroom at 12:30 on 27 October 2004

Superorbital Variation in X-ray Pulsars: Exploring the Accretion Flow.

Lukasz Stawarz ( CfA ) in Pratt at 12:30 on 3 November 2004

Multifrequency Radiation of Extragalactic Large-Scale Jets

Large-scale extragalactic jets, observed to extend from a few to a few hundred kiloparsecs from active galactic nuclei, are now studied over many decades in frequency of electromagnetic spectrum, from radio until (possibly) TeV gamma rays. For hundreds of known radio jets, only about 30 are observed at optical frequencies. Most of them are relatively short and faint, with only a few exceptions, like 3C 273 or M 87, allowing for detailed spectroscopic and morphological studies. Somewhat surprisingly, the large-scale jets can be very prominent in X-rays. Up to now, about 30 jets were detected within the 1 - 10 keV energy range, although the nature of this emission is still under debate. In general, both optical and X-ray jet observations present serious problems for standard models regarding the considered objects. We summarize information about multiwavelength emission of the large-scale jets, and we point out several modifications of the standard jet models -- connected with relativistic bulk velocities, jet radial stratification, particle energization and magnetic field amplification all the way along the jet, or finally intermittent jet activity of the central engine -- which can possibly explain some of the mentioned puzzling observations. We also comment on gamma-ray emission of the discussed objects.

Simone Migliari ( Universiteit van Amsterdam ) in Pratt at 12:30 on 10 November 2004

Disk-jet coupling in X-ray binaries: neutron stars vs. black holes

A universal X-ray/radio luminosity correlation has been established for black hole (BH) systems over ten orders of magnitude in X-ray luminosity and mass scale, from X-ray binaries (XRBs) to Active Galactic Nuclei. This relation is interpreted as representing the disc-jet coupling in the systems. We found that an analogous correlation seems to hold also for low-magnetic field neutron star (NS) XRBs. Focusing on the disc-jet coupling in X-ray binaries, I will compare BHs and NSs and discuss similarities and differences.

Sudip Bhattacharyya ( University of Maryland ) in Pratt at 12:30 on 17 November 2004

Implications of Surface Atomic Spectral Lines from Weakly Magnetized Rotating Neutron Stars

The report by Cottam et al. (2002) of iron lines in the thermonuclear burst spectrum of EXO 0748-676 motivates detailed studies of the information about neutron star structure and emission geometry that can be obtained from spectral line profiles in future observations. We calculate the structures of surface atomic spectral lines from rotating neutron stars, considering the full effects of general relativity (including light-bending and frame-dragging). We find that, even for spin frequencies up to 600 Hz, the stellar mass to radius ratio can be inferred from surface line profiles to better than 5%, which is the precision required for strong constraints on the equation of state of neutron stars. Our results also indicate that a signature of frame-dragging may be detected with future instruments in surface line profiles.

Anna Szostek (N. Copernicus Astronomical Center, Warsaw, Poland) in Pratt at 12:30 on 24 November 2004

A Study of Cygnus X-3 in X-Rays and the Radio

Cygnus X-3 is a high mass X-ray binary system which contains a Wolf-Rayet companion and a compact object of an unknown nature (black hole or neutron star). It is a persistently bright X-ray source with a 4.8 hour orbital modulation and is the brightest X-ray binary observed at radio wavelengths. I will describe and discuss the spectral properties and the correlated X-ray - Radio behavior of Cygnus X-3.

Elena Gallo ( Universiteit van Amsterdam ) in Pratt at 12:30 on 1 December 2004

Accretion modes and jet production in black hole X-ray binaries

I will review our current understanding of the radio properties of black hole X-ray binaries and discuss them in the framework of the recently proposed unified model for the jet/accretion coupling in these systems. I will further report on the discovery of a low surface brightness, jet-powered radio nebula around the stellar black hole in Cyg X-1, and how such structure can be used as an effective calorimeter for the jet kinetic power.

Saku Vrtilek ( CfA ) in Pratt at 12:30 on 8 December 2004

The Chandra View of X-ray Binaries

This talk will present some highlights from the many remarkable studies of X-ray binary systems that have been conducted using Chandra. Chandra's unprecedented spatial resolution and sensitivity have enabled us to determine luminosity functions for entire classes of X-ray binaries as observed in other galaxies and in globular clusters within our own and nearby galaxies, and to measure and analyze scattering halos around X-ray binaries at an accuracy high enough to provide a new method for measuring cosmic distances. It has been used for identifications of sources through accurate x-ray positions, and to place constraints on the chemical state of interstellar matter by measuring absorption lines in X-ray binaries, to measure the speed of powerful X-ray winds with the first detections of X-ray P-Cygni features, and to determine the size and separation of the jet material in galactic microquasars by measuring X-ray line velocities to an accuracy comparable to that of optical spectroscopy.

Matteo Perri ( ASI Science Data Center, Frascati, Italy) in Classroom at 12:30 on 13 December 2004 (Monday)

Log-parabolic Spectra in Blazars: the BeppoSAX Wide Band X-ray View of Mkn 421, Mkn 501 and PKS 2155-304

Jeroen Homan ( MIT ) in Pratt at 12:30 on 15 December 2004

Jets and disks: the optical/IR emission states of the black hole X-ray binary GX 339-4

While X-ray studies of black hole X-ray binaries have been very successful in identifying different emission states of the central accretion flow, mutli-wavelength studies have provided new insights into the underlying properties of these states. In this talk I present the results of quasi-simultaneous X-ray and optical/IR observations of the transient black hole X-ray binary GX 339-4. Two distinct states of optical/IR-X-ray behavior were found. In one state the optical/IR emission is likely dominated by a jet outflow and in the other one by the accretion disk - the jet is inferred to switch off on a time scale of a few days. I compare the observations of GX 339-4 with those of other systems and discuss the importance of multi-wavelength campaigns for understanding accretion flows onto compact objects.

HEAD in Phillips at 12:30 on 5 January 2005

Dry-Run Session for AAS

We invite HEAD members attending the AAS to participate in a practice session. Each speaker has a 10 min slot to allow for questions/input from the audience/setting up computer etc. This is an opportunity to polish your presentation and give us all a sneak preview!

Gijs Roelofs (University of Nijmegen, NL) in Classroom at 12:30 on 24 January 2005 (Monday)

Compact accreting binaries: the AM CVn stars

AM CVn stars are mass-transferring binaries with orbital periods ranging from a few minutes to just over an hour. The population of these systems has important implications for binary evolution theory, in particular the physics of common-envelope evolution, the stability of mass transfer between white dwarfs, and the progenitors of Supernovae Ia. For the longer-period systems, the unique helium accretion disks and extreme mass ratios provide laboratories for studying the influences of chemical composition and tidal resonances on accretion disks. The shorter-period systems are currently the best known candidates for detection with LISA. In the emerging field of gravitational wave astronomy, being able to study the same known binaries in both gravitational and electromagnetic waves is of great importance for testing the expected (and complicated) LISA measurements.

Scott Wolk in Classroom at 12:30 on 26 January 2005

Chandra Looks at Regions of Massive Star Formation

The Chandra X-ray Observatory, launched in July 1999, is the premier instrument for high resolution X-ray astrophysics. The combination of spatial and spectral resolution allow us to study regions of massive star formation which had been inaccessible even from the ground until the last decade. I will survey the state of our understanding by discussing data from 3 massive star forming regions. Two of these are somewhat remote southern clusters, RCW 38 and RCW 108 and the third is the nearby Orion Nebular Cluster. These three ostensibly similar star forming regions show very diverse and complex environments. RCW 108 is the youngest of these and supports the hypothesis that previously extant density enhancements within a cold cloud are pushed to collapse by the ram pressure from a nearby young star. The O5 star at the heart of RCW 38 is actively compressing a nearby core. In our study of the ONC, the most intensively observed massive star forming region, we focus on the effect of X-rays and X-ray flares on the disks around G stars. Finally I will discuss the new database (ANCHORS) which is being prepared to deliver X-ray data on individual stars and star forming regions to the public.

Dan Harris ( CfA) in Pratt at 12:30 on 02 February 2005

The Latest news from Chandra (and others) about the Jets in the Radio Galaxies M87, 3C120 (and others).

We present recent results from our monitoring of the M87 jet, including the current flaring of a knot close to the nucleus. For 3C120 we review the peculairities of the resolved knot 25'' from the core. Some recent data on 3C 273 will also be shown in order to compare properties of a low power jet with a high power jet.

Frank Krennrich (Iowa State) in Pratt at 14:00 on 07 February 2005 (Monday)

TeV Gamma-Ray Observations of Active Galaxies

Ground based Gamma-Ray Astronomy provides highly sensitive instrumentation to detect TeV photons from blazars, a sub-class of active galactic nuclei. I will review the status of TeV gamma-ray observations of blazars and their high energy spectra and discuss the implications for physics in their relativistic jets. TeV beams from extragalactic sources do also constrain the diffuse infrared background radiation and I will present the current evidence for gamma-ray absorption by the IR background. Furthermore, I will present the future prospects of blazar science with the next generation gamma-ray telescopes VERITAS and GLAST which together cover an energy range of about a GeV to 100 TeV.

Andrew Friedman (CfA) in Pratt at 12:30 on 09 February 2005

The Present and Future of GRB Cosmography

At least in the Gamma-Ray Burst (GRB) field, there has been a great deal of excitement --- and controversy --- recently concering the possibility of actually turning GRBs into standard candles and using them to constrain the cosmological parameters in parallel with Type Ia supernovae. We briefly discuss the history of GRB standard candles derived from energetics, and highlight the most promising current GRB standard candle, constructed from the newly discovered correlation between the peak energy in the rest frame prompt burst spectrum and the beaming-corrected gamma-ray energy. Although GRB standard candles have many potential advantages over SNe Ia, we show that the current GRB data are not yet cosmographically competitive, mainly due to the small sample and strong sensitivity to input assumptions. There is some hope that this relation and others may yield reliable standard candles with future data, but, at present, we urge caution concerning claims of the utility of GRBs for cosmography.

Hermine Landt ( CfA) in Pratt at 12:30 on 23 February 2005

What Types of Jets Does Nature Make? A New Population of Blazars

We have recently discovered a population of strong-lined blazars with jet synchrotron emission peaks in the UV/X-ray regime. So far, only radio quasars with lower synchrotron energy cut-offs (and so X-rays dominated by inverse Compton emission) were known. Our discovery challenges theories which posit that particle cooling by an external radiation field, such as the one produced by, e.g., an accretion disk, controls the jet synchrotron spectral cut-off. In this talk I will present first VLA maps and preliminary results from XMM spectroscopy of these new class of blazars and discuss their relation to the high-energy peaked BL Lacertae objects.

Gamil CASSAM-CHENAI ( CEA/Saclay) in Phillips at 12:30 on 16 February 2005

Thermal and Nonthermal X-ray Emission in Supernova Remnants

In supernova remnants (SNRs), the matter heated to millions of degrees produces X-ray thermal emission. This emission contains information on the chemical composition of the ejected matter and on the ambient medium, as well as on the hydrodynamical evolution of the SNR. Besides, the SNR shocks are believed to accelerate particles to very high energy (at least to the knee of the cosmic-ray spectrum). X-ray synchrotron radiation from accelerated electrons is then expected. We have investigated the X-ray thermal and nonthermal components in SNRs by observational and modelling aproaches. The observational part will deal with two SNRs - Kepler and G347.3-0.5 - observed by the European satellite XMM-Newton. The modelling part will emphasize the synchrotron emission obtained from a hydrodynamical model coupled with a nonlinear particle acceleration model which takes into account energy losses of the accelerated electrons.

Dan Schwartz ( CfA) in Pratt at 12:30 on 02 March 2005

Chandra Observations of Powerful Relativistic Jets in AGN

I will review Chandra survey observations of jets in quasars and FR II radio sources. We adopt interpretation of the X-ray emission in terms of inverse Compton scattering on the Cosmic Microwave Background. Both the SED and X-ray/radio morphologies support such a supposition, in many cases. This requires bulk relativistic motion of the jets at distances of 100's of kpc from the quasar, and allows estimates of the rest frame magnetic fields, all under the conditions of minimum energy. The kinetic flux carried by the jets is very large and efficient. Such jets in clusters would carry more than enough energy to balance cooling flows. The IC/CMB mechanism implies that jets should maintain a constant surface brightness to arbitrarily large redshifts -- no specific evidence exists yet to support this expectation.

Alastair Sanderson (University of Illinois) in Pratt at 12:30 on 9 March 2005

AGN Shock Heating in the Cool Core Galaxy Cluster Abell 478

The Chandra X-ray telescope has revealed clear signs of interaction between active galactic nuclei (AGN) in the cores of some galaxy clusters and the surrounding gaseous intracluster medium (ICM). However, there is surprisingly little evidence of direct shock heating of the gas by AGN jets or outflows. Moreover, AGN heating is a promising mechanism for explaining the lack of very cool gas in the centers of cool core clusters, which would otherwise be present if cooling is uninhibited. This talk will focus on the cluster Abell 478, where we have discovered 4 hot spots in the cool core, which appear to be associated with AGN activity.

Alessandro Baldi ( CfA) in Pratt at 12:30 on 16 March 2005

The complex hot ISM of the Antennae galaxies observed with Chandra: discovery of chemical enrichment

I will present an analysis of the properties of the hot interstellar medium (ISM) in the merging pair of galaxies known as The Antennae (NGC 4038/39), performed using the deep coadded ~411 ks Chandra ACIS-S data set. These deep observations and Chandra's high angular resolution allow us to investigate the properties of the hot ISM with unprecedented spatial and spectral resolution. Through a spatially resolved spectral analysis, we find a variety of temperatures (from 0.2 to 0.9 keV), densities (from 3x10^-2 to 3x10^-1 cm^-3), and Nh (from Galactic to a 2x10^21 cm^-2). Metal abundances for Ne, Mg, Si, and Fe vary dramatically throughout the ISM from sub-solar values (~0.2) up to ~20-30 times the solar abundance. Measures for the hot-gas mass (~10^7 M_sun), cooling times (10^7-10^8 yr), and pressure are derived. In the two nuclei the hot-gas pressure is significantly higher than the CO pressure, implying that shock waves may be driven into the CO clouds. Comparison of the abundances with the average stellar yields predicted by theoretical models of SN explosions points to Type II SNe as the main contributors of metals to the hot ISM. No evidence of correlation between radio-optical star formation indicators and the metal abundances is found. Although uncertainties in the average density cannot exclude that mixing may have played some important role, the time required to produce the observed metal masses (~3 Myr) suggests that the correlations are unlikely to be destroyed by the presence of efficient mixing. More likely a significant fraction of Type II SNe ejecta may be in a cool phase, in grains, or escaping in the wind. This work is supported in part by NASA contract NAS8-39073 and NASA grants GO1-2115X and GO2-3135X.

Shami Chatterjee ( CfA) in Pratt at 12:30 on 30 March 2005

Relativistic Winds from Neutron Stars: New Surprises from Chandra

The intense magnetic fields and rapid rotation of neutron stars drive a prodigious energy outflow into the interstellar medium, with the radiated Poynting flux being converted into particle flows in relativistic winds. Bow shock nebulae are uniquely well constrained systems in which we can investigate the interplay between neutron star relativistic winds and the interstellar medium. I present new Chandra observations that reveal a diverse variety of phenomena in these fascinating systems, showing that the interaction is even richer and more complex than expected.

Ronnie Hoogerwerf ( CfA) in Pratt at 12:30 on 04 April 2005 (Monday)

The mass, accretion disk, and accretion column of EX Hydrae. Joint Seminar with the Solar, Stellar, and Planetary Sciences Division

We present a CHANDRA HETG observation of EX Hydrae, an Intermediate Polar (IP) type Cataclysmic Variable (CV). In IP-type CVs the primary is a magnetic white dwarf whose field controls the accretion flow close to the white dwarf, leading to a shock and accretion column that radiate mainly in X-rays. We present (1) the first X-ray radial velocity curve for a white dwarf in a binary and derive its mass and (2) a new feature in the binary light curve, which leads to a detailed analysis of the temperature structure of the accretion disk. We will also discuss exciting new features in the white dwarf light curve. These features are directly related to the temperature and density profile of the accretion column and will provide a test for the standard model of magnetic accretion.

Yangsen Yao (UMASS Amherst) in Pratt at 12:30 on 13 April 2005

Warm-hot gas in and around the Milky Way: X-ray absorption line diagnostics

The existence of the warm-hot phase of the Galactic interstellar medium has been well established by the measurement of the diffuse soft X-ray background (SXB) emission and the detection of the UV absorption lines in the spectra of background Galactic stars. However, we have little knowledge of the chemical, physical, ionization states, as well as the amount and the extend of the absorbing gas. Here we present a systematic study of the hot interstellar medium (HISM) via the high resolution X-ray absorption line spectroscopy of 10 Galactic LMXBs and several extragalactix sources. We measure the temperature and the equivalent hydrogen column densities along multiple lines of sight, and investigate the possible origin of the SXB enhancement in the Galactic central region. We also compare our measurement with those from EM, UV absorption, and pulsar DM. In the end, we attempt to characterize the spatial distribution of the HISM and to examine the Galactic contribution to the observed z~0 AGN absorption lines which has been debated since their discoveries.

Jasmina Lazendic ( MIT ) in Pratt at 12:30 on 20 April 2005

Chandra HETG observations of Cas A

I will present Chandra High Energy Transmission Grating Spectrometer observations of the young supernova remnant Cas A. The high resolution X-ray spectrum reveals dominant emission lines from silicon and sulphur, and weaker lines from magnesium, argon, calcium and iron. I will discuss the difficulties in analyzing gratings data for extended sources and describe a new technique applied to the Cas A data. I will present results from Doppler shift measurements and plasma diagnostics of individual lines and discuss plasma conditions as a function of position throughout the remnant.

Li Tipei (Tsinghua University, Beijing ) in Phillips at 12:30 on 25 April 2005 (Monday)

Timing in the Time Domain: Rapid Variability in Accreting Black Holes

Variability study is an important tool to understand the physical processes in compact objects. I will introduce a new technique for studying variability in the time domain. With this technique, variation power densities, spectral lags and coherence for different timescales can be calculated directly from the observed light curves without using any time-frequency transformation. We have applied the new technique to study X-ray binaries and AGNs. Our results indicate that the time domain technique is sometimes more powerful than Fourier type analysis in revealing the underlying physics in non-periodic radiation processes. For making timing and imaging studies in the hard X-ray band, a high energy astrophysics mission Hard X-ray Modulation Telescope (HXMT) has been proposed and constructed in China. In the energy range of 10-250 keV, HXMT can perform full-sky survey with angular resolution smaller than 5' and sensitivity much better than Integral/IBIS and Swift/BAT, and can make high signal-to-noise ratio pointing observations of scientific hot spot sources for detailed temporal and spectral studies. The performances and status of HXMT project will be briefly introduced at the end of this talk.

John Swain (Northeastern) in Pratt at 12:30 on 27 April 2005

The Pierre Auger Observatory and the Mystery of High Energy Cosmic Rays.

Deep mysteries surround the highest energy cosmic rays and pose challenges to our understanding of fundamental physics. Particles carrying several Joules of energy strike the upper atmosphere producing showers of billions of subatomic particles covering areas of several square kilometers, and the Pierre Auger Observatory is a major international project whose aim is to study these showers. Detailed measurements of the fluoresence they produce as well as of the particles which reach the ground provide valuable data on the energies, origins, and composition of these mysterious particles. This talk reviews the puzzles of the field, motivation for the project, and the status of the Southern site of the observatory in Argentina.

Martin Elvis ( CfA) in Pratt at 12:30 on 11 May 2005

Doubting the Torus

If there is one image that everyone has in mind when thinking of what AGNs look like it is the 'donut' shaped torus from the Urry and Padovani review (1995 PASP 107, 803). While there is no doubt that a flattened obscuring structure exists in AGNs, it may well not have the geometry or kinematics normally ascribed to it. I look at recent evidence, and some old results, that cause us to doubt the torus, and may lead on to a more dynamic view of AGNs.

Glenn Allen (MIT) in Pratt at 12:30 on 18 May 2005

Evidence of an Inverse Compton Origin for the TeV Emission from the Supernova Remnant G347.3-0.5

We present the preliminary results of a joint spectral analysis of some radio, X-ray and gamma-ray data for the supernova remnant G347.3-0.5. The shell-type remnant was recently discovered in the ROSAT All-Sky Survey data. The X-ray flux is relatively large and dominated by synchrotron radiation. G347.3-0.5 is a fairly faint radio source and one of only a few remnants reported to emit TeV gamma rays. The physical process responsible for the TeV emission remains controversial. We review the three possible mechanisms---inverse Compton scattering, the decay of neutral pions and nonthermal bremsstrahlung---and argue that the results of our spectral analyses favor inverse Compton scattering. In this case, it is possible to determine the weighted mean values of the "maximum" electron energy and the magnetic field strength. The results also place tight constraints on the unmeasured velocity of the forward shock and the electron diffusion coefficient (i.e. rate of electron acceleration). The lower limit on the velocity helps constrain the disputed age and distance of the source.

Zhong Wang (CfA) in Pratt at 12:30 on 25 May 2005

Mid-infrared Observations of the Antennae Galaxies with Spitzer

The spectacular pair of galaxies NGC4038/4039 (also known as the Antennae) is one of the text-book examples of giant spirals experiencing close encounters. Studies with the Hubble Space Telescope and other facilities have revealed active star forming regions and a young stellar population in the system, possibly related to the on-going gravitational interaction between the two disks first modeled by Toomre and Toomre in the 1970s. We have used the two imaging cameras aboard the Spitzer Space Telescope to probe the mid-infrared emission in the Antennae, focusing on the effect of star-forming activities in regions enshrouded in dust. Combined with the results from radio, optical, near-IR and X-ray observations, we find evidence that gravitational disturbances trigger large-scale star formation in such galaxies in a sequential manner: individual parts of the system exhibit distinctive, yet continuously varying emission properties apparently related to their stages of evolution. The measurements of local intensity and colors of the different star forming regions in this system may have significant implications to our understanding of a range of phenomena such as galaxy mergers, formation of early-type galaxies, and the so-called ultra-luminous infrared galaxies (ULIRGs).

Aneta Siemiginowska (CfA) in Room M-340 Concord Avenue at 12:30 on 15 June 2005

AGN Feedback and Evolution of Radio Sources

OIR Talk in Pratt at 12:30 on 22 June 2005

Jonathan Gelbord (MIT) in Room M-340 Concord Avenue at 12:30 on 29 June 2005

Deep X-ray and Optical Observations of Quasar Jets

We present an update on our ongoing multiwavelength program to study high energy emission from extragalactic jets. We have selected several targets from our Chandra snapshot survey (Marshall et al. 2005, Marshall et al. in prep) for detailed follow-up observations with Chandra, HST, and ground-based optical and radio telescopes. In one of these systems, PKS 1421-490, we have discovered an enigmatic feature that may be most readily interpreted as a unique, optically-dominated jet knot. Another quasar (PKS 1055+201) exhibits a long, arcing X-ray jet; an unusual feature of this system is a broad swath of thermal X-ray emission that envelops both the jet and the (otherwise unseen) counter-jet. PKS 2101-490 is another system with a long, bent jet that shows evidence of diffuse X-ray emission between the core and counter-lobe. The diffuse emission around these jets provides direct evidence of the interaction between the relativistic flows and the surrounding medium.

Ben Williams (CfA) in Room M-340 Concord Avenue at 12:30 on 06 July 2005

Monitoring the X-ray Source Population of M31

Through an ongoing Chandra snapshot campaign, we have been studying the X-ray source population of M31. The program has produced high-resolution X-ray catalogs and an archive of data containing 5 years of variability information. Cross-correlation of the X-ray positions with optical catalogs has highlighted several X-ray binaries that lie suspiciously close to planetary nebulae. In addition, the high spatial resolution of the data has revealed the X-ray morphology of supernova remnants in M31 for the first time. Finally, the timing of the observations has provided a treasure trove of transient sources. Through a coordinated HST program, we have been able to search for optical counterparts for some of these X-ray transients. The combined power of the optical photometry and X-ray spectra provides new clues about the physical properties of the binary systems responsible for the outbursts.

Patrick Young (Los Alamos) in Phillips at 12:30 on 10 August 2005

The Dramatic Impact of Hydrodynamic Mixing on Supernova Progenitors

Recent multidimensional simulations have demonstrated the importance of hydrodynamic motions in the convective boundary and radiative regions of stars to transport of energy, momentum, and composition. The impact of these processes increases with stellar mass. Stellar models which approximate this physics have been tested on several classes of observational problems with excellent results. I will briefly describe the physics and its relevance to the solar composition problem as a prelude to the implications for supernova progenitors. The improved models predict substantially different interior structures at collapse, and subsequently very different explosions. I will present pre-supernova conditions and 3D explosion calculations for a range of initial models designed to explore the identity of the progenitor of Cassiopeia A.

Simona Giacintucci (Osservatorio Astronomico di Bologna, Italy) in Concord Av, 2nd Floor Conference Room at 13:00 on 17 August 2005

Catching the bulk of cluster radio halos with GMRT

I present the preliminary results of a deep radio survey at 610 MHz with the Giant Metrewave Radio Telescope (GMRT) of a complete X-ray flux-limited sample of 50 galaxy clusters at redshift z=0.2-0.4. The aim of this observational project is to test the predictions of a new statistical magneto-turbulent theoretical model for the formation of radio halos in galaxy clusters. It is expected that the bulk of the radio halo formation takes place in the redshift range 0.2-0.4, and that these sources are hosted in about 30% of the most massive clusters.

Elena Dalla Bonta' (University of Padova ) in Pratt at 12:30 on 14 September 2005

The Upper End of the Supermassive Black Hole Mass Function

We want to characterize the high mass end of the local supermassive black hole (SMBH) mass function. Indeed, it is in the high mass regime that the unavoidable link between the evolution of SMBHs and the hierarchical build-up of galaxies leaves its clearest signature. We carefully selected three brightest cluster galaxies (BCG). Their large masses, luminosities and stellar velocity dispersion, as well as their having a merging history which is unnmatched by galaxies in less crowded environments, make these galaxies the most promising hosts of the most massive SMBHs in the local Universe. We observed the BCG sample with the Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS). For each target galaxy we performed high-resolution spectroscopy of the H-alpha + [NII] emission lines at three slit positions, to measure the central ionized gas kinematics. Two galaxies, named ABELL 3565-BCG and ABELL 1836-BCG, show a regular rotation curve and a strong central velocity gradient. ACS images with three filters (F435W, F625W and FR656N) have been used to determine the optical depth of the dust, the stellar mass distribution near the nucleus and an intensity map. We used a dynamical model of the gaseous disk taking into account the whole bidimensional velocity field and the instrumental set-up. The extension of the high mass end of the local SMBH mass function is necessary to improve our understanding of how SMBHs, and their hosts, formed and evolved.

Hans-Jakob Grimm (CfA) in Pratt at 12:30 on 21 September 2005

Some statistical properties of the power law luminosity function

We study the statistical properties of the combined emission of a population of discrete sources. Namely, we consider the dependence of their total luminosity L_tot=Sum(L_k) and of total fractional rms of their variability on the number of sources N or on the normalization of the luminosity function. We show that due to small number statistics a regime exists, in which L_tot grows non-linearly with N, in apparent contradiction with the seemingly obvious prediction mean(L_tot)=Integral(dN/dL*L*dL) ~ N. In this non-linear regime, the rms_tot decreases with N significantly more slowly than expected from the rms ~ 1/sqrt(N) averaging law. Only in the limit of N much larger than 1 do these quantities behave as intuitively expected, L_tot ~ N and rms_tot ~ 1/sqrt(N). Using the total X-ray luminosity of a galaxy due to its X-ray binary population as an example, we show that the Lx-SFR and Lx-M* relations predicted from the respective ``universal'' luminosity functions of high and low mass X-ray binaries are in a good agreement with observations.

Philip M. Sadler (CfA ) in Pratt at 12:30 on 28 September 2005

AP Courses in American High Schools: Advanced Placement or Academic Padding?

NSF, DoEd, and NIH have funded our Science Education Department to study the transition of students from high school to college science. This $3M project is examining the factors predicting success (and failure) of students in their first year of college science, paying particular attention to aspects of their high school preparation. This is the first large scale study of its kind, involving 18,000 students at 100, randomly-chosen colleges and universities. An initial area of investigation focuses upon AP science courses, which are offered in an increasing number of U.S. high schools and enable students to "place out of" introductory college courses. Our findings do not support the claims of high levels of college success made by the AP's governing College Board. While students who take AP science do somewhat better in college than those who take less rigorous courses, this can be attributed primarily to other factors (e.g. math and reading skills and socio-economic indicators) and not their AP enrollment. We find strong evidence that AP students have not mastered the content of first semester college biology, chemistry, or physics courses and are handicapped in future courses if they are granted waivers. I will discuss other implications of this study and findings concerning improvement of the teaching of science at the pre-college level.

Don Ellison (North Carolina State University) in Pratt at 12:30 on 5 October 2005

The Production of Cosmic Rays in Young Supernova Remnants

While supernovae have long been believed to be the main sources of cosmic rays, it is only recently that clear evidence for the production of TeV particles in individual remnants has been obtained, particularly by imaging atmospheric Cherenkov telescopes such as HESS. I will give a brief review of the origin of cosmic rays and discuss the theory of particle acceleration in collisionless shocks, with emphasis on nonlinear effects, magnetic field amplification, and thermal particle injection. I will also discuss some recent observations that provide evidence for efficient particle acceleration in young supernova remnants.

Roberto Soria (CfA) in Pratt at 12:30 on 12 October 2005

POSTPONED

Matthias Vigelius (University of Melbourne) in Classroom at 12:30 on 19 October 2005

Gravitational Radiation from X-Ray Millisecond Pulsars

Observed spin frequency distribution of the fastest rotating neutron stars suggests that these objects are sources of gravitational waves: One possible mechanism to create a time-dependent quadrupole moment and therefore gravitational radiation is the formation of polar magnetic mountains. Such mountains are created when accreted material is confined at the poles by the magnetic tension of the stellar field. It is also widely believed that the observed reduction of the magnetic field of millisecond pulsars can be connected to the accretion phase during which the pulsar is spun up. A wide variety of reduction mechanisms have been proposed, including burial of the stellar field by magnetic mountains. In this talk, we will describe how to self-consistently model magnetic mountains and give a proof of their stability. The mountains effectively screen the magnetic dipole moment, reducing it by 90% after 10^-4 Msun have been added, and produce an associated reduced mass quadrupole moment of ~5x10^37 g cm^2 which is the correct size to explain the observed spin distribution. We will discuss the predicted spectrum of gravitational waves as well as the prospect of their detection with the new generation of long baseline interferometers. Finally, we will discuss the next step in these calculations, such as 3d non-ideal MHD simulations including sinking of the mountains, with the goal of a more accurate prediction of the gravitational wave signal.

Manami Sasaki (CfA) in Pratt at 12:30 on 26 October 2005

Shock-cloud interaction in the Galactic SNR CTB 109

We observed the Galactic supernova remnant (SNR) CTB 109 (G109.1-1.0) with XMM-Newton and Chandra. CTB 109 with its semi-circular shape is one of the most exotic objects in the X-ray sky. As neither X-ray nor radio emission is observed from the western part of the SNR shell, the SNR blast wave has apparently been stopped by a giant molecular cloud (GMC) complex located in the west. The EPIC data show remarkably little spectral variation across the remnant given the large intensity variations. There is an extended X-ray bright interior region known as the Lobe. This feature has previously been suggested as emission associated with the anomalous X-ray pulsar 1E 2259+586. However, the EPIC spectra show no indication of non-thermal emission. The Lobe is more likely enhanced emission from the interaction of the remnant with the GMC. The deep ACIS-I image reveals filamentary structures in the Lobe. Spatially resolved spectral analysis of the diffuse emission indicates variations in foreground absorption and plasma parameters in and around the Lobe.

Sebastian Heinz (MIT) in Pratt at 12:30 on 9 November 2005

Harrassing the Neighbors: How Jets Interact with their Environments

Jets can carry enormous amounts of kinetic energy and they are not shy about letting their environment know abut it. The prospect of heating the interstellar and intergalactic gas by firing off jets from growing black holes as a form of feedback to counteract radiative cooling in galaxies and galaxy clusters has recently brought new focus to the subject of jet-environment interactions. I will review our current understanding of this process for classical radio galaxies, launched by big, supermassive black holes, highlighting the considerable difficulties still present in modeling the heating of intergalactic gas. To this end, I will present a possible solution to this apparent `heating problem of cooling flows'.

Ben Chandran (University of New Hampshire) in Pratt at 12:30 on 16 November 2005

Are galaxy-cluster plasmas convective?

The specific entropy of the intracluster plasma in a cluster of galaxies increases outwards. If the convective stability criterion in such plasmas were the usual Schwarzchild criterion, the plasma would be convectively stable. However, magnetic fields and cosmic rays dramatically alter the convective stability criterion in such a way that clusters are convective throughout at least part of their volume. This talk will describe convective stability in clusters and present the results of a detailed mixing-length model of convective clusters, in which a central radio source produces cosmic rays which drive convection. Convection may play an important role in regulating a cluster's temperature profile and preventing plasma in cluster cores from cooling to low temperatures.

Yago Ascasibar (CFA) in PHILLIPS at 12:30 on 21 November 2005 (Monday)

Cold fronts in relaxed clusters

Chandra X-ray observations revealed the presence of cold fronts (sharp contact discontinuities between gas regions with different temperatures and densities) in the centres of many, if not most, relaxed clusters of galaxies with cool cores. In this talk, I will use the results of numerical simulations to try to convince you that these puzzling structures can be easily generated by the motion of satellite galaxies through the intracluster medium. We will also discuss the physical process in detail, as well as the observable imprints in X-rays and some of the numerical issues involved in its simulation.

Thanksgiving in at 12:30 on 23 November 2005

Kate Brand (NOAO) in Pratt at 12:30 on 30 November 2005

The accretion history of super-massive black holes in massive galaxies.

How did the mass of 10^9-10^10 solar mass super-massive black holes observed in the local Universe build up? Did the bulk of the growth happen in a luminous AGN phase? Or did a substantial fraction of SMBH growth occur in a dusty, obscured phase, visible as a luminous infrared galaxy? Has there been substantial SMBH growth in a radiatively inefficient regime after the more luminous AGN phase? These are particularly important questions given the tight relationship between the mass of galaxy bulges and their SMBHs, suggesting that the formation and evolution of galaxies are intimately linked to the accretion history of their SMBHs. I use the multi-wavelength data in the NDWFS Bootes field to address this issue. First, I will present an X-ray stacking analysis of ~20,000 red galaxies between z~0-1 to show that the nuclear accretion rates in these sources are either low or radiatively inefficient and are declining with time. I will then present my work on using the 24 to 8 micron ratio as a tool in determining the contribution of AGN to the mid-IR emission of luminous infrared galaxies and discuss the nature of an extreme, obscured population of ULIRGs with no comparable examples in the local Universe.

Anthony Piro (University of California at Santa Barbara) in Phillips at 11:00 on 2 December 2005 (Friday)

Recent Progress Relating the Theory and Observations of Neutron Star Oscillations

Neutron star surface layers can house a rich assortment of non-radial modes. Observations and modeling of such modes are powerful probes for learning about their interiors. I will review the basics of shallow ocean waves in the context of neutron stars, including interesting complications such as rotation, a solid crust, and a strong magnetic field. This will be related to the burst oscillations seen from accreting neutron stars and the exciting, recently discovered giant flare oscillations from soft gamma-ray repeaters.

Dan Evans (CfA) in Phillips at 12:30 on 7 December 2005

The Origin of X-Ray Emission in the Nuclei of Radio Galaxies

The physical origin of continuum X-ray emission in the cores of radio galaxies is widely debated. We present spectral results from Chandra and XMM-Newton observations of a sample of low-redshift FRI and FRII radio galaxies, and consider whether the emission originates from the base of a relativistic jet, an accretion flow, or contains contributions from both. We find that the nuclear X-ray spectra of FRI galaxies is dominated by unabsorbed emission from a jet. On the other hand, the nuclear spectra of FRII sources is heavily absorbed and likely to originate in an accretion flow. We discuss several models to account the differing nuclear properties of FRI- and FRII-type sources, and also demonstrate that both heavily obscured, accretion-related, and unobscured, jet-related components may be present at varying levels in all radio-galaxy nuclei.

Jennifer (Jeno) Sokoloski (CfA) in Pratt at 12:30 on 14 December 2005

The Ebullience of High Accretion Rate White Dwarfs

What happens when you accrete at a high rate onto a white dwarf? A. You get jets. B. You get quasi-steady thermonuclear shell burning on the white-dwarf surface. C. The white dwarf eventually approaches the Chandrasekhar limit and explodes as a Type Ia supernovae. D. The white dwarf experiences outbursts that are too energetic to be disk instabilities, yet too frequent to be classical novae. E. All of the above. I will discuss option E: all of the above. As part of this discussion, I will describe observational evidence for a new type of outburst that is triggered by a sudden burst of accretion, but powered by an increase in the rate of nuclear burning on the white-dwarf surface. This type of "combination nova" has in at least one case been linked with the production of jets. Furthermore, the association of classical symbiotic-star outbursts with combination novae has implications for whether the white dwarfs in symbiotic stars can gain enough mass to explode as Type Ia supernovae.

AAS Practice () in Phillips at 12:30 on 04 January 2006

TBA

Christian Leipski (Ruhr-Universitaet Bochum) in Pratt at 12:30 on 10 January 2006 (Tuesday)

The ISO-2MASS AGN Survey

We combined the ISOCAM Parallel Mode Survey at 6.7 micron with the 2MASS survey to obtain a powerful tool to search for AGN independent of dust extinction. Using moderate color criteria, we have selected 77 AGN candidates, amongst which optical spectroscopy reveals 30% type-1 QSOs, 12% type-2 AGN, and 57% red emission-line galaxies. Since one third of the type-1 sources show such red optical colors that they are missed in optical AGN surveys, the QSO surface density of the ISO--2MASS QSOs outnumber that of the SDSS quasar survey. We suggest that the red AGN resemble young members of the quasar population and that quasars spend much of their lifetime in a dust surrounded phase. However, mid-infrared spectroscopy with Spitzer of a sub-sample of the red type-1 QSOs do not show strong PAH emission from ongoing vigorous starbursts. While the emission-line galaxies were originally suggested to harbor a buried AGN due to their red NIR colors, the MIR spectroscopy do not support such an interpretation. These objects may resemble dusty, moderatley star-forming galaxies that seem to be very frequent in the nearby universe.

Carolyn Stern Grant (ADS Team) (CfA) in Pratt at 12:30 on 18 January 2006

How to make the most out of the ADS Abstract Service

It has been more than 12 years since the ADS Abstract Service was released to the astronomical community. During that time, the service has grown from several hundred thousand abstracts and no scanned pages or citations to more than 4 million abstracts, 3 million scanned pages, and more than 18 million citation pairs. I will discuss how to improve your searching, how to do some basic citation analysis, and the best ways to stay current on your favorite topics.

Aneta Siemiginowska and Vinay Kashyap (CfA) in Pratt at 12:30 on 08 February 2006

X-ray Astrostatistics: Bayesian Methods in Data Analysis

We will describe the California-Harvard AstroStatistics Collaboration, CHASC. We will provide an introduction to Bayesian methods in the context of some basic X-ray astrophysics problems, such as determining the source strength in the presence of background, and hardness ratios in the regime of (very) low counts. We will also discuss posterior predictive p-values (PPP), which are the preferred alternatives to the often abused F-tests used for model comparisons.

Ryan Hickox (CfA) in Phillips at 12:30 on 01 March 2006

Absolute measurement of the unresolved cosmic X-ray background in the 0.5-8 keV band with Chandra

We use the Chandra Deep Fields to measure the absolute intensity of the unresolved cosmic X-ray background (CXB). This measurement places new constraints on the total intensity of the CXB and the extent to which it has been resolved to date. I will present an overview of the measurement techniques, which involve exclusion of point and extended sources, and careful characterization and removal of the ACIS instrumental background. We find unresolved CXB intensities that are significant to 6 sigma in the 1-2 keV band and 2 sigma in the 2-8 keV band, which imply resolved fractions of the CXB of roughly 75-80%, smaller than previous estimates. The observed unresolved intensities suggest either a genuinely diffuse component (such as WHIM emission for E~1 keV) or a steepening of the logN/logS curve at low fluxes, which may be evidence for a new population of faint X-ray sources.

Rob Soria (CfA/UCL) in Pratt at 12:30 on 08 March 2006

Revising our view of ultraluminous X-ray sources

I will present updated results and speculations on three key issues for our understanding of this mysterious class of objects. (a) Some ULXs are associated with candidate radio lobes: this can help us determine the balance between mechanical (jets) and radiative luminosity. I shall compare this with the radio/X-ray behaviour of Galactic X-ray binaries. (b) The presence of a "soft excess" or disk component in ULX X-ray spectra is still a controversial issue: I will show how it is misleading to infer a mass from its fitted "temperature", and discuss alternative models. (c) I will briefly discuss what is arguably the closest example of the initial stages of ULX formation: a medium-size protocluster such as NGC 2264-C in our own Galaxy.

Dan Harris (CfA) in Pratt at 12:30 on 22 March 2006

Update on the Giant Flare in the M87 Jet

In 2005, the lightcurve of the knot, HST-1, began a sharp decay after having achieved an intensity more than 50 times larger than that observed in 2000. In the X-ray band, the decay timescale is similar to the characteristic rise time, suggesting that these timescales are controlled by light travel time across the emitting volume. Although uv and radio data are not yet complete, it appears that the uv lightcurve mimics the X-ray with little or no delay. We will describe the prospects for sorting out the LC decay at different bands in order to estimate the dominant mechanisms: light travel time; expansion; and energy loss process for the relativistic electrons. If this is successful, we should be able to determine an independent estimate of the average magnetic field strength as well as constrain the beaming factor. We will also show VLBA data which demonstrate superluminal motions downstream from the leading edge of HST-1.

Jan Egedal (MIT, PSFC) in Phillips at 12:30 on 7 April 2006 (Friday)

Magnetic Reconnection in Plasmas; a Celestial Phenomenon in the Laboratory

Plasmas -- ionized gas in lightning bolts, tube lights, and most of interstellar space -- are excellent conductors of electrical currents. Plasmas interact strongly with electric and magnetic fields and are generally frozen to magnetic field lines. However, the plasma can occasionally and rapidly break free and allow the magnetic field to change topology. This process is called magnetic reconnection and occurs in such diverse environments as the sun, the Earths magnetotail, and in magnetic fusion devices. Magnetic reconnection is responsible for, for examples, solar flares and the aurora borealis. An outstanding problem in reconnection theory is the discrepancy between the theoretical time scale predicted for magnetic reconnection and the much shorter observed time scale. Magnetic reconnection in the collisionless regime is studied on the Versatile Toroidal Facility (VTF) at MIT. The detailed evolution of the profiles of plasma density, current density, and electrostatic potential at the onset of driven reconnection is measured experimentally. The VTF device facilitates experiments with two distinct sets of boundary conditions: an open configuration for which the field lines intersect the vacuum vessel walls, and a closed configuration for which the magnetic field lines form closed loops inside the device. For the open configuration our studies reveal a new mechanism -- particle trapping -- responsible for fast reconnection. This mechanism is found to be consistent with unique spacecraft observations deep in the Earths magnetotail. The reconnection dynamics of the closed configuration differs significantly from that of the open, it is likely to be relevant to the solar plasma and fusion devices. In the talk I will discuss our experimental observations of magnetic reconnection in the open configuration, provide a theoretical explanation, and apply the theory to the spacecraft observations. I will also discuss preliminary results from the closed configuration.

Roy Kilgard (CfA) in Pratt at 12:30 on 12 April 2006

A Multiwavelength View of X-ray Populations in Galaxies

Observations of nearby galaxies with the Chandra X-ray observatory reveal a multitude of X-ray point sources, largely X-ray binaries and SNR. The X-ray data can provide crude classifications for these sources, but the only way to unambiguously classify a source is by observing a counterpart to that source in another wavelength. In addition, multiwavelength observations of X-ray source environments can provide a secondary means of classification. I will discuss a campaign to classify the discrete X-ray source population of nearby spiral galaxies using multiwavelength observations spanning the spectrum from radio through UV. I will further discuss the impact of the classification on three important X-ray diagnostics: first, that X-ray color can be used as a crude method of source classification; second, that the environment of X-ray sources within a host galaxy can help determine the formation history of the X-ray population; and third, that there may be universal luminosity functions of high- and low-mass X-ray binaries.

Paul Nulsen (CfA) in Pratt at 12:30 on 19 April 2006

AGN Heating of Clusters by Cavities and Shocks

It is now well established that AGN outbursts inject energy into surrounding gas in galaxies, groups and clusters. Also, AGN feedback is widely invoked to resolve issues of structure formation. However, while it has been demonstrated that AGN outbursts plays some part in solving the "cooling flow problem" (the lack of cooled gas at the centers of many systems with short cooling times) their overall significance and the heating mechanism remain subjects of debate. I will argue that the evidence is accumulating to show that AGN heating is the primary solution to the cooling flow problem. I will also argue that cavity enthalpy and shocks driven by expanding cavities both play important roles in heating the gas. Shocks are most effective close to expanding cavities, whereas cavity enthalpy is likely thermalized over more extended regions. Generally, some circulation is also required to prevent gas from cooling to low temperatures.

Patrick Slane (CfA) in Pratt at 12:30 on 26 April 2006

The Structure and Evolution of Pulsar Wind Nebulae

Pulsars steadily dissipate their rotational energy via relativistic winds. Confinement of these outflows generates luminous pulsar wind nebulae, seen across the electromagnetic spectrum in synchrotron and inverse Compton emission, and in optical emission lines when they shock the surrounding medium. These sources act as important probes of relativistic shocks, particle acceleration, and of interstellar gas. Here I review recent advances in the study of pulsar wind nebulae, with particular focus on the evolutionary stages through which these objects progress as they expand into their surroundings, and on morphological structures within these nebulae which directly trace the physical processes of particle acceleration and outflow.

Anne Lemiere (U. Paris VII) in Pratt at 12:30 on 3 May 2006

H.E.S.S. latest Galactic results and physical implications

H.E.S.S.(High Energy Stereoscopic System) is an array of telescopes exploiting the imaging Cherenkov technique, probing the gamma-ray universe between 100 GeV and 100TeV. Very high energy (VHE: E>10^11eV) gamma-rays are probes of the non-thermal universe providing access to energies far above accelerator energies on earth. We report here some results from the first sensitive survey of the inner part of the Milky-Way performed between 2003 and 2005 by HESS, which reveals a new population of VHE extended sources. While some of the sources can potentialy be associated with supernova remnants or pulsars wind nebula, at list two have no counterpart.

Marie Machacek (CfA) in Pratt at 12:30 on 10 May 2006

Outflows, Edges, Wakes and Tails: Snapshots of Galaxy Evolution in Cool Clusters and Groups

Galaxies in groups and clusters are subject to both tidal and hydrodynamical interactions that affect their evolution. While tidal interactions are identified by the appearance of disturbed stellar morphologies, the characteristic signatures of hydrodynamic processes, such as the action of ram pressure due to the galaxy's motion through the ambient medium, are imprinted on the hot X-ray emitting gas. These X-ray features include sharp surface brightness discontinuities (edges), 'horns' and 'tails' of gas turbulently stripped from the galaxy, and trailing wakes that track the galaxy's passage through the group or cluster core. I will use several nearby systems to show how detailed studies of these X-ray features, made possible by the high angular resolution of Chandra and XMM-Newton, not only reveal the nature of the gas-dynamical processes and feedback mechanisms working to transform the galaxy and its environment, but also constrain the three-dimensional motion of the galaxy as it passes through the group core, and may reveal high velocity encounters within these systems that are difficult to identify in any other way.

Kev Abazajian (LANL) in Phillips at 13:30 on 11 May 2006 (Thursday)

Hints at the nature of dark matter from dwarf galaxies to clusters of galaxies

Several observations of galaxy structure at small scales indicate the possibility of the need for modifications of the standard cold dark matter picture of structure formation. One such modification being actively considered is warm dark matter. Hidden in the neutrino sector of particle physics may be one or more fermions with no standard model interactions that nonetheless couple to neutrinos via their mass generation mechanism. Such a particle, a "sterile" neutrino, may be either cold or warm dark matter. I will discuss this candidate's production mechanism and its effects on galaxy-scale structure formation. Perhaps most interestingly, their production mechanism requires a coupling that leads to a radiative decay mode that may be observed by contemporary or future X-ray observations of Local Group galaxies or clusters of galaxies.

Ezequiel Treister (Universidad de Chile) in Pratt at 12:30 on 17 May 2006

AGN Unification and the X-ray Background

The AGN unification paradigm has been able to explain the observed properties of both obscured and unobscured AGN in the local Universe. However, whether this remains true at higher redshift is still unknown. Using a modified version of the AGN unification scheme, one in which the ratio of obscured-to-unobscured AGN changes with luminosity (more obscured AGN at lower luminosities) but does not evolve with redshift, we were able to explain the multiwavelength properties, ranging from infrared to X-rays, of the X-ray sources detected in the GOODS fields, which overlap with the deepest Chandra and XMM observations. This calculation assumes an average obscured-to-unobscured AGN ratio of 3:1, consistent with the observations of AGN in the local Universe. Allowing for an additional contribution from Compton-thick sources, we were able to explain the spectral shape and intensity of the X-ray background in the 1-100 keV range, where AGN emission is expected to dominate. The AGN contribution to the infrared background constrained from Spitzer observations of the GOODS fields is ~2\uffff^\uffff lower than previously expected, about 3-6% in the 3-24 microns range. Additionally, I will present the first results from a deep high-energy survey with INTEGRAL designed to obtain a complete sample of Compton-thick AGN in the local Universe.

Agnieszka Slowikowska (MPE Garching) in Pratt at 12:30 on 24 May 2006

The Studies of Three Rotation Powered Pulsars in Selected Energy Bands

The first part of the talk will be devoted to my radio and optical studies of the Crab pulsar. The most dramatic events in pulsar radio emission are so-called giant radio pulses (GRPs). They are a rare phenomenon, occurring in very few pulsars, including Crab. Our latest findings of new features of the Crab GRPs will be presented. In particular, an analysis of our Effelsberg data at 8.35~GHz shows that GRPs occur in all phases of rotation of its ordinary radio emission, including the phases of the two high frequency components (HFCs) visible only between 5 and 9~GHz. This suggests that similar emission mechanism may be responsible for the main pulse, the interpulse and the HFCs. Next, I will show and discuss phase-resolved polarisation characteristics of the Crab pulsar derived from our observations at Calar Alto using the high-speed photo-polarimeter OPTIMA. The intensity and polarisation were determined at all phases of rotation with higher statistical accuracy than ever, challenging theoretical models of pulsar emission. For the Crab twin from LMC, PSR B0540-69, I will show the latest INTEGRAL results for pulsed emission up to 100~keV. The analysis is based on observations of the LMC obtained in Jan. 2003 and Jan. 2004 with a total exposure of ~1.5 Ms (JEM-X and IBIS/ISGRI). Finally, the results for the closest and oldest X-ray ordinary pulsar, PSR B1929+10, will be presented. Pulsed emission was detected for combined ROSAT PSPC and HRI data. The X-ray spectrum can be satisfactorily described by a power-law or a double black-body model. With new XMM-Newton data of the source we confirm the existence of diffuse emission, with a trail lying in a direction opposite to the transverse motion of the pulsar. The pulsar's X-ray trail is likely formed by a ram-pressure confined pulsar wind.

Scott Randall (CfA) in Pratt at 12:30 on 31 May 2006

Constraining the Self-Interaction Cross-Section of Dark Matter with Numerical Simulations of 1E 0657-56

I will present results for constraining the self-interaction cross-section of dark matter, sigma, by comparing X-ray, optical, and strong and weak lensing observations of the galaxy cluster 1E 0657-56 (the so-called bullet cluster) with results from N-body simulations. This cluster shows a high-velocity merger in the plane of the sky with a prominent bow shock that gives a subcluster velocity of roughly 4800 km/s. A comparison of the X-ray image and weak-lensing mass map shows that the subcluster gas core lags the dark matter clump, which is coincident with the subcluster galaxies, indicating that the dark matter is not fluid-like. The observations allow for three independent methods for estimating sigma based on the relative offsets of the subcluster X-ray gas, galaxy, and total mass peaks, the high velocity of the subcluster, and its mass-to-light ratio. Analytic estimates based on these methods have previously been determined, though these estimates require simplifying assumptions that lead to conservative upper limits on sigma. I will show how tighter constraints on sigma are achieved by running detailed N-body simulations of the bullet cluster that include the effects of self-interacting dark matter. Additionally, I will describe how the observations alone provide evidence against some of the more popular versions of Modified Newtonian Dynamics.

Myriam Gitti (Ohio University) in Pratt at 12:30 on 7 June 2006

XMM-Newton view of the most powerful AGN outburst in a galaxy cluster: MS0735+7421

After a brief general overview of radio induced X-ray cavities observed in cooling flow clusters, I will report on the results of an XMM-Newton observation of the galaxy cluster MS0735+7421, where a giant cavity system has been discovered in a previous Chandra image showing that it hosts the most powerful AGN outburst currently known. XMM's large field of view and effective area allow us to accurately measure the temperature in the outskirts of the cluster, and to perform a detailed study of the mass profile reconstructed by using different methods. I will present several exceptional properties of this cluster and discuss their implications for the energetics of cooling flows, the "preheating" of clusters and the interaction of radio sources with the intra-cluster medium. I will also discuss the potential impact that these energetic AGN outbursts have on the general properties of clusters, like temperature profile and X-ray luminosity vs. temperature relation, which in turn can affect their utility as cosmological probes.

Misty Bentz (Ohio State University) in Pratt at 12:30 on 14 June 2006

Refining the Radius-Luminosity Relationship for AGN

The relationship between the size of the broad-line region (BLR) radius and the luminosity in AGN is the cornerstone for scaling relations that use single epoch spectra of high-redshift quasars to estimate their masses. We have undertaken various projects to improve the accuracy to which the radius-luminosity relationship is known. We account for host- galaxy starlight contributions to luminosity measurements using high-resolution HST images of the central regions of reverberation-mapped AGN. Initial results show that removing the starlight component results in a significant correction to the luminosity of each AGN, not only for the lower-luminosity sources but also for the higher-luminosity sources such as the PG quasars. We have also implemented new ground-based monitoring programs to replace earlier, inadequate BLR radius measurements for several reverberation-mapped AGN.

ESSENCE supernova meeting (CFA) in Pratt at 12:30 on 21 June 2006

NO TALK TODAY

Gianluca Israel (Rome) in Phillips at 13:00 on 22 June 2006 (Thursday)

Unveiling the AXPs/SGRs connection

In the latest years many new observational properties have been discovered, which changed our view of Anomalous X-ray Pulsars (AXPs), in great extent thanks to the new generation instruments. It is now evident that the multi-wavelength phenomenology of AXPs/SGRs is more complex than thought before. In this talk I will review the recently identified properties of AXPs comparing them with those of Soft gamma-ray Repeaters, with which AXPs are thought to be related at some level.A number of special cases which helped us in better understanding the class will be presented and discussed. Among others are the 27th December 2004 hyperflare from SGR1806-20 and the transient AXP XTEJ1810-197.

Simon Steel and Erika Reinfeld (CfA) in Pratt at 12:30 on 28 June 2006

A New Museum Exhibit on Black Holes

Following on from the successful 4-year traveling exhibit "Cosmic Questions," the Universe Forum at the CfA's Science Education Department is planning a new exhibit on black holes. We would like to work with the HEA division to brainstorm concepts that should be included in such an exhibit, and discuss ways in which scientists and engineers can become involved in development. Our presentation today will include a short talk about the proposed new exhibit and then break into a discussion about further planning.

Belinda Wilkes (CfA) in Pratt at 12:30 on 05 July 2006

The NASA Advisory Committee (NAC): Astrophysics Sub-Committee (APS)

NASA has recently re-constituted its Advisory Committees at the direction of Congress. I will summarise the status of the NAC, the membership of the APS, and the initial meeting in early May. The NASA science budget continues to be severely strained due to recent cuts and difficult decisions are being/have been made as to which missions can be supported. As your representative on the APS, I will summarise the information I have been given on the budget and the status of various missions, the current organisation of NASA Science and the Science Plan that is being drafted at present. The second meeting will take place 6-7 July. As your representative on the APS, I would like to obtain any feedback/input you may have, either before this meeting or at any time.

Firoza Sutaria (Pennsylvania State University) in Pratt at 12:30 on 09 August 2006

Chandra observations of AXP J1708

Observations of Anomalous X-ray Pulsar (AXPs) and Soft Gamma-ray Repeaters (SGRs) have opened up a host of questions regarding the underlying emission mechanisms from neutron stars with magnetic fields ~=10^{14} G (magnetars). In this talk I will present the results of deep Chandra observations of AXP J1708-4009, taken in the continuous clocking mode. J1708 is one of the two known glitching AXPs, and has been well studied in the past with RXTE, BeppoSAX and XMM-Newton observatories. The Chandra timing analysis reveals that this glitching source has been slowing down consistently for the last 3.5 yrs since the last glitch. However, the Chandra spectral observations show that the source spectrum and luminosity are indeed variable , and that the 8.1 keV absorption feature seen in the previous BeppoSAX observation is now absent, both in the phase-integrated, and in the phase-resolved spectra. Further, spectral analysis of multiwaveband (IR to Gamma-ray) spectra of J1708, suggests the need to revisit the question of absorption and extinction columns in the direction of this source. Finally, I present a comparison of the spectral and temporal properties of J1708 with other AXPs and discuss the implications of our observations for models of magnetar emission mechanisms.

Slavko Bogdanov (CfA) in Pratt at 12:30 on 20 September 2006

X-rays from Radio Millisecond Pulsars

Dennis Bodewits (KVI Atomic Physics Groningen) in Pratt at 12:30 on 5 October 2006 (Thursday)

Cometary X-rays: Solar wind charge exchange in cometary atmospheres

The interaction of the solar wind with the planets, moons and the interstellar medium is of key importance for understanding the evolution of our solar system. The interaction with Earth's atmosphere is best known for the northern light. In case of Mars, the interaction with the solar wind might have lead to the erosion of its atmosphere. Solar wind-atmosphere interactions can be studied particularly well in cometary atmospheres, because in that case the solar wind flow is not attenuated by a planetary magnetic field and interacts directly with its atmosphere, the coma. When solar wind ions fly through an atmosphere they are neutralized via charge exchange reactions with the neutral gaseous species. These reactions depend strongly on target species and collision velocity. The resulting X-ray and Far-UV emission can therefore be regarded as a fingerprint of the underlying reaction, with many diagnostic qualities. This seminar will address all aspects relevant for X-ray and FUV emission from comets: experimental studies of state-to-state charge exchange cross sections, observations of X-ray emission from comets using Chandra, XMM, and Swift, and theoretical modeling of the interaction of solar wind ions with cometary atmospheres and the resulting X-ray emission spectrum.

Belinda Wilkes (CfA) in Pratt at 12:30 on 11 October 2006

The NASA Advisory Committee (NAC): Astrophysics Sub-Committee (APS)

NASA has recently re-constituted its Advisory Committees at the direction of Congress. I will summarise the status of the NAC, the membership of the APS (which include myself) and the initial meetings, 3 so far. The NASA science budget continues to be severely strained due to recent cuts and difficult decisions are being/have been made as to which missions can be supported. I will summarise the information I have on the budget and the status of various missions, the current organisation of NASA Science and the Science Plan that is being drafted at present. As your representative on the APS, I would like to obtain any feedback/input you may have, either before this meeting or at any time.

Malcolm Coe (University of Southampton, UK) in Pratt at 12:30 on 8 November 2006

Wings, Bars and Star Formation in the SMC

Extensive X-ray observations from Chandra and RXTE have revealed a substantial population of high mass X-ray binaries, primarily in the Bar of the Small Magellanic Cloud. These observations over many years have recently culminated in a programme using Chandra to map the other major feature of the SMC - the Wing. This talk will present the early results from this Chandra programme together with follow-up optical studies of many of the 500 newly identified X-ray sources.

Richard Wilman (Durham) in Pratt at 12:30 on 15 November 2006

A low energy perspective on cooling flows

Our understanding of cluster cooling flows has been revolutionised since the turn of the millennium, with a sharp downward revision in X-ray cooling rates and the identification of radio-loud AGN as plausible heat sources for the cluster gas. In parallel with these high profile advances, significant progress has also been made in the search for cool molecular gas in cooling flows; the predicted molecular gas masses are now consistent with the observations, but the relationship between the various cool gas components is still uncertain. In this talk I will begin with a review of surveys for hot and cool H_2 in cooling flows. I will then present highlights from my recent optical and IR integral field spectroscopy of the ionized and molecular gas in several systems. These include: (i) the discovery of a 50-pc nuclear disk of hot H_2 in the core of NGC 1275 in the Perseus cluster and the measurement of the nuclear black hole mass; (ii) VLT-VIMOS IFU mapping of the optical line emission on scales less than 40 kpc in several H-alpha-luminous systems, and implications for the connection between this gas and the cool molecular gas.

Alexey Vikhlinin (CfA) in Pratt at 12:30 on 22 November 2006

Lack of "Cooling Flow" Clusters at z>0.5

We study the incidence rate of ``cooling flows'' in the high redshift clusters using Chandra observations of z>0.5 objects from a new large, X-ray selected catalog. We find that only a very small fraction of high-$z$ objects have cuspy X-ray brightness profiles, which is a characteristic feature of the cooling flow clusters at z~0. The observed lack of cooling flows is most likely a consequence of a higher rate of major mergers at z>0.5.

Guido Risaliti (CfA) in Pratt at 12:30 on 29 November 2006

X-ray observations of NGC 1365: Time-resolved eclipse of the X-ray source

I present the extraordinary X-ray observations of the Seyfert Galaxy NGC 1365, performed with XMM-Newton and Chandra. This source is unique in two respects: first, the X-ray spectra clearly show the presence of a highly ionized gas close to the source, responsible of Fe XXV and FeXXVI absorption lines; second, changes from Compton-thick to Compton-thin states have been observed in time scales of ~2 days, due to occultations by an intervening cloud. These rapid variation times have strong consequences for the unified model of AGN, implying an extremely compact structure of the circumnuclear absorber (within the BLR region). Moreover, they provide a direct measurement of the size of the X-ray emitting region, which, for reasonable velocities of the occulting cloud is less than ~10^14 cm, corresponding to a few gravitational radii according to the black hole mass estimates obtained both with M-sigma and M-L relations.

Steven Ritz (GSFC) in Pratt at 12:30 on 13 December 2006

GLAST Mission Overview and Opportunities

The Gamma-ray Large Area Space Telescope, GLAST, is a mission to measure the cosmic gamma-ray flux in the energy range 20 MeV to >300 GeV, with supporting measurements for gamma-ray bursts from 10 keV to 25 MeV. With its launch in 2007, GLAST will open a new and important window on a wide variety of high-energy phenomena, including black holes and active galactic nuclei, gamma-ray bursts, the origin of cosmic rays and studies of supernova remnants, and searches for hypothetical new phenomena. Along with the science, this talk will include a description of the instruments and their capabilities, the mission status, and the opportunities for Guest Investigators.

Howard Smith (CFA) in Pratt at 12:30 on 3 January 2007

The Herschel Space Telescope

The Herschel Space Telescope is a 3.5-m, passively cooled space telescope, that will be launched in ~late 2008 with a complement of three IR/submm instruments for imaging, photometry and spectroscopy between 57um and 625um; the maximum spatial resolution is 6arcsec, and the maximum spectral resolution (heterodyne) is 107. For more info see the webpages: http://www.rssd.esa.int/Herschel/ or http://www.ipac.caltech.edu/Herschel/index.shtml 68% of Herschel's programs will be in open time, with the large majority of these in dedicated "Key Projects." The deadline for Key Project proposals is October 15, 2007. Small observing proposals will also be considered, but the call for small programs will not be made until about 6 months AFTER launch in late 2008. Thus, anyone seriously interested in getting into the Herschel activities should think now about Key Project opportunities. NASA is supporting Herschel with instrument participation and software support at the Herschel Science Center at JPL and IPAC. *NASA will also support US investigators on Herschel Key Projects, probably in a significant way.* The OIR and RG communities are well aware of Herschel opportunities. In this talk I will present Herschel and its instruments to HEAD, and discuss the Guaranteed Time programs. My hope is to stimulate team collaboratins on extragalactic and galactic Herschel projects.

Fabio Gastaldello () in Pratt at 12:30 on 17 January 2007

X-ray bright Galaxy Groups as cosmological tools

We present radial mass profiles for 16 relaxed galaxy groups ( kT 1-3 keV) selected for optimal mass constraints from the Chandra and XMM data archives. The resulting mass profiles are described well by a two component model consisting of dark matter, represented by an NFW model, and stars from the central galaxy. For the first time we find that the NFW concentration parameter (c) for groups decreases with increasing virial mass (M) as expected in standard Lambda-CDM models. When combined with our own results for 7 elliptical galaxies and clusters from the literature, the X-ray c-M relation agrees with the relation produced by the flat, concordance Lambda CDM model provided the sample is comprised of the most relaxed, early forming systems, which is consistent with our selection criteria. The tilted, low sigma_8 model suggested by the 3-yr WMAP analysis is rejected at > 99.99% confidence, but it can be reconciled with the X-ray data by increasing the dark energy equation of state parameter to w = -0.8.

Lukasz Stawarz (Stanford University) in Phillips at 12:30 on 24 January 2007

On the Electron Energy Distribution in Jets, Hotspots and Lobes of Extragalactic Radio Sources

Understanding of extragalactic radio sources requires understanding of their multiwavelength emission, and thus energy evolution of the radiating particles. Unfortunately, many of the key issues regarding particle acceleration and generation of the non-thermal radiation in relativistic jets, their hotspots and lobes, are still hardly known. Here I will review some of the new results concerning these problems. In particular, I will discuss how an interplay between theoretical studies and the most recent multifrequency observations allow for constraining acceleration and radiative processes taking place in extragalctic radio sources, and thus for extracting crucial macroscopic parameters of the considered objects. I will emphasize that in many aspects these new results contradic/question standard models, assumptions and expectations. Paradigm of the diffusive shock acceleration and the resulting universal power-law form of the radiating electrons are the two examples of the challenged issues.

Pepi Fabbiano (CfA) in Pratt at 12:30 on 7 February 2007

The Chandra Source Catalog

The Chandra X-ray Center (CXC) has undertaken the production of the Chandra Source Catalog, a data-mining project that makes use of the continuously growing Chandra public archive. The distinguishing characteristic of Chandra, the NASA Great Observatory for exploring the universe in the X-rays, is its sub-arcsecond resolution, which provides the most sensitive and detailed view of the ~0.3-7 keV sky presently attainable. The Catalog will characterize the X-ray sky at high resolution and with accurate astrometry, making use of all the imaging Chandra data. It will provide a uniform reduction of the Chandra archive, that will be a major interface for the Virtual Observatory; will be continuously updated as more data enters the public domain during the on-going Chandra mission; and will enable a dynamic interaction to produce user-configured views with on-the-fly analysis workflows. In this paper we describe the Catalog, and the software and hardware needed for its realization.

Ryan Hickox (CfA) in Phillips at 12:30 on 21 February 2007

Resolving the unresolved cosmic X-ray background in the Chandra deep fields

We present a measurement of the intensity of the diffuse cosmic X-ray background (CXB) in the Chandra Deep Fields North and South (CDF-N and CDF-S), expanding on our previous analysis where we excluded X-ray sources detected in these ultra-deep pointings. Motivated by a recent X-ray stacking analysis by Worsley and collaborators, which showed that galaxies detected by hst but not by Chandra contribute a significant fraction of the unresolved CXB, we additionally excise HST and Spitzer IRAC sources. These sources account for most of the CXB flux above 1 keV that remained after the removal of X-ray detected sources. Some unresolved CXB still remains, although it is barely significant: (3.1+/-1.4)x10^-13 ergs cm^-2 s^-1 deg^2 in the 1--2 keV band and (2+/-9)x10^-13 ergs cm^-2 s^-1 deg^2 in the 2--5 keV band, or 7+/-3% and 2+/-9%, respectively, of the total CXB intensity. Galaxies with ``starburst'' colors in the optical account for ~=43% of the X-ray-unresolved CXB in the 1--2 keV band, while ``normal'' (non-starburst) galaxies contribute ~=21%. Below 1 keV, the CXB is dominated by diffuse Galactic and local emission. The unresolved CXB in the 0.65--1 keV energy band (just above the bright Galactic O VII line and including the Fe XVII lines) is (12+/-2)x10^-13 ergs cm^-2 s^-1 deg^2, which is getting interestingly close to the predictions for the average emission from the warm-hot intergalactic medium (WHIM). A WHIM simulation that accounts for the particular selection of the CDF pointings may thus provide constraints on the WHIM metallicity.

Jeremy Lim (Institute of Astronomy and Astrophysics (Taiwan)) in Pratt at 12:30 on 28 February 2007

Radially-Infalling Molecular Gas from an X-ray Cooling Flow

Radiative cooling of the hot X-ray gas surrounding galaxy clusters should result in an inflow of cool gas to the cluster center with mass-deposition rates of up to several thousands of solar masses a year. This traditional picture of X-ray cooling flows has long been challenged by the inability to find cooler gas (below 10^6 K) with masses anywhere close to the predicted levels. In recent years, the absence in XMM-Newton spectra of X-ray gas at temperatures below about one-third the ambient X-ray temperature, and the ubiquitous presence in Chandra images of strong disturbances in the X-ray gas at the centers of putative cooling-flow clusters, have demonstrated that AGN activity (radio jets) reheats the X-ray gas thus severely reducing if not quenching the cooling flow. Here, we present compelling evidence that the X-ray cooling flow in the Perseus cluster is not completely quenched, but has recently deposited molecular gas on kpc scales in the central cD galaxy but only in directions away from its radio jets. This molecular gas can be traced all the way in to the center of the galaxy, and is likely responsible for fueling the AGN.

Roberto Soria (CfA) in Pratt at 12:30 on 28 March 2007

Revisiting accretion-state evolution in ULXs and Galactic BHs

It has been suggested that the X-ray spectral and timing properties of ultraluminous X-ray sources (ULXs) resemble those of stellar-mass black holes (BHs) in their very high state (or steep-power-law state). I try to quantify this comparison. I show that Galactic BHs display two types of very high state. One is characterized by only a partial upscattering of the disk photons, with the innermost stable orbit still visible; the disk appears hotter and harder. The other occurs at even higher accretion rates, when the inner disk is completely covered, replaced, or drained of all its accretion power by a non-thermal medium. What is left of the (outer) disk appears much cooler and larger. I suggest that the latter state is the ULX branch. In this scenario, ULXs would have a mass ~ 50-100 solar, and an accretion rate ~ 10-20 times Eddington. Understanding the power budget of BHs in this accretion regime has also fundamental implications for galaxy formation.

Paulo Lopes (CfA) in Pratt at 12:30 on 11 April 2007

Scaling Relations of the NoSOCS Galaxy Clusters

Galaxy cluster properties such as total mass, X-ray temperature (TX) and luminosity (LX), velocity dispersion (sigma) and optical luminosity (Lopt) can be used to define scaling relations which are very helpful for studying the intra-cluster medium (ICM) and cosmology. An important tool for the latter is the cluster mass function and its evolution with cosmic time. However, the ability to compute the mass function for a large data set depends on the connection between cluster mass and an easily observable quantity. Underestimation of the scatter of the mass-observable relation could lead to controversial determinations of cosmological parameters. Here we investigate the connection between richness and X-ray properties of galaxy clusters, paying special attention to the impact of substructure on these relations. We have used literature data from BAX to prepare a list of X-ray emitting galaxy clusters in the northern sky and compare those to optically selected systems from DPOSS. We evaluated the recovery rate of the X-ray clusters in the optical as a function of richness, redshift and X-ray luminosity. Substructure alone can not explain the scatter in the richness to LX relation, but the comparison between richness and temperature is very sensitive to the exclusion of clusters showing signs of substructure. Additionally, we used SDSS data for the low redshift systems (z le 0.1) to estimate their velocity dispersion, mass and virial radius. The connection of these parameters to richness (and Lopt), as well as the X-ray properties, is further investigated.

Greg Sivakoff (Ohio) in Pratt at 12:30 on 18 April 2007

Low-Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies

The sub-arcsecond resolution of the Chandra X-Ray Observatory has resolved the X-ray content of nearby early-type galaxies into low-mass X-ray binaries (LMXBs) and diffuse interstellar gas. Extragalactic low-mass X-ray binaries probe the massive stars formed billions of years ago in early-type galaxies and probe the properties of dense stellar environments (globular clusters). The ~50-200 bright LMXBs per galaxy are an incredible complementary sample to the ~150 active Galactic LMXBs; large LMXB samples reveal general LMXBproperties and rare LMXB phenomena. I discuss results from single- and multi-epoch observations of individual galaxies. In particular, I highlight results comparing a sample of eleven early-type Virgo Cluster galaxies with Hubble Space Telescope Advanced Camera for Surveys observations of globular clusters. Finally, I discuss future plans to explore the X-ray content of early-type galaxies.

Arielle Phillips (Amherst) in Phillips at 12:30 on 25 April 2007

Chasing a WHIM: The Changing Picture of an Important Baryon Reservoir

The warm/hot intergalactic medium (WHIM) in the cosmic web may help solve the missing baryon problem. X-ray absorption observations of a WHIM component in the Local Group and at higher redshift, numerous detections of WHIM OVI absorption features in quasar spectra, as well as recent observations of the cosmic X-ray background in the Chandra Deep Fields North and South compel us to revisit earlier theoretical predictions for the WHIM. We use a new algorithm to ferret out and extract structures in a higher resolution large scale simulation which includes the effects of galactic superwind feedback and non-equilibrium ionization. The predicted temperature-density phase diagram for the intergalactic medium points to a new definition for the WHIM. A better understanding of the physical properties and extent of the WHIM (and therefore of the interpretation of its signature in current and future observations) is achieved by progressing beyond a threshold-based definition of this component to look at the topology and geometry of this substantial baryon reservoir.

Bettina Posselt (MPE) in Pratt at 12:30 on 2 May 2007

Where to Look for X-ray Thermal Isolated Neutron Stars

X-ray thermal neutron stars like the famous 'Magnificent Seven' allow us to study directly the neutron star's surface emission. This enables the determination of radii constraining the equation of state. The Magnificent Seven represent nearly half of the local neutron star population and may be interesting as a connecting link between pulsars, rotating radio transients and anomalous X-ray pulsars/Soft Gamma-ray repeaters. Despite many searches since the 90's no new candidates have be confirmed as X-ray thermal neutron star to date. New results from our recent population synthesis of neutron stars with thermal X-ray emission will be presented in this talk. Regions in the sky that are favourable for new searches are identified, which depend on the progenitor and ISM distribution. Our own search program with ROSAT and XMM is summarized with respect to the population synthesis result. The expected age and distance distributions of the X-ray thermal neutron stars will be discussed.

Rhaana Starling (Univ. of Leicester) in Pratt at 12:30 on 9 May 2007

Three things we can learn from Gamma-ray Burst afterglow spectral energy distributions

Gamma-ray Bursts (GRBs) are highly energetic phenomena occurring throughout cosmological history. A large fraction of them likely form in the core collapse of a massive star, and we find GRBs located in star-forming regions of faint, blue galaxies out to z=6.3. Spectral energy distributions (SEDs) of the GRB afterglow emission provides a means of probing the host galaxies and the GRB physics. I will describe three recent studies I have made using the SEDs of a sample of BeppoSAX GRBs, to measure the host galaxy extinction, the density structure of the circumburst media and further understand the physics of the blastwave.

Alessandro Baldi (CfA) in Pratt at 12:30 on 16 May 2007

A Chandra archival study of the temperature and metal abundance profiles in hot Galaxy Clusters at 0.1 lt z lt 0.3

We present the analysis of the temperature and metallicity profiles of 12 galaxy clusters in the redshift range 0.1-0.3 selected from the Chandra archive with at least ~20,000 net ACIS counts and kT gt 6 keV. We divide the sample between 7 Cooling-Core (CC) and 5 Non-Cooling-Core (NCC) clusters according to their central cooling time. We find that single power-laws can describe properly both the temperature and metallicity profiles at radii larger than 0.1 r180 in both CC and NCC systems, showing the NCC objects steeper profiles outwards. A significant deviation is only present in the inner 0.1 r180. We perform a comparison of our sample with the De Grandi and Molendi BeppoSAX sample of local CC and NCC clusters, finding a complete agreement in the CC cluster profile and a marginally higher value (at ~1 sigma) in the inner regions of the NCC clusters. The slope of the power-law describing T(r) within 0.1 r180 correlates strongly with the ratio between the cooling time and the age of the Universe at the cluster redshift, being the slope gt 0 and tau_c/tau_{age} ~ 0.6 in CC systems.

Somak Raychaudhury (CfA) in Pratt at 12:30 on 23 May 2007

The Star Formation Properties of Galaxies in Clusters

It has long been known that galaxies in the cores of clusters generally have very little ongoing star formation, and that the star formation rate of a galaxy is correlated with the density of galaxies in its immediate environment. On the other hand, there have been several recent sightings of galaxies with unusually high star formation on the outskirts of clusters. This talk will summarize observational evidence of how the star formation properties of galaxies change as they fall into a cluster, and the insight this gives into the physical processes that are involved.

David Cohen (Swarthmore College) in Pratt at 12:00 on 11 June 2007 (Monday)

Quantitative Analysis of Resolved X-Ray Emission Line Profiles of O Stars

The resolved X-ray emission line profiles of O stars carry a significant amount of information about the kinematics of the hot plasma in these stars' massive stellar winds. They provide important clues about the X-ray production mechanism, and via the effects of continuum absorption, also place key constraints on conditions in the bulk, cool wind. For normal O stars, the broad X-ray emission line profiles resolved by Chandra and XMM - which are more symmetric than expected - can help disentangle the competing effects of mass-loss rate reduction and large-scale wind clumping. Initial results indicate that O star mass-loss rates must be significantly lower than has commonly been supposed, whereas there is little evidence for large-scale clumping and the associated wind porosity. A subset of hot stars show X-ray emission lines that are much narrower than those of typical O stars. I will briefly discuss magnetically channeled wind models that can explain these narrower emission lines (and the observed harder and stronger X-ray emission) in young O stars like theta1 Ori C, and also a different scenario that can explain the very soft X-rays and narrow emission lines in the Chandra and XMM spectra of early B stars.

Andisheh Mahdavi (University of Victoria, Canada) in Phillips at 12:30 on 18 June 2007 (Monday)

Combining Multiwavelength Observations of Galaxy Clusters

Clusters of galaxies are dominated by dark matter. We can see the gravitational effect of this dark material on the orbits of cluster members, the thermodynamics of the hot gas, and the shapes of galaxies behind the cluster. I will show that combining multiwavelength data for a single relaxed cluster can yield powerful constraints on its dark matter distribution. JACO, a parallel code for joint modeling of X-ray, lensing, SZ, and optical data, will soon be publicly available for this purpose. At the same time, as the bullet cluster teaches us, multiwavelength observations of merging clusters can yield significant and perhaps even more interesting constraints on dark matter properties. Both relaxed and merging clusters are well-represented in the Canadian Cluster Comparison Project, an upcoming survey of fifty gt 5 keV clusters at z~0.25. I will conclude by discussing an unusual, massive, X-ray bright region devoid of galaxies at the core of Abell 520.

Stefania Carpano (European Space Astronomy Centre, Spain) in Pratt at 12:30 on 27 June 2007

Wolf-Rayet/Black-Holes X-ray binaries

A small number of high-mass binary systems should end up with a compact neutron star or black hole orbiting a Wolf-Rayet star. An even smaller number would be strong accreting X-ray sources with luminosities of about 1038 erg/s. Until recently, the famous Cyg X-3 was the only recognised example for such system. Few years ago, the brightest source in the nearby galaxy IC 10, has been discovered to be the first extragalactic counterpart. Thanks to XMM data, we recently discovered the second extragalactic candidate in the spiral galaxy NGC 300. Furthermore, Swift observations of these objects, NGC 300 X-1 and IC 10 X-1, have shown them both to have very similar periods of 32.8+-0.4 and 34.8+-0.9 hours, respectively, that are probably the orbital periods of 30 or 40 solar mass black holes around almost identical Wolf-Rayet stars. It seems a surprise that these modulations are so similar and yet so large compared to the short 4.8- hour period of Cyg X-3. How do these systems form and become such bright X-ray sources?

Agnieszka Janiuk (University of Nevada) in Pratt at 12:30 on 11 July 2007

Black hole accretion with low angular momentum

I will discuss the evolution of small angular momentum gas accreting onto a black hole. The problem might be relevant to various types of astrophysical objects, from active galactic nuclei to the collapsing massive stars as the progenitors of gamma ray bursts. I will present the results of the 2D and 3D non-axisymmetric hydrodynamical simulations, showing how the rotationally supported torus forms around the central black hole and how much angular momentum is crucial for the torus to close. I will also discuss the dependence of this critical angular momentum amount on the black hole mass, and the implications of the Wolf Rayet star models for the durations of long gamma ray bursts.

Raanan Nordon (Technion, Israel) in Pratt at 12:30 on 18 July 2007

Coronal X-ray flares, abundances and human color vision

The study of stellar X-ray flares has been given a great boost since the launch of the hi resolution telescopes XMM-Newton and Chandra. It has since been observed that many stellar coronae exhibit abundance fractionation different from Solar. While in the Solar corona, low first ionization potential (FIP) elements seem to be enriched compared with the photosphere, some other stars show an inverse effect - a depletion of low FIP elements compared with high FIP elements. A few studies have linked this change of abundance pattern with X-ray activity. We analysed a sample of flares from XMM-Newton and Chandra archives looking for evidence of abundance variations during large flares. The results are discussed.

Saku Vrtilek (CfA) in Pratt at 12:30 on 25 July 2007

The X-ray photoionized wind of Cygnus X-1 during X-ray high/soft states

High-resolution ultraviolet observations of the black hole X-ray binary Cygnus X-1 were obtained using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Observations were taken at two epochs roughly one year apart; orbital phase ranges around $\phi_{orb}$ = 0 and 0.5 were covered at each epoch. We analyze the characteristics of a selection of P Cygni profiles and note, in particular, a strong dependence on orbital phase for the high ionization material: the profiles show strong, broad absorption components when the X-ray source is behind the companion star and noticeably weaker absorption when the X-ray source is between us and the companion star. We fit the P~Cygni profiles using the Sobolev with Exact Integration method applied to a spherically symmetric stellar wind subject to X-ray photoionization from the black hole. Of the wind-formed lines, the Si\,IV doublet provides the most reliable estimates of the parameters of the wind and X-ray illumination. Our models determine parameters that may be used to estimate the accretion rate onto the black hole and independently predict the X-ray luminosity. Our predicted L$_x$ matches that determined by contemporaneous RXTE ASM remarkably well, but is a factor of 3 lower than the rate according to Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the energy of accretion may go into powering a jet. We test our model by comparing our predicted X-ray luminosity with contiguous observations by the RXTE ASM.

Bram Boroson (CfA Visitor) in Philips at 12:30 on 17 August 2007 (Friday)

Problem Means Opportunity: X-ray Binaries Distort Winds and Reveal Them

I will review HST and Chandra observations of X-ray binaries can test models of stellar winds and disk winds. When X-rays disturb the system, we can use that disturbance to reveal the kinematics. I will point out ways these methods can be improved, and put what we can learn in the context of the energy balance between the emitted X-rays, jets, and accretion rate.

Joey Neilsen (Harvard University) in Pratt at 12:30 on 12 September 2007

The Eccentric Accretion Disc of the Black Hole A0620-00

In 1975, at a peak flux of 50 Crab, A0620-00 became the brightest X-ray nova ever detected. I will present new optical spectroscopic observations of this now-quiescent black hole binary, taken on the Magellan telescopes at Las Campanas Observatory. We use absorption lines from the secondary to measure its radial velocity and rotational broadening, as well as the systemic mass ratio and the fraction of light contributed by the disk. Although quiescence implies little accretion activity, we find that the disc contributes 56 +/- 7 per cent of the light in B and V, and is subject to significant flickering. Doppler maps of the Balmer lines reveal bright emission from the gas stream-disc impact point and unusual crescent-shaped features. We also find that the disc centre of symmetry does not coincide with the predicted black hole velocity. Instead, it appears to wobble in the corotating frame. By comparison with SPH simulations, we identify this source with an eccentric precessing disc. With high S/N, we pursue modulation tomography of H-alpha and find that the aforementioned bright regions are strongly modulated at the orbital period. We interpret this modulation as a superhump phenomenon, and discuss relevant cases for the accretion disc evolution.

Kisha Delain (University of Minnesota) in Phillips at 12:30 on 19 September 2007

Diffuse Radio Emission in Groups of Galaxies

I present results of new diffuse radio sources likely associated with groups of galaxies and without apparent AGN. We have discovered these sources through an unbiased search of the WENSS and WISH catalogs. Until now, this type of source was found exclusively in rich clusters of galaxies, probably due to selection effects. The radio halos and 'relics' of rich clusters are thought to be powered by shocks and turbulence from infall into their deep potential wells. Our detection of similar sources within the shallow potential wells of groups of galaxies challenges this model. Their radio luminosities are approximately two orders of magnitude higher than expected from the extrapolation of the apparent rich cluster radio/X-ray luminosity relation.

David Angelo Rapetti (Stanford) in Phillips at 12:30 on 24 September 2007 (Monday)

The potential for constraining dark energy with X-ray cluster mass fraction measurements

We examine the ability of a future X-ray observatory, with capabilities similar to those planned for the Constellation-X mission, to constrain dark energy via measurements of the cluster X-ray gas mass fraction, f_gas. We find that f_gas measurements for a sample of ~500 hot (kT>~5keV), X-ray luminous, dynamically relaxed clusters, to a precision of ~5 per cent, can be used to constrain dark energy with a Dark Energy Task Force (DETF; Albrecht et al. 2006) figure of merit of 20-50. Such constraints are comparable to those predicted by the DETF for other leading, planned `Stage IV' dark energy experiments. Our analysis uses a Markov Chain Monte Carlo method which fully captures the relevant degeneracies between parameters and facilities the incorporation of priors and systematic uncertainties in the analysis. We explore the effects of such uncertainties, for scenarios ranging from optimistic to pessimistic. We conclude that the f_gas experiment offers a competitive and complementary approach to the other best large, planned dark energy experiments. In particular, the f_gas experiment will provide tight constraints on the mean matter and dark energy densities, with a peak sensitivity at redshifts midway between those of supernovae and baryon acoustic oscillation experiments.

Edwin Kellogg and Joy Nichols (CfA) in Pratt at 12:30 on 26 September 2007

R Aquarii: a small binary with big ambitions

Observations with X-ray and far-UV telescopes and the VLA in the past several years reveal a rich variety of features in the historically well studied but still mysterious symbiotic binary, R Aquarii. We discuss jets, lobes, and spectral features reminiscent of AGN plus detection of a 29 minute period, both from the central region of this fascinating and apparently unique system.

Sudip Bhattacharyya (GSFC) in Pratt at 12:30 on 3 October 2007

Neutron Stars and Thermonuclear X-Ray Bursts

Many aspects of extreme physics can be studied only by observing and understanding neutron stars, as these problems cannot be addressed by doing experiments in laboratories. One such problem is the lack of knowledge of the nature of super-dense cold matter in the neutron star cores, and only the accurate measurements of the mass, radius and spin period of a neutron star can resolve this. A promising way to measure these stellar parameters is to study type I X-ray bursts, which are produced by thermonuclear burning of matter accumulated on the surfaces of accreting neutron stars. This is because, these intense bursts, which sometimes exhibit timing features (e.g., millisecond period brightness oscillations), and may show surface spectral features, contain detailed information about these stars. Moreover, X-ray bursts can be helpful for constraining the stellar atmospheric parameters, and for understanding the thermonuclear flame spreading under extreme physical conditions that exist on neutron star surfaces. I will discuss some of the diagnostic merits of these bursts.

Peter Eisenhardt (JPL) in Pratt at 12:30 on 17 October 2007

Clusters of Galaxies in the First Half of the Universe from the Spitzer/IRAC Shallow Survey

We have identified 335 galaxy cluster and group candidates, 105 of which are at z > 1, using a 4.5mum selected sample of objects from a 7.25 deg^2 region in the Spitzer Infrared Array Camera (IRAC) Shallow Survey. Clusters were identified as 3-dimensional overdensities using a wavelet algorithm, based on photometric redshift probability distributions derived from IRAC and NOAO Deep Wide-Field Survey data. The significance of the detections is such that less than 19 (and less than 10 of those at z greater than or equal to 1) should arise by chance. To date 10 of the z > 1 candidates have been confirmed spectroscopically, at redshifts from 1.06 to 1.41. Although not selected to contain a red sequence, some evidence for red sequences is present in the spectroscopically confirmed clusters, and brighter galaxies are systematically redder than the mean galaxy color in clusters at all redshifts. The mean I - [3.6] color for cluster galaxies up to z ~1 is well matched by a passively evolving model in which stars are formed in a 0.1 Gyr burst starting at redshift z_f = 3. At z > 1, a wider range of formation histories is needed, but higher formation redshifts (i.e. z_f > 3) are favored for most clusters.

Prajval Shastri (Indian Institute of Astrophysics, Bangalore) in Pratt at 12:30 on 24 October 2007

Different Angles on AGN: Some Empirical Tests of the Unification Paradigm

Much of current AGN research is worked into the operating framework of the 'Unification Paradigm', which is based on the notion that orientation plays a dominant role in the appearance of active galaxies. I will describe the results of a variety of empirical tests of unification, in both the radio-loud and radio-quiet regimes.

Andrea Comastri (Osservatorio Astronomico di Bologna, Italy) in Pratt at 12:30 on 29 October 2007 (Monday)

The quest for the most obscured Supermassive Black Holes

A sizable population of heavily obscured Compton Thick AGN (NH > 10^24 cm-2) appears to provide the most likely explanation for the unresolved X-ray Background above 5--6 keV. Compton Thick AGN are also needed to reconcile the relic SMBH mass function, obtained by integrating the X-ray luminosity function, with the local one, estimated through the local M_BH - sigma / M_BH - M_bulge relationships and bulges luminosity function. While abundant in the local Universe, only a handful of them are known at cosmological distances. I will review the present efforts aimed at obtaining a complete census of CT AGN using multiwavelength selection criteria, and in particular hard (> 10 keV) X-ray and deep FIR surveys. Perspectives for future observations with both present and foreseen facilities will also be discussed.

Brandon Kelly (University of Arizona) in Pratt at 12:30 on 31 October 2007

Observational Constraints on the Dependence of Radio-Quiet Quasar X-ray Emission on Black Hole Mass and Accretion Rate

The extraordinary activity associated with quasars involves accretion onto a supermassive black hole, with the UV/optical emission arising from a geometrically thin, optically thick cold accretion disk, and the X-ray continuum arising from a hot, optically thin corona that Compton upscatters the disk UV photons. Recent hydrodynamic calculations of accretion flows have found that the efficiency of the quasar in driving an outflow depends on the fraction of energy emitted through the UV/disk component as compared to the X-ray/corona component. I will discuss our recent efforts at using the X-ray and optical properties of a sample of 318 quasars, drawn mostly from the SDSS, to constrain the dependence of quasar accretion disk structure on black hole mass and accretion rate. Specifically, we investigate the dependence of the ratio of optical/UV flux to X-ray flux on black hole mass and Eddington ratio, as well as the dependence of the X-ray spectral slope on black hole mass and Eddington ratio. I will discuss our results within the context of accretion models with comptonizing corona, and discuss the implications for quasar feedback.

Neelima Sehgal (Rutgers) in Pratt at 12:30 on 7 November 2007

Measuring the Growth of Structure with Multi-Wavelength Surveys of Galaxy Clusters

Current and near-future galaxy cluster surveys at a variety of wavelengths may provide a promising way to obtain precision measurements of structure growth over cosmic time. This in turn would serve as an important precision probe of cosmology. However, to realize the full potential of these surveys, systematic uncertainties in cluster mass estimates and sample selection must be well understood. This work follows two different approaches to understand these uncertainties. 1.) X-ray and weak-lensing mass estimates are compared for shear-selected galaxy clusters in the Deep Lens Survey (DLS) to study possible biases in using cluster baryons or weak-lensing shear as tracers of the cluster dark matter. Results are presented for four galaxy clusters that comprise the top-ranked shear-selected system in the DLS. 2.) Cluster sample selection is investigated in the context of upcoming arcminute-resolution millimeter-wavelength surveys. Large-area, realistic simulations of the microwave sky are constructed and cluster detection is simulated using a multi-frequency Wiener filter to separate the galaxy clusters, via their Sunyaev-Zel'dovich (SZ) signals, from other contaminating microwave signals.

Dan Evans (CfA) in Phillips at 12:30 on 19 November 2007 (Monday)

Hot and Cold Gas Accretion Modes and Feedback Processes in AGN

The interaction between AGN, jets, and their environments in radio galaxies has important implications for black-hole accretion and the role of feedback. I present the results of all Chandra and XMM observations of z<0.5 3CRR radio galaxies and quasars, and demonstrate that Bondi accretion of the hot, X-ray emitting phase of the IGM is sufficient to power all `low-excitation radio galaxies' (FRI sources as well as some more powerful FRIIs), while `high-excitation' sources are powered by accretion of cold gas that is in general unrelated to the hot IGM. This model explains a number of properties of the radio-loud active galaxy population, and has vital consequences for the energy input and feedback processes of AGN jets into the hot phase of the IGM: (1) the energy supply of powerful high-excitation sources does not have a direct connection to the hot phase, (2) the Fanaroff-Riley dichotomy is a function of jet power and environment, and not accretion-flow mode, and (3) obscuring `tori' are not required in all AGN, requiring modification of unified schemes. I will also describe new work that aims to connect black-hole accretion with jet formation processes for both radio-loud and radio-quiet AGN.

Harold Francke (Yale/U. de Chile) in Pratt at 12:30 on 21 November 2007

Clustering of Intermediate Luminosity X-ray selected AGN at z~3

I will present preliminary clustering results of X-ray selected AGN at z~3 in the MUSYC (MUltiwavelength Survey by Yale/Chile). Using Chandra X-ray imaging and UVR optical colors from MUSYC photometry in the ECDF-S field, we selected a sample of 58 z~3 AGN candidates and 1385 LBG at 2.8 < z < 3.8. We performed auto-correlation and cross-correlation analyses, and here I will present results for the clustering amplitudes and dark matter halo masses of each sample. For the LBG we find a correlation length of r_0,lbg = 6.7+/-0.5 Mpc, implying a bias value of 3.5+/-0.3 and dark matter (DM) halo masses of log(Mmin/Msun) = 11.8+/-0.1. The AGN-LBG cross-correlation yields r_0,agn-lbg = 8.7+/-1.9 Mpc, implying for AGN at 2.8 < z < 3.8 a bias value of 5.5+/-2.0 and DM halo masses of log(Mmin/Msun) = 12.6+0.5/-0.8. Evolution of dark matter halos in the Lambda CDM cosmology implies that today these z~3 AGN are found in high mass galaxies with a typical luminosity 6 +4/-2 L*.

Andrey Kravtsov (University of Chicago) in Pratt at 12:30 on 28 November 2007

Cosmological simulations of clusters of galaxies: status, problems, challenges

I will describe high-resolution self-consistent cosmological simulations of clusters forming in the concordance Cold Dark Matter model with vacuum energy. The resolution of the simulations is sufficiently high to resolve formation and evolution of cluster galaxies and their impact on cluster gas. We use these simulations to study the effects of galaxy formation on the global properties of clusters, such as the shape of cluster dark matter halo and its density profile, the baryon fractions, gas density and temperature profiles. I will present comparisons of simulations results with the recent X-ray Chandra, Sunyaev-Zeldovich, and optical observations of clusters with highlights of both successes and problems of the models. I will show that despite complexities of their formation and uncertainties in their modeling, clusters of galaxies both in observations and numerical simulations are remarkably regular and consistent outside of their core region (~5% of the virial radius), which holds great promise for their use as cosmological probes.

Norbert Werner (SRON, The Netherlands) in Classroom A-101 at 13:00 on 3 December 2007 (Monday)

Possible non-thermal nature of the excess soft X-ray emission in the cluster of galaxies Sersic 159-03

We studied new Suzaku data and two archival XMM-Newton data sets of the cluster of galaxies Sersic 159-03, which has a strong excess soft X-ray emission component. The Suzaku observation confirms the presence of the soft excess emission, but it does not confirm the presence of redshifted O VII lines in the cluster. We derived radial profiles and 2D maps which show that the soft excess emission has a strong peak at the position of the central cD galaxy and has no significant azimuthal variations. We concluded that the spatial distribution of the soft excess is neither consistent with the models of inter-cluster warm-hot filaments, nor with models of clumpy warm intra-cluster gas associated with infalling groups. Moreover, the XMM-Newton RGS observation does not show OVII line emission, which we would expect to see if the centrally peaked soft emission was of thermal origin. We concluded that a non-thermal model provides the best explanation for the observed properties of the soft excess in Sersic 159-03. This non-thermal emission might be due inverse-Compton scattering of the cosmic microwave background photons on relativistic electrons. The total energy in relativistic electrons needed to explain the excess emission within the radius of 600 kpc does not exceed 1x10^61 erg, while the total thermal energy within the same radius is 3x10^63 erg. Furthermore, we discuss the prospects of a search for the missing baryons in the warm-hot phase of the inter-cluster filaments with the current instruments using pairs of clusters of galaxies, in which the filament connecting them has a favorable geometry.

Joachim Moortgat (University of Rochester) in Phillips at 12:30 on 5 December 2007

Particle-in-cell simulations of collisionless reconnection in GRB outflows

One of the open issues in GRB models is whether the outflow dynamics are dominated by the particles or by the electromagnetic Poynting flux, and the related question of the jet composition, i.e. a pure pair plasma or a plasma loaded with a certain fraction of baryons. Few observational diagnostics exist to constrain either the magnetic fields or the jet composition. Whichever is the case close to the source, observations indicate that at larger distances the particles are accelerated to highly relativistic velocities and dominate the energy budget. Magnetic reconnection is a likely mechanism to operate in the magnetized GRB jet plasma and is known to efficiently accelerate particles at the expense of the free magnetic energy. This is fairly well studied in the less energetic environments of the solar corona and the Earth's magnetotail, but to a much lesser extend in the relativistic regime and for pair plasmas. To address some of these issues, we are developing numerical simulations of magnetic reconnection under conditions applicable to the GRB outflows. Here we present some of our first results using a fully explicit and highly parallelized relativistic particle-in-cell code, Osiris. We study a double Harris current sheet configuration in 2 dimensions with periodic boundary conditions on all sides and high spatial and temporal resolutions. By tuning the plasma's Alfven velocity from non-relativistic to highly relativistic values and, similarly, studying both a pure electron-positron plasma and a plasma whith an increasing proton fraction we intend to obtain different particle acceleration spectra that could be used as distinguishing observational diagnostics of the jet's nature.

Barbara Ercolano (CfA) in Pratt at 12:30 on 12 December 2007

Fully 3D photoionisation and radiative transfer modelling from the IR to the X-ray

Photoionized environments characterize a wide range of astrophysical problems involving sources of X-radiation. With the advent of new technology used for instruments on board of (e.g.) XMM-Newton and Chandra, high resolution spectroscopy of such environments has become a reality. A number of 1D photoionization codes continue to represent powerful analytical tools for the analysis of astrophysical spectra from the X-ray to the infrared regime; however very few real X-ray sources are spherically symmetric. The first fully 3D photoionization and dust RT code, MOCASSIN (Ercolano et al. 2003, 2005), that use a Monte Carlo approach to the transfer of radiation, was developed to remedy these shortcomings, and has recently been extended to the X-ray regime (Ercolano et al. 2007). In this talk I will review the basics of radiative transfer using Monte Carlo techniques. Recent application to the modelling of fluorescence emission from stellar photospheres and their potential as geometry and abundance diagnostics will also be presented.

Harsha Raichur (Raman Research Institute) in Phillips at 12:30 on 17 December 2007 (Monday)

Long term superorbital studies and orbital evolution in X-ray binaries

The talk will be divided mainly into two parts, first part will discuss the long term superorbital studies (mainly Cen X-3 aperiodic intensity variations) and the second part will discuss the orbital evolution and apsidal motion of a few X-ray binary pulsars. Cen X-3 shows aperiodic long term intensity variations with timescales of a few days to a hundred days. This is in contrast to the superorbital intensity variations observed in other X-ray binary pulsar systems that are periodic (Her X-1, LMC X-4) or quasi-periodic (SMC X-1) and understood to occur due to obscurtion of the central X-ray source by a warped or inclined precessing accretion disk. The Cen X-3 QPOs, orbital modulation and the pulsed fraction measurements in different source intensity states provide clues to understanding the aperiodic intensity variations. The results of these studies will be discussed in the talk. The orbital evolution of few high mass X-ray binaries will be presented. Tidal interaction, mass loss from the binary system and mass transfer from the normal companion star to the neutron star all contribute to orbital evolution. Orbits of some X-ray binaries which are eccentric also allow for measuring the rate of apsidal motion of the binary orbit. Measuring the rate of apsidal motion allows one to estimate the apsidal motion constant of the companion star which in turn can be a test for stellar structure models. The results of these studies will be presented.

Sabina Bucher (CfA) in Pratt at 12:30 on 19 December 2007

The Prospects for a 15-Year Chandra Mission

The viability of the Chandra spacecraft for a fifteen year mission will be determined by: (1) vehicle health, (2) the evolution of the Chandra orbit, and (3) science return. The spacecraft hardware is in a favorable position to support a fifteen year mission, but there are thermal concerns that cannot be ignored. We will walk through a top level summary of the state of the spacecraft hardware and an overview of the thermal concerns and the outlook for each. As the Chandra orbit continues to evolve it will bring the spacecraft to lower altitudes and closer to the magnetic poles than it has been to date. These changes will bring challenges and potential benefits. We will discuss the changes to the orbit and the impacts of thereof. As the hardware ages, the thermal conditions change and the orbit evolves mission scheduling must adapt, generally through introduction of new constraints. Many observations are now split due to constraints and in 2005 science time efficiency began to suffer. We will look at the state of current constraints and how they are expected to change. We will also discuss how constraints impact observation duration, science time efficiency and target availability. Finally we will use thermal trends and to preview allowed observation times into the future.

Joe Shields (Ohio Univ.) in Pratt at 12:30 on 15 January 2008 (Tuesday)

Recent Results on Star Clusters in Galaxy Nuclei

Recent surveys have revealed that a large fraction of galaxies host compact star clusters in their centers. These sources trace dissipational processes that generate mass concentrations in galaxy nuclei, and represent a possible vehicle for creation of "seed" black holes that are the precursors for luminous accreting systems. This talk will summarize recent observations bearing on the stellar content of nuclear star clusters, and possible connections to galaxy bulges and central black holes.

Marie Machacek (CfA) in Pratt at 12:30 on 23 January 2008

The Active Nucleus of IC 4970: A Nearby Example of Merger-Induced Cold-Gas Accretion

Observations of nearby interacting galaxies in moderately massive groups offer a unique window into the dynamical processes that may trigger nuclear activity and promote the coevolution of black hole and host galaxy at earlier epochs, when galaxies were rapidly transforming. I will present results from Chandra X-ray and Spitzer mid-infrared observations of one such example, the interacting galaxy pair NGC6872/IC4970 in the Pavo galaxy group, that show the smaller companion galaxy IC4970 hosts a highly obscured active nucleus (AGN). I will use X-ray data to place limits on possible accretion modes for the AGN, and argue that nuclear activity in IC4970 is most likely triggered and fueled by cold gas driven into the nucleus during IC4970's ongoing off-axis encounter with the dust- and gas-rich spiral galaxy NGC6872.

Francesco Massaro (CfA) in Pratt at 12:30 on 6 February 2008

X-ray Spectral Evolution of Very High Energy Emission from TeV BL Lacs

Many of the extragalactic sources detected in gamma rays at TeV energies are BL Lac objects. In particular, they belong to the subclass of "high frequency peaked BL Lacs" (HBLs), as their spectral energy distributions exhibit the synchrotron peak in the X-ray band. At a closer look, their X-ray spectra appear to be generally curved into a log- parabolic shape, in terms of three parameters: the SED energy peak, the height of the SED at this energy and the curvature. In a previous investigation of Mrk 421, based on a sample of observation spanned over eleven years, two correlations were found between these spectral parameters and they have been interpreted in terms of synchrotron emission from relativistic electrons, accelerated in statistic/stochastic processes. Subsequently, the whole sample of X-ray observations of all TeV HBLs, obtained with the BeppoSAX, XMM-Newton and Swift satellites have been considered for a similar analysis of their behaviour. I focus on five sources whose X-ray observations warrant detailed searching of correlations or trends. I found that four out of five sources, namely PKS 0548-322, 1H 1426+418, Mrk 501 and 1ES 1959+650, follow similar trends as Mrk 421 while PKS 2155-304 differs. The trends can be useful to warrant discussing predictions from the X-ray spectral evolution to that of TeV emissions.

Ashley Ruiter (New Mexico State University) in Pratt at 12:30 on 13 February 2008

Evolutionary Pathways Leading to SN Ia Progenitors, and the Impact of White Dwarf Binary Populations on LISA

I will discuss the results of two different projects carried out using population synthesis methods: i) white dwarf binaries as sources of gravitational waves for the Laser Interferometer Space Antenna (LISA) and ii) delay times of Type Ia Supernovae and the nature of their progenitors. Discussion related to the first topic will focus on the characteristics of the Galactic double white dwarf populations (Milky Way disc, bulge and halo) and how they contribute to the LISA gravitational wave signal. For the second topic, I will show expected SN Ia delay times for our standard model calculations for three SN Ia formation channels: Single Degenerate Scenario, Double Degenerate Scenario and the AM CVn channel, and discuss these (preliminary) results in context of recent observationally-derived delay times of SN Ia.

Bozena Czerny (Copernicus, Warsaw) in Pratt at 12:30 on 20 February 2008

Determination of the black hole mass in active galactic nuclei

I will review review the standard methods of mass determination. In particular, I will discuss the problems met in case of the reverberation approach, like the possible (and likely) dependence on the source inclination. Later I will present the mass measurement method for Seyfert 1 galaxies based on X-ray excess variance which we currently develop, and I will address the problems we met in case of the Narrow Line Seyfert 1 galaxies. I will conclude with an issue of the most suitable classification of AGN.

Anca Constantin (CfA) in Pratt at 12:30 on 27 February 2008

Accretion onto SuperMassive Black Holes in Cosmic Voids

I will present recent results of a study of the nebular emission activity in galaxy nuclei of the most underdense regions of the universe, the voids. I will show evidence that active supermassive black holes are just as common in void galaxies as they are in the rest of the universe, the walls. Comparisons of void and wall systems based on a variety of physical properties, near neighbor statistics, and spatial clustering calculations, reveal however differences in their nuclear activity. An interesting finding is that both small and large scale environment influence the interplay between AGN and nuclear stellar activity, and thus the optically dominant power source. I will present these ideas in the context of a potential H II -> Seyfert/Transition Object -> LINER evolutionary sequence.

Branden Allen (CfA) in Pratt at 12:30 on 5 March 2008

Characterization and Observation of TeV sources with Milagro

Milagro is a wide field water Cherenkov detector / array that has been in continuous operation over the past 8 years. I will discuss the Milagro observations of the galactic plane, the Crab nebula,Mrk 421 and the recent detection of Mrk 501, and compare these with recent observations from the various ACT's currently in operation.

Nico Cappelluti (MPE) in Pratt at 12:30 on 19 March 2008

The X-ray cluster-AGN correlation in the COSMOS survey

XMM-COSMOS is deepest wide field X-ray survey performed by XMM-Newton. Its unprecedented X-ray sensitivity has been also improved by 1.8 Ms Chandra high angular resolution imaging. Moreover the COSMOS survey accounts onto a unprecedented set of multi-wavelength observations, including HST. Thanks to the combination of optical and X-ray observations we measured the cluster AGN-cross correlation function which revealed that AGN are strongly clustered around galaxy clusters. This result has important consequences on our understanding of the physics of galaxy cluster. Swift-Chandra and XMM-Newton observations of nearby galaxy clusters pointed out that the power of non thermal emission due to accelerated particles is much reduced with respect to previous measurement when considering AGN emission and multi-temperature gas in clusters. Finally I will give an update on the status of the eROSITA mission and the first simulations of its all-sky 100.000 clusters survey for which XMM-COSMOS can be considered the best pathfinder.

HEAD (CfA) in Pratt at 12:30 on 26 March 2008

HEAD Dry Run - CANCELLED

Martin Rosvall (Univ. of Washington) in Phillips at 13:30 on 7 April 2008 (Monday)

Mapping Science

To comprehend the multipartite organization of large-scale biological and social systems, we introduce an information theoretic approach that reveals community structure in weighted and directed networks. We use the probability flow of random walks on a network as a proxy for information flows in the real system and decompose the network into modules by compressing a description of the probability flow. The result is a map that both simplifies and highlights the regularities in the structure and their relationships. We illustrate the method by making a map of scientific communication as captured in the citation patterns of more than 6,000 journals.

Diana Hannikainen (Metsahovi Observatory, Finland ) in Phillips at 12:30 on 9 April 2008

INTEGRAL monitoring of GRS 1915+105

Since March 2003, we have been monitoring the black hole X-ray binary GRS 1915+105 with INTEGRAL. GRS 1915+105 has been "on" since its discovery in 1992 and apparent superluminal ejections have been observed on several occasions. This LMXRB hosts a 14 solar mass black hole and is notorious for exhibiting a plethora of variability patterns. The aim of our monitoring program is to catch GRS 1915+105 in as many different variability X-ray/gamma-ray states. I shall present some results from our monitoring, concentrating on those observations for which we had radio coverage.

Chris O'Dea (Rochester Institute of Technology) in Pratt at 12:30 on 16 April 2008

Spitzer Observations of Brightest Cluster Galaxies

I present Spitzer IRAC and MIPS observations of 62 brightest cluster galaxies (BCGs) with optical line emission located in the cores of X-ray luminous clusters. We find that at least half of these sources have signs of excess infrared emission. The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines. Excluding several systems dominated by an AGN, the excess mid-infrared emission in the remaining brightest cluster galaxies is likely related to star formation. The mass of molecular gas (estimated from CO observations) is correlated with the IR luminosity as found for normal star forming galaxies. The gas depletion time scale is about 1 Gyr. The physical extent of the infrared excess is consistent with that of the optical emission line nebulae. This supports the hypothesis that the star formation occurs in molecular gas associated with the emission line nebulae and with evidence that the emission line nebulae are mainly powered by ongoing star formation. We find a correlation between mass deposition rates estimated from the X-ray emission and the star formation rate estimated from the infrared luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates suggesting that the re-heating of the ICM is generally very effective in reducing the amount of mass cooling from the hot phase but does not eliminate it completely.

Matteo Murgia (Observatory of Cagliari) in Pratt at 12:30 on 23 April 2008

Dying radio galaxies

I will present new radio and X-ray observations of a sample of fading radio galaxies recently discovered in the Westerbork Northern Sky Survey. These sources have been selected on the basis of their extremely steep broad-band radio spectra, which is a strong indication that these objects belong to the rare class of dying radio galaxies. Very Large Array and observations confirmed that in these sources the central engine has ceased to be active for a significant fraction of their lifetime although their extended lobes have not yet completely faded away. We found that many dying sources of our sample are located at the center of an X-ray emitting cluster of galaxies, suggesting that the pressure of a dense gaseous environment prevented a quick liquidation of the fossil radio lobes through adiabatic expansion. At last, I will show the results we obtained from a Chandra observation of one of these clusters.

Ryan Hickox (CfA) in Pratt at 12:30 on 30 April 2008

Host galaxies, clustering, and evolution of AGN at z less than 1

I will present studies of active galactic nuclei and their host galaxies at z less than 1 using data from the 9 deg^2 multiwavelength Bootes survey, with redshifts from MMT/AGES. AGN selected in different wavebands (radio, X-ray, infrared) have distinctly different host galaxy and clustering properties, and likely represent different modes of supermassive black hole accretion. I will discuss these various AGN modes in the context of the cosmological evolution of galaxies and their central black holes.

Sarah Blake, Dan Calvelo, Chris Heale and Richard Hextall (University of Southampton/CfA) in Pratt at 12:30 on 7 May 2008

Southampton/SAO MPhys (Hons) Astrophysics Thesis Talks

This year's group of Southampton Masters students are all working with HEAD scientists, so instead of having a special talk scheduled for presentation of their theses (thesii?), we will do their presentations as a regular HEAD lunch. If you are interested in having a student next year, visit the SAO/Southampton web page and submit a project - the program was not intended to be limited to HEAD scientists only! Sarah Blake: "The Spectral and Temporal Variability of Low Mass X-ray Binaries in the nearby Elliptical NGC 3379". Dan Calvelo: "Doppler and modulation tomography of XTE J1118+480 in quiescence" Chris Heale: "The Spitzer Interacting Galaxy Study : IRS spectroscopy". Richard Hextall: "The X-Ray Variability and Flaring of M31*"

Manodeep Sinha (Penn State) in Tea Room at 14:00 on 9 May 2008 (Friday)

Hot Halo Gas in Galaxy Merger Simulations

Galaxy merger simulations have explored the behaviour of gas within a galactic disk, yet the dynamics of hot gas within the galaxy halo has been neglected. We report on the initial results of high-resolution hydrodyanamic simulations of colliding galaxies with hot halo gas. We explore a range of mass ratios and orbital configurations to constrain the shocks and the dynamics of the gas within the progenitor halos. Preliminary results indicate that a strong shock with a temperature of about 3x10^6 K is produced in the halo of the galaxies before the first passage, increasing the temperature of the gas by almost an order of magnitude. About 12% of the initial gas mass is unbound from the galaxies and ends up at distances greater than 1 Mpc from the merger remnant. We discuss the implications of these results for galaxy evolution, and their role in the formation and enrichment of the Warm-Hot Intergalactic Medium.

Gianfranco Brunetti (Universita di Bologna) in Pratt at 12:30 on 14 May 2008

Nonthermal components and particle acceleration in clusters of galaxies

Radio observations of diffuse synchrotron emission from galaxy clusters are the most important evidences for non thermal components (relativistic particles and magnetic fields) in the intergalactic medium. Understanding the origin and evolution of these components is crucial because they are sources of pressure, they control particle transport in the intergalactic medium and they are likely related to the dark matter- driven cluster-cluster mergers. In the first part of the talk I will review the observational and theoretical "status of the art" on this topic. Several observational facts suggest that turbulence driven by cluster mergers in the intergalactic medium may play an important role in the acceleration process of relativistic electrons (and protons). Thus in the second part of the talk I will focus on this physics and on the most important expectations of this scenario in different observational bands. Finally I will discuss the importance of future observations, at low radio frequencies (with LOFAR, LWA) and in the gamma rays (GLAST), in addressing the physics of non thermal components in galaxy clusters, and also the importance of X-ray observations with future hard X-ray telescopes.

Elena Rasia (U. Michigan) in Pratt at 12:30 on 21 May 2008

The M-Yx relation for galaxy clusters during extreme mergers

In the next few years, large sky surveys will start identifying galaxy clusters over the large portion of the sky. The astronomical community will deal with tens of thousands of clusters observed in the optical, millimeter and X-ray bands. Given these expectations, now is the time to investigate the systematics that could affect the statistical studies that will be performed. To address questions related to cosmology and the smaller-scale cluster astrophysics, we are using simulations to derive relations between the various observable cluster properties, their intrinsic quantities and the underlying mass distributions, including their evolution with redshift. I will present some first results based on simulations of two extreme cluster mergers and on a large cosmological sample.

Trevor Weekes (CfA) in Pratt at 12:30 on 11 June 2008

The first year of observations with VERITAS

The advent of advanced systems of atmospheric Cherenkov imaging telescope arrays has opened the relativistic universe to observations with high sensitivity. Somewhat surprisingly, the ground-based techniques match, or exceed, the sensitivity of space telescopes at lower energies and hence provide complementary observations to the EGRET, AGILE and GLAST missions. VERITAS (the Very Energetic Radiation Imaging Telescope Array System) in southern Arizona came on-line in April, 2007 and has met its technical specifications. VERITAS consists of four telescopes of 12 m aperture and cameras with 499 pixels. Preliminary results from the first year of operation of VERITAS will be presented. More than a dozen TeV sources of gamma rays have been detected. There is now evidence that the emission of very high energy gamma rays is ubiquitous with evidence for more than 20 extragalactic sources; hence TeV gamma-ray astronomy promises to be a fertile new discipline in high energy astrophysics. Of particular interest is emission from distant blazars which exhibit time variations on times scales of minutes.

Franco Vazza (INAF/Institute for Radioastronomy, Bologna) in Pratt at 12:30 on 18 June 2008

Shocks and turbulence in (simulated) galaxy clusters

Despite a number of theoretical indications that chaotic motions (i.e. shocks and turbulence) in the intra cluster medium are important to explain galaxy clusters as we observe, only few direct observations of these phenomena are presently available. Numerical simulations can play a key role in describing those motions as produced in the standard cosmic evolution scenario. I will present results on the level of turbulent motions and shocks detected in a large sample of galaxy clusters simulated with two of the most widely used cosmological code on the market: the eulerian code ENZO and the lagrangian code Gadget. Novel techniques are presented to detect and study turbulent motions and shocks within simulated galaxy clusters. Data are then coupled with recipes to follow the injection and evolution of Cosmic Rays protons and electrons in the simulated intra cluster medium, with the aim of giving a bettere understanding of the complex interplay of chaotic phenomena causing non-thermal emission from real galaxy clusters.

Nicky Brassington (CfA) in Pratt at 12:30 on 25 June 2008

Deep Chandra Observations of LMXB Populations in Normal Elliptical Galaxies

Low mass X-ray binaries provide unique information on the formation and evolution of binary stars in elliptical galaxies. Here, I present the results of deep Chandra monitoring observations of the two nearby elliptical galaxies NGC 3379 and NGC 4278, which allow us to probe the LMXB populations to luminosities reaching to <10^36 erg/s. These observations reveal 98 sources in NGC 3379, and 180 in NGC 4278. From these sources we have characterized the properties of the LMXB populations (spatial distribution, spectra and X-ray colors, time variability and the X-ray luminosity function) and have used HST observations to identify GC correlations. Here I will discuss the properties of these populations from the two galaxies, highlighting their differences, which may be related to the different GC specific frequencies in these optically similar galaxies. In this discussion I will focus on the transient behaviour that has been observed in both populations, providing important information that can constrain the nature of these LMXBs. I will also discuss the LMXB-GC connection which furthers our understanding of the relation of LMXBs to the underlying stellar population.

Daryl Haggard (University of Washington) in Pratt at 12:30 on 2 July 2008

Revealing a Population of Obscured Quasars/AGN with Spitzer MIR Surveys

A substantial population of obscured QSO/AGN is required to explain current models of the cosmic X-ray background, unification theories (both geometric and evolutionary), and to disentangle questions concerning the density evolution of AGN. However, the expected population of obscured AGN has proved elusive in deep optical and X-ray surveys. The advent of the Spitzer Space Telescope provides a valuable new opportunity to search for these objects. Here I will review recent Spitzer mid-IR survey results and their implications for a population of obscured QSO and AGN. I will cover arguments for obscuration, multi-wavelength selection strategies, SED fitting, as well as several high-z and heavily obscured, Compton-thick samples.

Alessandro Rettura (Johns Hopkins University) in Pratt at 11:00 on 7 July 2008 (Monday)

Formation Epochs and Morphologies of Massive Early-Type Galaxies in Cluster and Field Environments at z ~1 : Insights from the Rest-Frame UV

I am presenting a study in which we derive stellar masses, ages and star formation histories of massive early-type galaxies in the z=1.237 RDCS1252.9-2927 cluster and compare them with those measured in a similarly mass-selected sample of field contemporaries drawn from the GOODS South Field. Additionally, I present new, deep U -band photometry of both fields, giving access to the critical FUV rest-frame, in order to constrain empirically the dependence on the environment of the most recent star formation processes. I also analyze the morphological properties of both samples to examine the dependence of their scaling relations on their mass and environment.

REU Summer Interns (CfA) in Phillips at 09:00 on 13 August 2008

Dave Henley (University of Georgia) in Pratt at 12:30 on 17 September 2008

The Soft X-ray Background from the Solar System to the Galactic Halo

The soft X-ray background (SXRB) below ~1 keV band is mainly due to line emission from highly ionized metals. Traditionally, this emission was thought to come from ~1-3 million degree gas in the Local Bubble and the Galactic halo. X-ray spectroscopy of the SXRB enables us to determine the physical conditions in the hot gas, providing clues to its origin and evolution. Unfortunately, in recent years it has become apparent that these measurements are hampered by the emission of X-rays from within the Solar System, via solar wind charge exchange (SWCX), which causes time-varying contamination of SXRB spectra. I will describe recent observations of the SXRB with XMM-Newton and Suzaku, and will discuss some of the implications of these results for models of the various components of the SXRB.

Aurora Simionescu (MPE, Garching) in Pratt at 12:30 on 24 September 2008

Sources of metals and AGN-induced metal transport in cluster cooling cores.

Sources of metals and AGN-induced metal transport in cluster cooling cores. One particular feature of cool-core clusters of galaxies is the presence of a metallicity peak coinciding with the cluster center. It is thought that the metals in this peak are produced by the brightest cluster galaxy (BCG), which is usually also located at the position of the peak. However, the distribution of metals is more extended than the light of the central galaxy, leading to the question of what mechanism drives metal transport through the cluster medium. I will show a collection of radial profiles and maps characterizing the distribution of several elements in a set of cool core clusters for which deep X-ray data and detailed spectral modeling is available. I will discuss reproducing the observed abundance ratios with combinations of different type Ia and core-collapse supernova yield models. I will evaluate the metal contributions from supernovae and stellar winds needed to reproduce the observed metal peaks to test whether all observed metals can be produced by the central galaxy alone. Moreover, I will estimate the amount of iron which is being transported by interaction between the active galactic nucleus (AGN) and the cluster medium, one probable mechanism which broadens the metal peak with respect to the light distribution in the BCG.

William Joye (CfA) in Pratt at 12:30 on 1 October 2008

Catalogs, RGB images, and Movies, Oh My! Exploring Advanced features of SAOImage DS9

It has been 10 years since the introduction of SAOImage DS9. While the current GUI would be familiar to users 10 years ago, many advanced features have been added to support today's research requirements. In addition to the many "under the hood" improvements which have been implemented to support today's computer platforms, operating systems, and astrophysical data sets, entirely new advanced features have been added such as catalogs, MPEG movies, and RGB images. In this talk, I will discuss and demonstrate a number of these new features. In particular, there will be detailed sessions on utilizing regions and catalogs, creating RGB, binned, and data cube images, and saving images and movies. I will close with a brief discussion on current and future development efforts.

Joanna Kuraszkiewicz (CfA) in Classroom A-101 at 12:30 on 8 October 2008

What makes an AGN red?

The IR-to-X-ray spectral energy distributions (SEDs) of the red (J-Ks>2) AGN selected from the 2MASS survey are red with little or no blue bump. The near-IR color selection isolates the reddest subset of AGN that can be classified optically. The 2MASS AGN optical colors are strongly affected by reddening, host galaxy emission, redshift, and in few, highly polarized objects, also by scattered AGN light. The levels of optical, X-rays, and far-IR obscuration estimated from our detailed modeling, are all consistent and imply N_H<10^23 cm^-2. This, combined with the [OIII]5007 emission line equivalent widths, suggest a predominance of inclined objects in which obscuration/inclination allows us to see/study weaker emission components which are generally swamped by the direct AGN light. PCA analysis of the SED and emission line properties showed that, while the parameters listed above are important, the dominant cause of variance in the sample (eigenvector~1) is the L/Ledd ratio. This analysis also distinguishes two sources of obscuration: the host galaxy and a circumnuclear absorption.

Jan-Uwe Ness (Arizona State University) in Phillips at 12:30 on 15 October 2008

Classical Novae in X-rays

Classical Novae (CNe) are members of the class of Cataclysmic Variables in which the radiative output is powered by nuclear burning. Nuclear burning occurs in a thermonuclear runaway explosion that is triggered after an episode of accretion. The emitted radiation comes from the optically thick, expanding, shell of gas. As the density drops, the outer layers become optically thin, and the photosphere moves inward while the source remains at constant bolometric luminosity. The effective temperature therefore gradually increases until the peak of the spectral energy distribution moves into the X-ray regime. X-ray spectra taken at this time are classified as supersoft source (SSS) spectra since they resemble those of systems like CAL 83. I present Chandra and XMM-Newton high-resolution X-ray spectra of three galactic novae during their SSS phase. I also show some X-ray observations taken before the SSS phase to illustrate the different nature of the spectra. Time permitting, I will discuss some approaches how to model the spectra.

Julie Comerford (UC Berkeley) in Pratt at 12:30 on 22 October 2008

Inspiralling AGN in the DEEP2 Survey: A New Signpost for Galaxy Mergers

Galaxy mergers and AGN are thought to play key roles in driving galaxy evolution. However, galaxy mergers are difficult to identify observationally and it is unclear to what extent galaxy mergers initiate AGN activity. I will introduce a new technique, based on inspiralling AGN in galaxy merger remnants, for identifying galaxy mergers and probing the connection between mergers and AGN. I search the DEEP2 Galaxy Redshift Survey for galaxy spectra that exhibit AGN emission lines that are offset in velocity relative to the mean velocity of the host galaxys stars, suggesting that the AGN are inspiralling in the host galaxy due to a recent merger. I discovered 37 such AGN, suggesting that half of early-type galaxies hosting AGN are also merger remnants. This result implies that mergers may trigger AGN activity in early-type galaxies and sets a merger rate of ~3 mergers/Gyr for early-type galaxies at 0.3 < z < 0.8.

Ofer Cohen (CfA) in Pratt at 12:30 on 29 October 2008

Magnetic Fields and Particles meet in the Numerical Playground Modeling of Astrophysical Plasmas

Magnetic fields and their interaction with ionized gas appear in wide range of astrophysical phenomena such as solar and stellar atmospheres, interplanetary space and interstellar medium, as well as different astrophysical accretion disks. The MagnetoHydroDynamic (MHD) approximation, in which the gas is described as a conducting fluid is appropriate for the description of the large scale evolution of these astrophysical conditions. In recent years, high-performance, state-of-the-art numerical models have been developed to study astrophysical MHD problems introducing capabilities that are beyond the analytical solutions. Nevertheless, these models are not perfect and there are some unique issues related to the their performance. In my talk I will describe the scientific benefits of using numerical models in astrophysics and will present some numerical applications for the solar and stellar coronae, accretion disks and extra-solar planets.

Rene Fassbender (MPE-Garching) in Pratt at 12:30 on 5 November 2008

New X-ray Luminous Galaxy Clusters at z>~1 and their Galaxy Populations

Investigating X-ray luminous galaxy clusters at high redshift provides a challenging but fundamental constraint on evolutionary studies of the largest virialized structures in the Universe, the baryonic matter component in form of the hot intracluster medium (ICM), their galaxy populations, and the effects of the mysterious Dark Energy. I will discuss the status and prospects of the XMM-Newton Distant Cluster Project (XDCP), a new generation serendipitous X-ray survey focused on the most distant galaxy clusters at z>1. Based on the analysis of 80 deg2 of deep XMM-Newton archival X-ray data, we have selected some 250 distant cluster candidates, followed-up by optical/NIR two-band imaging, and the ongoing spectroscopic confirmation of currently about two dozen systems. I will present multi-wavelength properties of recently discovered high-z clusters and discuss preliminary implications for the evolution of BCGs and early type galaxies in cluster environments.

Paolo Grigis (CfA) in Pratt at 12:30 on 12 November 2008

XRT observations of quiet Sun nanoflares

Hinode is a satellite observing the Sun built by a Japan/USA/UK collaboration. After a brief description of its capabilities and its 3 instruments (EIS, SOT and XRT), I will present soft X-ray observations of the dynamic evolution of the quiet Sun corona, focusing on variability on short timescales. Small explosive events called nanoflares are observed ubiquitously, mostly taking place near the location of the photospheric network. I will present an analysis of the properties of these nanoflares in XRT images.

Chris Deloye (Northwestern) in Pratt at 12:30 on 19 November 2008

Weighing the Pulsar in SAX J1808.4-3658

SAX J1808.4-3658 is a unique object in many respects. It was the first accreting millisecond pulsar ever discovered, providing evidence for the evolutionary connection between low-mass X-ray binaries and the radio millisecond pulsars. Its recurrent disk outbursts have allowed measurement of the underlying neutron star's spin and the binary's orbital period evolution. And, its neutron star's cooling rate may be the most rapid known. All of these have implications for our understanding of binary evolution and of neutron star structure (in particular, matter's equation of state at supranuclear densities). In this talk, I'll discuss our analysis of recent Gemini data taken while SAX J1808 was in quiescence. Combined with constraints on SAX J1808's distance, analysis of the optical light curves allows us to place constraints on the system's binary properties, in particular the masses of the secondary and neutron star. I'll place these constraints in the context of expectations for the neutron star's structure (as relates to its rapid cooling), the radio pulsar properties of the system during quiescence, and the system's binary evolution and discuss how it continues to surprise us in several of these regards.

Adam Mantz (Stanford) in Pratt at 12:30 on 3 December 2008

Cosmological Tests Using X-ray Observations of Clusters of Galaxies

X-ray emission from massive clusters of galaxies offers two independent and complementary methods for studying cosmology. The first uses measurements of the ratio of gas to total mass in hot, dynamically relaxed clusters to provide a standard ruler, directly tracing the expansion of the Universe. This procedure produces constraints on dark energy and the mean dark matter density that are competitive with those from type Ia supernova studies. The second method uses measurements of the distribution and growth of cosmic structure observed through the cluster X-ray luminosity function. These studies place tight constraints on the amplitude of the density perturbation power spectrum and provide an independent probe of dark energy. I will review recent results from these experiments and describe the prospects for improvement in the future.

Glenn Kacprzak (Swinburne) in Pratt at 12:30 on 17 December 2008

Dynamics of Extended Gaseous Halos of Galaxies: From Both Observations and LCDM Simulations

We lack a thorough understanding, both observationally and theoretically, of how feedback from star formation, winds and inflows precisely affect the dynamics of galaxies and their extended halos. Here, we attempt to disentangle the rather complex coupling between these processes using both observations and simulations of extended gaseous galaxy halos. MgII absorption lines detected in the spectra of background quasars can be used to probe the kinematics and physical conditions in the halos of foreground galaxies. By comparing halo gas kinematics to the dynamics of the host galaxies themselves, a clear picture of the galaxy-halo relationship begins to emerge. We demonstrate that by comparing high quality absorption features and galaxy spectra with similarly-analyzed LCDM cosmological simulations, we can now begin to understand the dominant processes that drive the halo gas kinematics. Thus, mock observations through cosmological simulations provide a powerful means for interpreting observational data and understanding how halos dynamically change as a function of time.

Francesca Civano (CfA) in Pratt at 12:30 on 14 January 2009

The Chandra-COSMOS survey

The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8Ms, program that has imaged the central 0.5 sq.deg of the COSMOS field for 160 ksec, and an outer 0.4 sq.deg. area at 50 ksec, to limiting source detection depths of 2e-16 cgs in the soft (0.5-2 keV) band, 8e-16 cgs in the hard (2-7 keV) band, and 6e-16 cgs in the full (0.5-7 keV) band. Adding the Chandra coverage to the COSMOS survey allows us to address several of the issues concerning the co-evolution of SMBH and their host galaxies and to fully characterize the SED of faint AGNs and starburst galaxies not detected in the XMM-COSMOS survey. I will give an overview of the COSMOS survey and I will present the details of the Chandra COSMOS survey. I will focus on the optical identifications of the C-COSMOS X-ray sources. The multiwavelength properties of these sources can be determined by fully exploiting the unique COSMOS multiwavelength dataset (including HST, Spitzer, multiband optical and near-IR photometry, and deep IMACS and VLT optical spectroscopy). Moreover, I will focus on a few examples of the most interesting classes of objects (obscured AGN, close pairs, off-nuclear sources).

Fabrizio Nicastro (CfA/INAF) in Pratt at 12:30 on 21 January 2009

The WHIM in the X-rays: Current Evidence, Controversies and Future Prospects

One of the most firm predictions from models for Large Scale Structure (LSS) formation in the Universe, in the framework of our Lambda-CDM Standard Cosmological paradigm, is the existence of a diffuse web of hot intergalactic matter at z < 1, where the majority of the baryons hide: the so called Warm-Hot Intergalactic Medium (WHIM). During the last 7 M-years, these baryons have continuously fed new structures in the process of forming and, at the same time, received positive feedback from such structures, via galaxy winds and AGN outflows that contributed to enrich the WHIM with metals. Detecting, counting and studying the physics of these baryons in the WHIM, is therefore not only a necessary condition to validate our Standard Cosmological Model, but also a unique way to discriminate between galaxy/AGN feedback models. However, baryons in the WHIM are hard to detect. This is mostly because H and He in the WHIM are almost fully ionized, and so detectable WHIM signatures can only be produced by the interaction of highly ionized metals with radiation. The main electronic transitions from these ions fall in the Far-UV and X-ray bands, where the limited capabilities of current instrumentation make these study extremely challenging. Here I will first briefly review all current observational evidence of the WHIM, and will then try to assess the controversy on the two highest statistical significance detections reported so far: two candidate WHIM filaments along the line of sight to the nearby blazar Mkn 421. The second part of the talk will instead be dedicated to future prospects for WHIM studies with the International X-ray Observatory (IXO).

Manuel Linares (Amsterdam) in Pratt at 12:30 on 28 January 2009

X-ray variability in neutron star LMXBs

Accretion onto neutron stars powers the brightest X-ray sources in the sky, offering a unique opportunity to test strong gravity and ultra dense matter. The luminosity and energy spectrum of neutron star low-mass X-ray binaries vary in a wide range of timescales. On the short timescale side (seconds to milliseconds), I will show and discuss results on the rapid X-ray variability of two accreting millisecond pulsars: kilohertz quasi-periodic oscillations in XTE J1807-294 and extremely strong broadband noise in IGR J00291+5934. On much longer timescales (hours to weeks) these systems switch between different accretion states, which reflect different configurations of the accretion flow. I will present a systematic study of the luminosity, spectral and variability properties of such accretion states and discuss the main results of this work: i) the luminosity of state transitions varies by more than one order of magnitude among different systems and ii) some of the variability frequencies show a universal anti correlation with the hardness of the energy spectrum.

Monica Young (CfA) in Pratt at 12:30 on 4 February 2009

First Results from the SDSS/XMM-Newton Quasar Survey

We have cross-correlated the DR5 Sloan Digital Sky Survey (SDSS) quasars with the XMM-Newton archive to obtain ~800 optically-selected quasars with X-ray observations, of which ~500 have X-ray spectra. The large sample size enables both large, statistical studies as well as closer looks at interesting sub-populations. The talk will present this two-pronged approach to understanding quasar physics. First, we use the full sample to investigate the relation between optical and X-ray emission, parameterized as the correlation between the optical-to-X-ray slope (alpha_ox, defined at 2 keV and 2500A), and the UV luminosity. Optical and X-ray spectra enable a new look at this well-studied relation by defining alpha_ox at different frequencies than those traditionally used, thereby revealing clues about the underlying physics. We also present the rare sub-population of intrinsically red quasars. We find that 7 out of 17 of the reddest SDSS quasars can be classified as probable "intrinsically red" objects. Low accretion rates, rather than absorption may explain their steep optical continua.

Jun LIN (CfA/SSP) in Pratt at 12:30 on 11 February 2009

An MHD Model for the Formation of Episodic Jets

Episodic ejection of plasma blobs have been observed in many black hole systems. While steady, continuous jets are believed to be associated with large-scale open magnetic fields, what causes the episodic ejection of blobs remains unclear. Here by analogy with the coronal mass ejection on the Sun, we propose a magnetohydrodynamical model for episodic ejections from black holes associated with the closed magnetic fields in an accretion flow. Shear and turbulence of the accretion flow deform the field and result in the formation of a flux rope in the disk corona. Energy and helicity are accumulated and stored until a threshold is reached. The system then loses its equilibrium and the flux rope is thrust outward by the magnetic compression force in a catastrophic way. Our calculations show that for parameters appropriate for the black hole in our Galactic center, the plasmoid can attain relativistic speeds in about 35 minutes.

Tanya Urrutia (IPAC -Caltech) in Pratt at 12:30 on 18 February 2009

What host obscured quasars can tell us about quasar evolution

I will present results on a multiwavelength campaign to identify the nature of dust-reddened Type 1 quasars. These quasars were selected by matching FIRST radio sources, 2MASS point sources and very red optical counterparts with r'-K > 5. From X-ray observation and Balmer decrement measurements, the obscuring dust is most likely located in a cold absorber such as the host galaxy, rather than from a torus near the AGN. Hubble ACS imaging of a subsample of these sources showed a very high fraction of interacting and merging systems. The quasars appear to be very young in which dust from the merging galaxies is still settling in. I will also show Spitzer IRS and MIPS data, that show LIRG-like star formation signatures and deep Silicate absorption features in the Mid-IR on many of the red quasars. I will end with an outlook on IFU spectroscopy on these objects, with emphasis on finding signatures of quasar feedback, especially because at high redshift we find an unusually high fraction of Low Ionization Broad Absorption Line quasars in our samples, implying that the wind in young quasars is most probably responsible for shutting down star formation in the host galaxy.

Angela Bongiorno (MPI for extraterrestrial Physics) in Pratt at 12:30 on 25 February 2009

Evolution of AGN seen through the COSMOS survey

Active Galactic Nuclei (AGN) play an important role in many aspects of the modern cosmology and of particular interest is the issue of the interplay between AGN and galaxies and their strong related evolution. Many studies have been done and many results achieved in the last decade but many things remain still unclear and need to be studied using larger sets of data. The Cosmic Evolution Survey (COSMOS) is currently the largest HST survey ever undertaken (~2 sq.deg). COSMOS observations include also the full coverage of the field with multi-band photometry (from UV, Optical, NIR to MIR and FIR), in combination with a multi-wavelength data-set from radio to X-rays and a spectroscopic coverage obtained using VIMOS, a multi-object spectrograph mounted at VLT. Using this powerful data-set we explored the properties of a sample of AGN and we studied their evolution also in relation with the evolution of their host galaxy. I will present some final and some preliminary results achieved in this project.

Mathieu Servillat (CfA) in Pratt at 12:30 on 11 March 2009

Multiwavelength identification of X-ray sources in globular clusters and their role in clusters dynamical evolution

I will present a multiwavelength study of the Galactic globular clusters NGC 2808 and M 22 based on XMM-Newton and Chandra observations. These are strengthened by HST (ultraviolet), VLT (optical spectroscopy) and ATCA (radio interferometry) observations in order to identify the X-ray sources associated with these globular clusters. One quiescent neutron star low-mass X-ray binary is detected in NGC 2808, in agreement with the correlation already observed between the number of these objects and the stellar encounter rate in the core of globular clusters. I find a possible deficit of X-ray sources in NGC 2808 compared to 47 Tuc which could be related to the metallicity content and the complexity of the evolution of NGC 2808. Using X-ray to ultraviolet ratios of cataclysmic variable candidates in NGC 2808, and optical spectroscopic observations of two cataclysmic variable candidates in M 22, I suggest different approaches to tackle the issue of the deficit of cataclysmic variable outbursts already observed in globular cluster. NGC 2808 is supected to contain an intermediate mass black hole. However, the non detection in X-rays and radio leads to mass constrains of several hundred of solar masses for such an object.

Bram Boroson (CfA) in Pratt at 12:30 on 18 March 2009

My God, It's Full of Stars!

We address the effect of stellar X-ray emission on the ISM properties, particularly in X-ray faint elliptical galaxies. Measuring gas properties (amount, temperature, abundance) versus location in the galaxy requires accurate information of active binaries (such as LMXBs, CVs, and RS CVn), because they can be confused with gas emission. We estimate stellar contributions based on Milky Way sources and nearby X-ray faint galaxies which are expected to have little hot gas. We apply this method to a sample of elliptical galaxies observed with Chandra and XMM in a self-consistent manner.

Shikui Tang (UMASS Amherst) in Pratt at 12:30 on 25 March 2009

Stellar Feedback and Impact on Galaxy Evolution

Supernovae and stellar winds from galactic bulges, which may be the primary feedback source in nearby low- and intermediate-mass early-type galaxies, can play an essential role in galaxy evolution. However, the conventional 1-D SN-driven bulge wind model is hardly consistent with the truly diffuse X-ray emission from such galaxies. To aid the interpretation of the observations and to determine the role of the feedback on galaxy evolution, we have carried out 1-, 2-, and 3-D hydrodynamic simulations on various galactic scales. On galactic bulge scales, we simulate the collective effect of SNe with their blastwaves resolved, which demonstrates that the sporadic explosions produce a wealth of substructures in the diffuse hot gas and significantly affect the spectroscopic properties of the X-ray-emitting gas. On galactic halo scales, the stellar feedback can play an essential role in shaping the galactic gaseous structure and evolution. The galactic bulge wind can later evolve into a subsonic quasi-stable outflow as the energy injection decreases with time, which may provide a solution to several long-standing puzzles, including the so-called missing stellar feedback and over-cooling problems.

Zhiyuan Li (CfA) in Pratt at 12:30 on 8 April 2009

The circumnuclear environment in M31

Galactic circumnuclear environments, in which stars and ISM are present in a dense state, are of vast astrophysical interests. We present a multiwavelength study of the central ~500 parsec region of M31, based on high sensitivity and high resolution observations, from infrared to X-ray bands. With the quantifications of the multi-phase ISM, in particular for the first time for the hot gas and dusty cold gas, we show clear evidence for a dearth of the ISM, despite the expected mass input from evolved stars as well as gas inflowing from the outer disk. This deficiency, along with the starving of the central super-massive black hole (SMBH), is best explained in the context of the ISM being heated to form an outflow, a picture fully consistent with the observed morphology of the hot gas on large-scales. Our study strongly argues that stellar feedback, particularly in the form of energy release from SNe Ia, may play an important role in regulating the evolution of SMBHs and the ISM in galactic bulges.

Susmita Chakravorty (IUCAA, India) in Pratt at 12:30 on 15 April 2009

Stability of Warm Absorber in AGN

Warm absorbers (WA) are found in soft X-ray spectra of about half of all Seyfert1 galaxies and in some quasars and blazars. We use the thermal equilibrium curve generated by the photoionization code CLOUDY to study the influence, of the shape of the ionizing continuum, density and the chemical composition of the absorbing gas, on the existence and nature of the WA. We have shown that use of, recently derived, more reliable, dielectronic recombination rates gives different results, necessitating revision of these analyses. Systematic stability curve analysis shows that i) the value of the spectral index of the X-ray power-law ionizing continuum need to be more than 0.2 for the WA to exist and should be about 0.8 for its multiphase nature ii) thermal and ionization states of highly dense WA are sensitive to their density if the ionizing continuum is sufficiently soft, i.e. dominated by the ultraviolet a significant new result opening new avenues for density estimation; iii) absorbing gas with super Solar metallicity and/or rich in iron and associated elements is more likely to have a multiphase nature iv) the soft excess component in the ionizing continuum in the form of a blackbody with temperature in the range 100 to 200 eV increases the stability of 10^5 K gas. The final test is to include magnetic fields, of appropriate structure, in this analysis which is likely to influence the dynamics of the gas and stabilizes the WA - providing a robust description of the system.

Valsamo Antoniou (CfA) in Pratt at 12:30 on 22 April 2009

Probing the link between star-formation history and young X-ray binary populations: the case of the Small Magellanic Cloud

Using Chandra, XMM-Newton and optical photometric catalogs we study the young X-ray binary (XRB) populations of the Small Magellanic Cloud (SMC). We find that the Be/X-ray binaries (Be-XRBs) are observed in regions with star-formation (SF) rate bursts ~30-70 Myr ago, which coincides with the age of maximum Be-star formation. We also find evidence for correlation of the number of Be-XRBs with the strength of the SF at this age, i.e. ~40 Myr ago, while regions with strong but more recent SF (e.g. the Wing) are deficient in Be-XRBs. In addition, we examine the spectral-type distributions of Be field stars and Be-XRBs in the Magellanic Clouds and the Milky Way. In this comparison, we use the spectral types of 20 SMC Be-XRBs obtained with the 2dF spectrograph of the Anglo- Australian Telescope (AAT).

Todd Boroson (NOAO) in Pratt at 11:00 on 27 April 2009 (Monday)

Finding the Goodies in the SDSS QSO Archive

The Sloan Digital Sky Survey archive is a treasure trove of information, but the sheer volume of data demands new approaches for managing one's path through it. This talk will describe an approach that builds on the Karhunen-Loeve Transform (a type of Principal Component Analysis) to quantify the information in 12,000 spectra of low-redshift quasars. The procedure has two significant results: (1) it improves the signal-to-noise (by a factor of 5-6) of the objects that are noisiest, and (2) it identifies the objects that don't fit in the sample. These may be contaminants, or they may be important prototypes that have rare or extreme properties. One such object in this last category turns out to be a promising candidate for a sub- parsec supermassive black hole binary system, which has interesting implications for the nature and evolution of supermassive black hole binary mergers.

Predoc Series I (CfA) in Pratt at 12:30 on 29 April 2009

Pre-doctoral Research Presentations I

Elizabeth Bartlett Timing and Spectral Analysis of the Unusual Soft X-ray transient XTE J0421+560/CI Camelopardalis Stacie Powell Model independent mass determinations for a sample of Galaxy Clusters Lindley Lentati Field and Globular Cluster LMXBs in NGC 4278 Annamaria Donnarumma (5 min) Testing the Discrepancy between X-ray and lensing mass measurements for relaxing Galaxy clusters

Charlotte Feldman & Carolyn Atkins (U. Leicester; U. C. London, U.K.) in Pratt at 12:30 on 6 May 2009

Active X-ray optics for the next generation of X-ray space telescopes

The immediate future for X-ray astronomy is the need for high sensitivity, requiring the combination of large apertures and collecting areas, the newly formed NASA, ESA and JAXA mission IXO (International X-ray Observatory) is specifically designed to meet this need. However, looking beyond the next decade, there have been calls for an X-ray space telescope that can not only achieve this high sensitivity, but could also boast an angular resolution of 0.1 arc-seconds, a factor of five improvement on the Chandra X-ray Observatory. NASA's proposed Generation-X mission is designed to meet this demand; it has been suggested that the X-ray optics must be active in nature in order to achieve this desired resolution. The Smart X-ray Optics (SXO) project is a UK based consortium looking at the application of active/adaptive optics to both large and small scale devices, intended for astronomical and medical purposes respectively. With missions such as Generation-X in mind, an active ellipsoidal prototype has been designed by the SXO consortium to perform single reflection, point-to-point X-ray focussing, whilst being able to simultaneously manipulate the optical surface to improve its initial resolution. Following the completion and successful X-ray testing of the large scale SXO prototype, presented is an overview of the production and operation, including prototype metrology, finite element analysis and a introduction into the recently analysed results.

Ohad Shemmer (University of North Texas) in Pratt at 12:30 on 13 May 2009

Multiwavelength Insights into the Nature of Weak Emission-Line Quasars at High Redshift

The Sloan Digital Sky Survey has recently discovered ~50 quasars at z=2.7-5.9 with weak or undetectable high-ionization emission lines in their UV spectra (WLQs). I will present multiwavelength spectroscopic observations that provided insights into the nature of these remarkable sources. I will show that WLQs are unlikely to be dust-obscured quasars, broad-absorption line quasars, or high-redshift galaxies with apparent quasar-like luminosities due to gravitational lensing amplification. Additional monitoring data suggests that the weakness of the lines in WLQs cannot be explained by microlensing that amplifies the continuum relative to the emission lines in ordinary quasars. I will also argue against the idea that WLQs are the long-sought high-redshift BL Lacertae objects. Instead, I suggest that WLQs are quasars with extremely high accretion rates that suppress the formation of the high-ionization emission lines. Finally, I will discuss X-ray and near-infrared observations required to test this scenario with implications for emission line formation and the accretion process in active galactic nuclei.

predoc series II (CfA) in Pratt at 12:30 on 21 May 2009 (Thursday)

predoc series II

Daniel Castro: Cosmic Ray Acceleration at Supernova Remnant Shocks Yuan Liu: Suzaku Monitoring of the Iron K Emission Line in the Type 1 AGN NGC 5548 Nicholas Wright: The Massive Star Forming Region, Cygnus OB Herbert Pablo: Cygnus X-3: Unraveling the Mystery through Spectral Analysis

Laura Brenneman (NASA/GSFC) in Pratt at 12:30 on 27 May 2009

Investigating Black Hole Spin and its Role in AGN

Determining the spins of black holes allows us to begin probing General Relativity in the strong field limit. Until very recently, however, astronomers have lacked the theoretical and observational tools necessary to robustly constrain black hole spin. I will describe the methods that can be used to make these measurements, focusing in particular on the spectral modeling of relativistically broadened iron lines fluoresced from the inner accretion disk as a means of constraining spin. I will discuss recent spin measurements made in both stellar and supermassive black hole systems, including my current work on determining the spins of black holes in a sample of type-1 AGN using data from Chandra, XMM-Newton and Suzaku. I will also address the role of black hole spin in shaping an AGN's surroundings through the transfer of angular momentum. Highlighting current multiwavelength studies of the LINER NGC 1052, I will describe new methods being used to investigate the connection between black hole spin, the inner accretion flow and jet production in AGN.

Tim Roberts (Durham, UK) in Pratt at 12:30 on 25 June 2009 (Thursday)

Ultraluminous X-ray sources and the Ultraluminous State

The underlying nature of ultraluminous X-ray sources (ULXs) has been one of the more compelling mysteries of X-ray astronomy in the Chandra and XMM-Newton era. The central controversy has been whether they represent evidence for the hitherto unseen (but long sought-after) class of intermediate-mass black holes, or whether they are instead the most extreme accretors amongst the more prosaic stellar-mass black hole population. I will briefly review evidence in favour of both scenarios, and then focus on new analyses of the highest quality XMM-Newton X-ray spectral and timing data on ULXs. These studies suggest that ULX behaviour does not explicitly resemble the behaviour of any of the well-known sub-Eddington black hole accretion states. The data instead point towards the majority of bright ULXs operating in a new super-Eddington "ultraluminous state", where a massive wind driven by the super-Eddington accretion flow becomes increasingly important. This points to ULX black hole masses being relatively small; I will discuss steps towards confirming this via follow-up of the optical counterparts to nearby ULXs.

Laura Trouille (U. Wisconsin-Madison) in Pratt at 12:30 on 29 July 2009

OPTX Project: X-ray versus Optical Spectral Type and the Search for Highly Obscured AGNs

Our uniformly selected and highly spectroscopically complete sample of Chandra X-ray sources (the OPTX sample) provides an illuminating probe of the obscuring material around AGNs. I will discuss how our data confirm that one cannot use X-ray spectral classifications and optical spectral classifications equivalently, suggesting that until a better understanding is reached for how the X-ray and optical classifications relate to the obscuration of the central engine, the use of a mixed classification scheme can only complicate the interpretation of X-ray AGN samples. I will also present our use of optical emission line ratio diagnostics (trained on our OPTX sample) to search for highly obscured AGNs.

Ned Douglass (Boston Univ.) in Pratt at 12:30 on 28 August 2009 (Friday)

The Cluster Environment of Wide Angle Tail Radio Sources

Due to their frequent association with galaxy clusters and their connection to intracluster medium ram pressure, wide angle tail (WAT) radio sources have proven to be reliable tracers of high-density environments at a range of redshifts, as well as possible indicators of dynamical activity within their host systems. To better understand the degree to which WATs trace their environmental conditions, we have performed an in depth analysis of the X-ray properties of two WAT clusters of differring X-ray and radio morphologies (Abell 562, Abell 1446). We conclude that merger induced ICM ram pressure is the likely cause of the bending of both WAT sources. In an effort to determine whether the X-ray properties of WAT clusters define them as a population, we have examined a sample of 11 WAT clusters observed with the Chandra X-ray Observatory, which are publicly available in the archive. Many of these clusters are found to display both merger signatures and evidence of cool gas coincident with the WAT host galaxy, suggesting a possible relation between the formation of WATs and the presence of perturbed cool cores. To examine if there is a distinction between the X-ray properties of WAT clusters and WAT-less clusters we compare the results with those of an identical analysis of an archival sample of cool core and non-cool core clusters in which WAT radio sources are not present.

Al Ibrahim (American University in Cairo/MIT) in CLASSROOM at 12:30 on 2 September 2009

Quasi-Periodic Oscillations in the Recurrent Bursts From SGR 1806-20 Bursts

Quasi-Periodic Oscillations (QPOs) in two magnetar flares from SGR 1806-20 and SGR 1900+14 were recently discovered, offering direct detections of seismic vibrations from neutron stars (NSs) and making possible to use seismology to study the interior structure and composition of NSs and to constrain their EOS. We present evidence for QPOs in the more common type of magnetar emission, the recurrent bursts, using Rossi X-ray Timing Explorer (RXTE) observations. By searching a large sample of bursts from SGR 1806-20 for timing signals at the frequencies of the QPOs discovered in the 2004 December 27 giant flare from the source, we find three QPOs at 85, 105, and 648 Hz in three different bursts. The first two QPOs lie within $7.6\%$ and $14\%$, respectively, from the 92 Hz QPO detected in the giant flare. The third QPO lie within $3.5\%$ from the 625 Hz QPO also detected in the same flare. These QPOs are detected in archival observations that took place eight years before the giant flare. Given the fast rise time and short duration of the recurrent bursts, we employed Monte-Carlo simulations to assess the statistical significance of the QPOs. The simulations ruled out or weakened some candidates that we dismissed but supported the aforementioned QPOs whose confidence levels are set to a lower limit of 4.3$\sigma$. We also find evidence for QPO candidates at higher frequencies in other bursts at 1096, 1230, 2785 and 3690 Hz with a lower significance. The detected QPOs are found in bursts with different durations, morphologies, and brightness. The fact that we can find evidence for QPOs in the recurrent bursts near the frequencies of the giant flare QPOs is intriguing and can offer insight about the proposed origins of the oscillations. The reported QPOs are detected during the bright X-ray emission of the bursts and are not seen in the decaying phase, arguing against arising from a passive debris disk around the neutron star. The QPOs are also seen in short (< 0.1 s), relatively dim bursts and also in long (> 0.1 s), bright bursts that in the magnetar picture would have significantly contrasted signatures in the magnetosphere, making a magnetospheric oscillation less likely. We attribute the QPOs to toroidal modes driven by seismic vibrations in the neutron star crust and utilize them to constrain the magnetar properties.

Mahboubeh Asgari (UCL,UK) in Pratt at 12:30 on 16 September 2009

Self-organized braiding and the structure of Coronal Loops

We examine the effect of reconnection on the structure of a braided magnetic field. A prominent model for both heating of the solar corona and the source of small flares involves reconnection of braided magnetic flux elements. Much of this braiding is thought to occur at as yet unresolved scales, for example braiding of threads within an EUV or x-ray loop. However, some braiding may be still visible at scales accessible to TRACE or the EIS imager on Hinode. We suggest that attempts to estimate the amount of braiding at these scales must take into account the degree of coherence of the braid structure. We demonstrate that simple models of braided magnetic fields which balance input of topological structure with reconnection evolve to a self-organized critical state. An initially random braid can become highly ordered, with coherence lengths obeying power law distributions. The energy released during reconnection also obeys a power law. Our model gives more frequent (but smaller) energy releases nearer to the ends of a coronal loop.

Poshak Gandhi (RIKEN Cosmic Radiation Lab, Japan) in Pratt at 12:30 on 28 September 2009 (Monday)

Fundamental new constraints on accretion in stellar and super-massive black holes from multi-wavelength observations

As a broad introduction to my research, I will talk about two separate aspects of recent multi-wavelength work on accreting black holes. First, I will present the discovery of correlated optical and X-ray variability in the low/hard state of black hole X-ray binaries, found in a very fast multi-wavelength timing study. Our simultaneous VLT/Ultracam and RXTE/PCA data reveal intriguing patterns with characteristic peaks, dips and lags down to very short timescales (~150 ms) in two sources, GX 339-4 and Swift J1753.5-0127. Various pieces of evidence seem to rule out simple linear reprocessing as the origin of the aperiodic optical power. Instead, the variability may be driven by synchrotron emission from the inner flow regions, with energy division between the various physical components (e.g. jet and corona) creating the complex time correlations. Such rapid multi-wavelength observations give independent constraints for modeling the inner regions of accreting stellar sources. Next, I will discuss new results suggesting that resolved mid-infrared (mid-IR) imaging of active galactic nuclei (AGN) is a precise, isotropic probe of their intrinsic luminosities. We have obtained 8-m telescope diffraction-limited mid-IR imaging of radio-quiet AGN in the local Universe, using which we find a strong correlation between observed mid-IR (12 micron) and intrinsic X-ray (2-10 keV) luminosities. The relation holds true for Seyferts of all types, including unobscured, obscured, as well as heavily Compton-thick sources, which means that the mid-IR is an excellent proxy for the intrinsic AGN power. The dispersion in the mid-IR:X-ray relation is tight enough to provide sensitive discrimination between physical models of clumpy vs. smooth dusty tori geometries, and also provides a good pathway for estimating the intrinsic powers of Compton-thick AGN.

Akos Bogdan (MPA-Garching) in Pratt at 11:00 on 29 September 2009 (Tuesday)

Progenitors of type Ia Supernovae in Early-type Galaxies

Although there is a general agreement that type Ia Supernovae are associated with the thermonuclear disruption of a CO white dwarf, the exact nature of their progenitors is still unknown. In the double degenerate scenario, coalescence of two white dwarfs spiraling onto each other in a compact binary leads to the thermonuclear runaway and Supernova explosion. Alternatively, the single degenerate scenario envisages a white dwarf accreting matter from a non-degenerate companion in a binary system. Gravitational and nuclear energy of the accreted matter is released in the form of electromagnetic radiation and/or kinetic power of Classical Nova explosions prior to the Supernova event. We show that combined X-ray output of SNIa progenitors and statistics of Classical Novae predicted in the single degenerate scenario are inconsistent with observations of nearby early-type galaxies. No more than ~5-10% of SNeIa associated with old stellar population can be produced via single degenerate evolutionary channel, unless our understanding of accretion and nuclear fusion on the white dwarf surface is fundamentally flawed.

Tibor Torok (Observatory of Paris Meudon) in Pratt at 12:30 on 30 September 2009

Formation of 3D nullpoint topologies, torus-unstable flux ropes, and erupting sigmoids in the solar corona

In this talk I will present two recent studies which have been undertaken in collaboration with the SSXG group at SAO. The first study addressed the formation of 3D nullpoint topologies in the solar corona by combining Hinode/XRT observations of a small dynamic limb event, which occurred beside a non-erupting prominence cavity, with a 3D zero beta MHD simulation. To this end, we model the boundary-driven 'kinematic' emergence of a compact, intense, and uniformly twisted flux tube into a potential field arcade that overlies a weakly twisted coronal flux rope. The expansion of the emerging flux in the corona gives rise to the formation of a nullpoint at the interface of the emerging and the pre-existing fields. We unveil a two-step reconnection process at the nullpoint that eventually yields the formation of a broad 3D fan-spine configuration above the emerging bipole. The first reconnection involves emerging fields and a set of large-scale arcade field lines. It results in the launch of a torsional MHD wave that propagates along the arcades, and in the formation of a sheared loop system on one side of the emerging flux. The second reconnection occurs between these newly formed loops and remote arcade fields, and yields the formation of a second loop system on the opposite side of the emerging flux. The two loop systems collectively display an anenome pattern that is located below the fan surface. The nature and timing of the features which occur in the simulation do qualititatively reproduce those observed by XRT in the particular event studied. Moreover, the two-step reconnection process suggests a new consistent and generic model for the formation of anemone regions in the solar corona. In the second study, we analyzed the physical mechanisms that form a 3D coronal flux rope and cause its eruption, using a zero beta MHD simulation of an initially potential bipolar field that evolves by means of simultaneous slow magnetic field diffusion and shearing motions in the photosphere. As in similar models, flux cancellation driven photospheric reconnection in a bald-patch (BP) separatrix transforms the sheared arcades into a slowly rising stable flux rope. A transition from a BP to a quasi-separatrix layer (QSL) topology occurs later on in the evolution, while the flux rope keeps growing and slowly rising, now due to coronal tether-cutting reconnection. As the rope reaches the altitude at which the overlying field drops sufficiently fast for the onset of the ideal MHD torus instability, it starts to accelerate rapidly upward. Thus we find that photospheric flux-cancellation and tether-cutting coronal reconnection do not trigger CMEs in bipolar magnetic fields, but are key pre-eruptive mechanisms for flux ropes to build up and to rise to the critical height above the photosphere at which the torus instability causes the eruption. Simplified synthetic soft X-ray images, obtained from the distribution of the electric currents in the simulation, allowed us a qualitative comparison with an erupting sigmoid recently observed by Hinode/XRT, which will be briefly discussed.

Jonathan Trump (U Arizona) in Pratt at 12:30 on 7 October 2009

The Growth of Supermassive Black Holes in COSMOS

The Cosmic Evolution Survey (COSMOS) provides a unique opportunity to study the growth of supermassive black holes (SMBHs) with deep multiwavelength observations over 2 square degrees. I will present results from a 3-year spectroscopic survey of X-ray selected AGNs in COSMOS using the Magellan/IMACS and MMT/Hectospec instruments. I will show how our observations reveal accretion rate as a driver of SMBH activity. Type 1 AGN in COSMOS shows that broad emission lines are produced only at high accretion rates (L_bol/L_Edd gt 0.01). COSMOS also contains the largest sample of "optically dull" AGN, which are X-ray bright AGN that lack emission lines in their optical spectra, and many of these objects are best described as radiatively inefficient accretors. In this framework of SMBH activity, the optical/UV continuum can no longer support ionized emission lines at low accretion rate (L_bol/L_Edd lt 10^-4). I will show how "obscured" AGNs might instead be intrinsically weak AGNs with low accretion rates, and how accretion rate is an important part of any AGN "unified model."

David Garofalo (JPL) in Pratt at 12:30 on 21 October 2009

Why galaxies know about black hole mass but listen to black hole spin

I will try to motivate the idea that the behavior of the space between accretion disks and black holes is crucial in determining the morphology, energetics and time evolution of supermassive black hole jets and thus the cosmological evolution of radio-loud AGN. These ideas suggest a fundamental importance to general relativity in AGN evolution.

Doron Lemze (Tel Aviv Univ.) in Pratt at 13:00 on 28 October 2009

Galaxy clusters: their use as probes of the large scale structure and a multi-wavelength study

Galaxy clusters can be used as probes of the large scale structure (LSS) of the universe, and, since they are essentially closed boxes because their gravitational potential wells are so deep, it is possible to study and compare between the cluster dynamical properties. I will show a combined theoretical and observational analysis of galaxy clusters, utilizing high quality multi-wavelength data using a new modeling method. I'll determine precisely the dynamical properties of galaxy clusters. In addition, I'll show that their contribution to the X-ray background can be used for constraining alternative non-Gaussian models and the concentration parameter.

Rasmus Voss (MPE) in Pratt at 12:30 on 4 November 2009

Populations of LMXBs in Nearby Galaxies

Chandra observations of nearby galaxies make it possible to study statistical properties of the LMXBs, such as the spatial distribution and the luminosity function. Two distinct populations of LMXBs were known from the Milky Way: field LMXBs believed to be formed from primordial binaries and globular cluster LMXBs believed to be formed in dynamical encounters. Recent studies find the luminosity functions to be different with a clear lack of faint sources in the globular clusters. A third population of LMXBs has been found in the dense central regions of galaxies. Their spatial distribution indicates a dynamical origin and their luminosity function is consistent with the one of globular cluster LMXBs. Despite decades of study, both the formation and evolution of LMXBs are very poorly understood, especially in globular clusters where there are three possible formation channels. The Chandra observations provide useful constraints with the potential to significantly improve our understanding of LMXBs.

Enrico Landi (Naval Research Lab) in Pratt at 12:00 on 9 November 2009 (Monday)

Physical conditions in a CME from Hinode, STEREO, and SOHO observations

Aleks Diamond-Stanic (U. Arizona) in Pratt at 12:30 on 18 November 2009

Luminosity Indicators for Active Galactic Nuclei

The growth of supermassive black holes can be traced via the luminosities of active galactic nuclei, but for most sources the line of sight is blocked by gas and dust. Commonly used luminosity indicators (e.g., X-ray continuum, optical line emission) are often attenuated by several orders of magnitude, and it is difficult to accurately estimate extinction corrections. The [O IV] 26 micron line is more robust because it probes high-ionization gas and suffers little dust attenuation. Using Spitzer measurements of [O IV] for a complete sample of 90 local Seyfert galaxies, we find that the luminosity distributions of obscured and unobscured AGNs are indistinguishable, even though the obscured sources are systematically fainter in terms of [O III] optical and 2-10 keV X-ray emission. In addition, as part of of our work to calibrate the relationship between [O IV] and AGN intrinsic luminosity, we find that even hard (10-200 keV) X-rays are biased tracers, particularly for Compton-thick sources. This has important implications for the census of black hole growth from future X-ray surveys.

John Hughes (Rutgers) in Pratt at 12:30 on 2 December 2009

First Results from the Atacama Cosmology Telescope: SZ Detections of Galaxy Clusters

The Atacama Cosmology Telescope (ACT) is a 6-m diameter telescope with a sensitive mm-wave band camera custom designed to survey the cosmic microwave background (CMB) on scales of 1.5 arcmin. The camera observes simultaneously in three bands at frequencies of 145 GHz, 220 GHz, and 280 GHz. One of the goals of the project is to find galaxy clusters through the Sunyaev-Zeldovich effect, which arises when hot electrons in the cluster inverse Compton scatter cold CMB photons. The SZ effect is manifest as a decrement in ACT's low frequency channel, an increment in its high frequency channel and a null in the middle channel. In this seminar I will present results on clusters detected by ACT and discuss our on-going multiwavelength follow-up studies of the ACT cluster sample.

Erin Bonning (Yale Univ.) in Pratt at 12:30 on 9 December 2009

How to catch a kicked black hole: searching for post-merger recoiling black holes in astronomical data

Numerical simulations of binary black holes through coalescence and merger have shown that the product of such a merger may gain a kick of several thousand kilometers per second. This velocity is astrophyscially interesting due to the fact that it is comparable to the escape velocity of a typical galaxy. Since all massive galaxies likely contain a central supermassive black hole and are believed to have undergone one or more mergers in their lifetime, it is possible that such kicked black holes may be detected in current observations. I will discuss observational methods for finding kicked black holes, including proposed candidates, as well as the difficulties faced in interpretation of the data.

Dimitra Koutroumpa (NASA/GSFC) in Pratt at 12:30 on 10 December 2009 (Thursday)

The local diffuse soft X-ray background: Solar Wind Charge Exchange emission, Local Hot Bubble emission and other implications

I will present an overview of our current knowledge of the solar wind charge exchange (SWCX) X-ray emission and its contribution to the local diffuse background. I will particularly detail the application of self-consistent calculations of the heliospheric SWCX component to observations made with the ROSAT (in the 1/4 keV band) and XMM-Newton, Chandra and Suzaku (in the 3/4 keV band) observatories. I will discuss in what extend the contribution of the heliospheric SWCX emission affects the presence/temperature of the Local Bubble hot gas. Finally, I will expose the needs in theoretical calculations and experimental measurements in order to enrich the atomic data input in the 1/4 keV band from high Z solar wind ions.

Giulia Macario (INAF-IRA, CFA) in Pratt at 12:30 on 16 December 2009

X-ray and radio observation of a shock front in the galaxy cluster A754

I will present a new deep Chandra ACIS-I observation of the merging cluster A754. Previous X-ray observations suggested the presence of a shock wave in the intergalactic medium, and the new observation was obtained to test this hypothesis. Our analysis reveals both a sharp gas density edge and a convincing temperature jump that unambiguosly confirms the existence of such a shock. The derived Mach number is ~1.5. This is only the third clear example of a merger shock front in galaxy clusters. I will also present new low frequency GMRT radio observations of the cluster, which reveals that the cluster radio halo has an edge coincident with the shock front. This feature may be produced by electron acceleration by the shock.

Hugh Hudson (UC Berkeley) in Pratt at 12:30 on 13 January 2010

Flare waves and the impulsive phase.

A solar flare requires large-scale mass motions in order to extract energy from the coronal magnetic field. Global waves in the corona and in the solar interior result from this process. I discuss the behavior of the lower solar atmosphere during the impulsive phase of a flare, emphasizing the possible relationships between the large scales of these global waves and the small scales of the observed energy release in the white-light flares and hard X-ray sources.

Hans Moritz Guenther (CfA) in Pratt at 12:30 on 27 January 2010

Accretion, winds and jets: High-energy emission from young stellar objects

This talk deals with the high-energy emission from young stellar objects. Stars of spectral type B and A are called Herbig Ae/Be (HAeBe) stars in this stage, all stars of later spectral type are termed classical T Tauri stars (CTTS). I present high-resolution X-ray grating data from the CTTS V4046 Sgr and the HAeBe HD 163296. The first one shows a high-density ratio in the He-like triplets, in the later the O VII exhibits line ratios similar to coronal sources, indicating that neither a high density nor a strong UV-field is present in the region of the X-ray emission. Using these and similar observations, it can be concluded that at least three mechanisms contribute to the observed high-energy emission from CTTS: First, those stars have active coronae similar to main-sequence stars, second, the accreted material passes through a strong accretion shock at the stellar surface, which heats it to a few MK, and, third, some CTTS drive powerful outflows. Shocks within these jets can heat the matter to X-ray emitting temperatures. The first is already well characterised; for the latter two models are presented. The shocks in jets are shown on the example of the CTTS DG Tau, which is heavily absorbed and the observed soft X-ray emission originates spatially offset from the star. DG Tau is an exceptionally interesting object, just two weeks ago it was reobserved with Chandra for 360 ks.

Simone Dall'Osso (INAF, Osservatorio Astronomico di Roma) in Pratt at 12:30 on 3 February 2010

Magnetars: from X-rays to Gravitational Waves (and back)

Significant observational and theoretical efforts have been devoted in the last decade to the study of Anomalous X-ray Pulsars (AXPs) and Soft Gamma Repeaters (SGRs), galactic X-ray sources now commonly thought to host ultramagnetized neutron stars, according to the so-called magnetar model. A wealth of observational evidence has indeed accumulated over the years in favour of the magnetar model, while arguing against most proposed alternative scenarios. The magnetar model has thus become a paradigm in interpreting observations of AXPs and SGRs of increasing detail and complexity. I will briefly discuss some recent findings by our group that have provided new strong hints as to the ultramagnetized nature of these source, namely the detection of QPOs in the lightcurve of SGR 1806-20 after the 2004 Giant Flare and the high-time resolution spectral studied of the 2006 ``burst forest'' from SGR 1900+14 made possible by Swift data. Bearing on the magnetar hypothesis I introduce a state-of-the-art scenario for the dynamical evolution of such objects soon after they are formed, showing that newly born magnetars hold the potential to become strong sources of Gravitational Waves (GWs) in their very first days. Signals might be so strong indeed to be detectable up to Virgo cluster distances with the forthcoming generation of GW detectors, thus also occurring at an interesting rate for actual detection in a few yrs. This conclusion depends on the precise role of yet poorly constrained aspects of the astrophysics of rapidly rotating, ultramagnetized neutron stars (NSs). In highlighting all of them, I will discuss to what extent they can be presently treated taking into account theoretical as well as observational constraints (when they are available) and how future studies can help in better assessing the above argument.

Ed Cliver (Hanscom Airforce Base) in Pratt at 12:30 on 10 February 2010

The solar Moreton wave of 6 December 2006: Evidence for a CME driver

We analyzed ground- and space-based observations of the eruptive flare (3B/X6.5) and associated Moreton wave (~850 km/s, ~270 degree azimuthal span) of 6 December 2006 to determine the wave driver - either flare pressure pulse or coronal mass ejection (CME). While the Moreton wave observations themselves are excellent, there are key gaps in coronagraph and EIT/SXR coverage. Thus kinematic arguments cannot rule out either a flare or CME origin, although they are less constraining for the CME scenario. Support for a CME driver is based primarily on the correspondence between wave evolution and the inferred eruption of a coronal arcade overlying a region of weak magnetic field to the west of the principal flare. -alpha footpoint brightenings, disturbance contours in off-band images, and He I 10830 flare ribbons are used to trace the development of the eruption from 18:42-18:45 UT as it moved southwest along the arcade. At 18:45 UT, the Moreton wave exhibited two separate arcs, one off each flank of the tip of the arcade, that merged and coalesced by 18:47 UT to form a single smooth wave front having its maximum amplitude in the direction of the arcade central axis. We suggest that the erupting arcade (i.e., CME) expanded laterally to drive the coronal shock responsible for the Moreton wave. We attribute a darkening in H-alpha from a region underlying the arcade to absorption by faint unresolved post-eruption loops.

Daniel Wik (University of Virginia) in Pratt at 12:30 on 17 February 2010

Inverse Compton Emission in Clusters: the View with Suzaku and Swift

Coexisting with the X-ray emitting, thermal gas in galaxy clusters is a nonthermal phase made up of relativistic electrons, ions, and magnetic (B) fields. This phase is revealed in merging clusters through diffuse, Mpc-scale radio synchrotron emission; the same electrons producing this radiation also lose energy through inverse Compton interactions with CMB photons, which should be detectable as a nonthermal spectral component at hard X-ray energies for small enough values of B. In this talk, I will present Suzaku/HXD and Swift/BAT searches for inverse Compton emission from clusters, particularly from Coma and Abell 3667, which contradict previous results and imply that relativistic components in the intracluster medium could be energetically significant. Also, I will introduce an XMM-Swift survey of two samples: all clusters with diffuse, radio synchrotron emission and the clusters in the flux-limited HIFLUGCS survey.

Guido Risaliti (CfA) in Pratt at 12:30 on 24 February 2010

The structure of AGNs from X-ray eclipses

Eclipses of the X-ray source in Active Galactic Nuclei are rather common. We discovered the first example of these events a few years ago in the Seyfert Galaxy NGC 1365, and we now have a sample of about 10 sources with clear evidence of rapid X-ray occultations due to clouds crossing the line of sight. I will show several examples of these eclipses, in both type 1 and type 2 AGNs, and I will discuss the physical and geometrical structure of the central region of AGNs as inferred from these observations. Finally, I will show how X-ray eclipses may provide a strong test of general relativistic effects in AGNs, through the analysis of the changes of the iron line profile during the occultations. Such experiment may be possible even today with present observatories (XMM and Suzaku) under particularly favourable conditions, and will be relatively straightforward with future large-area X-ray observatories like iXO.

Bram Boroson (CfA) in Pratt at 12:00 on 10 March 2010

Long term monitoring of X-ray pulsars

The value of long-term monitoring of stellar X-ray sources through the RXTE ASM and Swift BAT (and earlier instruments) increases with the length of the monitoring because (1) the statistics improve, (2) rarely visited states recur, (3) we can correlate changes in the source over decades-long periods (similar to the solar activity period for example). We focus on Hercules X-1 and LMC X-4, X-ray pulsars with month long periods from global disk wobble. With decade scale monitoring data, we correlate period changes with the X-ray luminosity, pulsar period, and orbital period, and test models of radiation-induced disk warping and wobbling.

Shin'ya Yamada (University of Tokyo) in Pratt at 12:30 on 17 March 2010

New observational insights into from Low/Hard to Very High state of black hole binaries with Suzaku satellite

The X-ray satellite Suzaku, launched in 2005, has enabled us to study the wide-band and timing properties of black-hole binaries (BHBs): from one with a low accretion rate (Low/Hard state) to a moderate (High/Soft state), and to a high accretion rate (Very High state). The BHBs in Low/Hard state (Cyg X-1 and GRO J1655-40) shows the inner radius of thin disk, Rin, ~ 15 Rg and ~ 8 Rg, respectively, consistent with mildly broadened Fe-K line (Makishima et al. 2008, Takahashi et al. 2008) in terms of thermal comptonization, as well as being associated with highly inhomogeneous coronae. The BHB in Soft state (4U1630-472) gives a constant radius independent of luminosity (Kubota et al. 2007), and the BHB in Very High state (GX 339-4) gives Rin > several Rg (Yamada et al. 2009). These results suggest that the inner radius of thin disk is getting closing to a BH as the luminosity increases, and that the coronae surrounding a BH is inhomogeneous and rapidly varying. In addittion, I'd like to show my work on applying a sophisticated disk model, developed mainly by Jeff and Narayan group, to Suzaku data. And lastly, I will briefly introduce a current status of Suzaku, MAXI, and Astro-H.

Ignazio Pillitteri (CfA) in Pratt at 12:30 on 24 March 2010

X-rays from star forming regions explored with two XMM large programs (DROXO and SOXS).

Very young stars are characterized by intense X-ray emission. The study in X-rays band allows us to gain knowledge on deeply embedded stars still accreting and contracting during the Pre-Main Sequence phase. I will review some of the results from two XMM-Newton large programs devoted to the study of two different environments. DROXO has obtained a deep 10 days long observation of the nearby Rho Ophiuchi star forming region characterized by strong and non uniform absorption. SOXS has collected a series of 7 XMM observations in synergy with Spitzer in a large strip south of ONC aimed at characterizing the population of young stars dispersed in small clusters in this region.

Bev Wills (University of Texas at Austin) in Pratt at 12:30 on 31 March 2010

Exploring Magnetic Field Alignment in AGN Jets using Radio and Optical Polarization

The Fermi era is ushering in a wealth of detailed studies of the time variability of SEDs and polarization for blazars. For comparison, we travel back in time, comparing radio and optical polarization position angle data obtained over many years, sometimes tens of years, to explore magnetic fields in the jets of AGN. The optical data, while sparsely sampled, can be compared with well-sampled radio data from UMRAO and elsewhere. We discuss the relationship of magnetic field alignment with radio structure, for both blazar classes -- the BL Lac objects, and quasar-blazars.

NO LUNCH TALK (ADS Meetings) in Pratt at 12:30 on 14 April 2010

Dai Takei (Rikkyo University) in Pratt at 12:30 on 21 April 2010

X-ray Spectroscopy of Classical Novae

I will introduce our recent studies of classical novae with the Suzaku X-ray satellite. Classical novae occur in binary systems consisting of a white dwarf and a late-type companion, which fills the Roche-Lobe and thus transfers hydrogen-rich material by its overflow. When the amount of accreted material reaches a critical mass, sudden outbursts occur by nuclear fusion of hydrogen on the white dwarf surface. X-ray observations are well-suited to understanding their nature and evolution. However, observing classical novae is difficult because of their faintness in quiescence and their transient behavior. We therefore utilized monitoring observations of novae obtained by the Swift satellite and conducted several ToO observations with the Suzaku satellite. Swift provides frequent opportunities to observe an unpredictable target, while Suzaku can obtain high signal-to-noise ratio spectra with moderate energy resolution in a wide energy band. The two observatories make a powerful combination for studying classical novae. For example, in the classical nova V2491 Cyg, we successfully detected power-law emission extending up to 70 keV, 9 days after the outburst - the first detection of a non-thermal signature from classical nova explosions in the X-ray band. I will show the results of X-ray observations of classical novae obtained over the last several years, and will discuss future strategies to study novae using Swift, Suzaku, Chandra, and other missions.

Soma De (University of Oklahoma) in Pratt at 12:30 on 22 April 2010 (Thursday)

Time dependent radiative transfer in SNe atmosphere and cosmic recombination epoch: Effect on spectra and transition probabilities in a true multilevel framework

The radiative transfer equation (RTE) is effectively the Boltzman equation for photons. This requires one to account for all possible transition rates connecting all energy levels in a given system of species (involving atoms, ions, or molecules). These rates depend on the local radiation intensity and vice versa. This makes the nature of the equations involved in such a scenario highly coupled and the size of the problem very large, especially in a multi-level, multi-species system. I will describe the scope of our general purpose stellar atmosphere code PHOENIX to handle such problems focusing on the cases of SNe and cosmic recombination. I will emphasize my results on core-collapse supernova 1999em and SN1987A atmospheres describing how time dependence in the rate equation arising from long recombination time of hydrogen could affect the Balmer line profile in a supernova's lifetime. I will also describe the effect of escape, photo-ionization probabili ties and forbidden transitions in a true multi-level calculation where no assumptions of equilibrium between the higher bound states were made. PHOENIX can also be applied during cosmic recombination epoch. I will present preliminary results on the electron fraction during the the recombination epoch which is obtained by following a time dependent RTE through cosmic recombination. These calculations should shed light on the understanding of CMB power and polarization spectra.

Alex Richings (CfA) in Phillips at 12:15 on 28 April 2010

The Hot Interstellar Medium of the Interacting Galaxies NGC 4485 and NGC 4490 (SAO/Southampton Masters thesis)

In this talk I will present an analysis of the hot interstellar medium (ISM) in the interacting pair of galaxies NGC 4485 and NGC 4490, using ~100 ks of Chandra ACIS-S observations. The high angular resolution of Chandra enables us to remove discrete sources and perform spatially resolved spectroscopy for the star forming regions and associated outflows, allowing us to look at how the physical properties of the hot ISM such as temperature, hydrogen column density and metal abundances vary throughout these galaxies. The abundance ratios of Ne, Mg and Si with respect to Fe are found to be consistent with those predicted by theoretical models of type II supernovae. The thermal energy in the hot ISM is just ~4-5% of the total mechanical energy input from supernovae, so it is likely that the hot ISM has been enriched and heated by type II supernovae. The X-ray emission is anticorrelated with the H-alpha and mid-infrared emission, suggesting that the hot gas is bounded by filaments of cooler ionized hydrogen mixed with warm dust.

Laura Brenneman (CfA) in Pratt at 12:30 on 5 May 2010

Complex absorption and a broad iron line in NGC 3783

The Seyfert galaxy NGC 3783 has long been known to harbor complex intrinsic "warm" absorption as well as a robust, broad iron line. An ongoing Suzaku Key Project has observed NGC 3783 in July 2009 for 200 ks, the deepest non-grating X-ray exposure to date. This observation, along with the 900 ks HETG data in the Chandra archive, has allowed us to measure the properties of the continuum, warm absorber, and iron line region with unprecedented precision. Though the continuum flux varies by a factor of two over a ~35 ks period, we do not observe any significant variation in the warm absorber during the Suzaku observation. This is consistent with previous results that indicate absorber variability timescales on the order of a month. By contrast, both distant and inner disk reflection components do vary during the observation. These variations allow us to place constraints on the locations of the reflecting material, and strongly indicate that the accretion disk emits well within the Schwarzschild radius of marginal stability, implying that the supermassive black hole at the core of NGC 3783 possesses significant angular momentum. I will expand on these results, focusing in particular on both the time-averaged and time-resolved Suzaku spectra.

Gianfranco Brunetti (University of Bologna) in Pratt at 12:30 on 19 May 2010

The nature of radio halos in galaxy clusters

Radio and X-ray observations support a scenario where cluster-scale synchrotron emission is generated in connection with mergers between massive galaxy clusters. Galaxy clusters are unique laboratories to study many aspects of the physics of particle acceleration. Acceleration by MHD turbulence and shocks, generated during cluster mergers, provide a viable scenario to explain the origin of the non thermal activity. Radio Halos are the most spectacular non thermal sources in galaxy clusters. In the last few years, radio observations of large X-ray cluster samples and their follow up at low radio frequencies allow for a step forward in our understanding of the nature of these sources. These observations suggest that turbulence plays a role in the acceleration of relativistic particles in galaxy clusters, in which case the majority of Radio Halos in the Universe should have very steep spectra and glow up preferentially at low radio frequencies. After discussing the most relevant observational and theoretical aspects of the non thermal activity in galaxy clusters, I will focus on new results based on turbulent acceleration and on the most promising possibilities to test present models with both low frequency surveys (GMRT, LOFAR, LWA) and multifrequency (radio -- X-ray -- gamma) observations.

Rachel Osten ( STSCI ) in Pratt at 12:30 on 25 May 2010 (Tuesday)

Fast and Furious: The Very Local Population of "Gamma-Ray Bursts"

The Swift spacecraft was designed to detect and study emission from gamma-ray bursts. In practice, its capabilities make it a useful tool for studying a wide range of transients. One interesting area where Swift has been able to make significant progress is in the identification and study of large stellar flares on nearby, normal stars. The Swift satellite has now detected several flares from nearby, magnetically active stars in the guise of gamma-ray bursts. Due to their large "superflare" flux levels ( >10-9 erg/cm2/s above 15 keV), variations in these flares can be studied on timescales of a few hundred seconds. The detection by Swift of Iron Kalpha (6.4 keV) emission and evidence for nonthermal hard X-ray emission mark two firsts in stellar flare astronomy. Understanding the formation of the 6.4 keV emission line is important because it allows for a relatively model-independent determination of coronal scale sizes. I will discuss some of the results obtained from these Swift-detected stellar superflares and implications.

Paul Martini (Ohio State University) in Pratt at 12:30 on 26 May 2010

The Evolution of AGN in Groups and Clusters of Galaxies

The past decade has illustrated the dramatic importance of AGN feedback on the hot gas in groups and clusters. While these AGN are generally associated with the brightest member of the group or cluster, AGN are also present in other galaxies in these environments. I will present new results on the distribution of AGN in local group and cluster galaxies, the relative growth rates of the black holes and their host galaxies, and the evolution of AGN in dense environments out to high redshift. The demographics of local group and cluster AGN shed new light on the origin of nuclear activity in galaxies, as well as their ability to retain cold gas, while their evolution provides important new information on the co-evolution of black holes and galaxies as a function of environment.

Andrea Comastri (INAF-Osservatorio Astronomico di Bologna) in Pratt at 12:30 on 2 June 2010

Ultra-deep XMM observations in the Chandra Deep Field South and the evolution of obscured accretion.

The main scientific goal of the ultra-deep (~ 3 Ms) XMM_newton survey in the Chandra Deep Field South (CDFS) is the detection and the spectral characterization of the most obscured Supermassive Black Holes (SMBH) at cosmological distances (up to z ~ 1-2). Together with the large body of deep multiwavelength observations in the CDFS, XMM observations will allow us to investigate the role of heavy obscuration in the growth and evolution of Supermassive Black Holes and the role played by Compton Thick absorption as a feedback mechanism. At the time of writing about 50 sources were detected with more than 300 pn counts in the 0.5-8 keV band. We will present the X-ray spectral analysis of a sizable sample of obscured and candidate Compton Thick AGN in the CDFS making use of about 1.5 Ms of XMM data. The X-ray data will be complemented by multiwavelength observations to build broad band Spectral Energy Distribution (SED). Implications for the SMBH growth and evolution will be also discussed.

Lee Townsend (Southampton University) in Pratt at 12:30 on 9 June 2010

Broadband studies of Be/X-ray Binaries in the Small Magellanic Cloud

In recent years it has been discovered that the Small Magellanic Cloud (SMC) is home to a plethora of accreting binary systems. Being at a known distance, in an environment of known metallicity and spatially very close, this population is ideal for studying star formation, binary evolution and accretion onto compact objects. Born out of our long-term monitoring programme with RXTE, it is becoming clear that many SMC Be/X-ray binary (BeXRB) systems exhibit substantial spin-up during phases of accretion onto the neutron star. A newly discovered 11.5s BeXRB pulsar is used as a prime example of how orbital information can be extracted from X-ray observations and how this information can be used along with optical data to describe many aspects of the system. The simultaneous study of both optical and X-ray data are key to furthering our knowledge of the general population of binary systems in the SMC.

Silas Laycock (Gemini Observatory, HI) in Pratt at 14:30 on 15 June 2010 (Tuesday)

X-ray Binaries and Black Holes in Dwarf Starbursts: A New Lab for Astrophysics

Accretion powered X-ray binaries (XRBs) containing black holes and neutron stars are the relics of the most massive and short-lived stars. The stellar companion acts as chronometer, scale, and fuel source, making the compact relic visible in X-rays at intergalactic distances. XRBs efficiently provide fundamental astrophysical parameters (e.g. orbital periods, spin, mass, mass-transfer rate, age, magnetic field) that are unobtainable at other wavelengths. Until recently we have been discovering and studying XRBs one by one, in many different locations throughout our Galaxy and beyond. We have now reached a threshold, where to make an analogy it is time to move from identifying and studying the trees, to a systematic study of the forest. The dwarf galaxies of the local group provide just such a forest, enabling "whole-galaxy" surveys with well-defined age and environmental attributes. The Milky Way by contrast is a tangled jungle, having been burned over and re-grown many times. To resolve the link between XRB and their parent populations, a virgin forest is needed to probe the very youngest ages. I will describe recent and ongoing efforts to discover and characterize the entire XRB populations (i.e. all spatial and temporal parameter spaces) of IC 10 (the most extreme starburst in the local universe) and the Small Magellanic Cloud. Including: the identification of a reservoir of quiescent HMXBs in the SMC, implications for XRB vs SFR scaling, and a new tool for population estimation (Capture-Recapture analysis) with wide applications in time domain astronomy.

Jan-Uwe Ness (XMM-Newton Science Operations Centre, ESA) in Pratt at 12:30 on 21 June 2010 (Monday)

X-ray and UV eclipse studies of the recurrent nova U Sco in outburst

The 2010 outburst of U Sco was observed in X-ray and UV/optical simultaneously with XMM-Newton. Two 60 ksec observations were taken, starting Feb-19 15:41 (day 22.9 after outburst) and Mar-03 14:34 (day 34.9 after outburst), respectively. The observations were centred around eclipse, covering phases 0.8-1.5 and 0.54-1.16 of the 1.23-day orbit, where phase 1.0 is defined as the white dwarf being behind the companion. On day 22.9, the X-ray light curve shows a high degree of variability with about 50 percent brightness changes, but only until phase 1.25 (quadrature). The remaining part of the observation is also variable, but at a much smaller amplitude, indicating that the large variations must somehow be connected to the eclipse. The X-ray light curve resembles those of X-ray dippers. The optical monitor (OM) was operated in grism mode, and 27 UV spectra were taken. While the nova was too faint for detailed UV spectroscopy, the observations were used to extract two light curves in different UV bands. Both UV light curves show clean eclipses. From hard UV via soft UV to optical (zero order), the width of the eclipse increases significantly, indicating a smaller effective radius for harder emission. During the second observation taken two weeks later, the nova was turning off. A significant decline can be identified from the start to the end of the observation. Clean eclipses are seen in X-rays and UV, indicating that the ejecta have become more homogeneous. High resolution X-ray spectra are available from the RGS. The spectra are dominated by high-ionization emission lines on top of a blackbody- like continuum. Little notable spectral change can be identified between in-eclipse and out-of-eclipse spectra, indicating that the emitting plasma is well mixed. The only difference is perhaps that during dips on day 22.9, the emission lines from higher ionization stages are weakened by more than those of lower ionization stages.

No lunch talk () in at 12:30 on 30 June 2010

Federica Govoni (INAF - Osservatorio di Cagliari) in Pratt at 12:30 on 9 July 2010 (Friday)

Magnetic field strength and gas temperature connection in galaxy clusters

Information on the intracluster magnetic fields can be obtained, in conjunction with X-ray observations of the hot gas, through the analysis of the rotation measure (RM) of radio galaxies in the background or in the galaxy clusters themselves. I will present a work aimed to establish a possible connection between the magnetic field strength and the gas temperature of the intracluster medium. For this purpose we investigated the RM in hot galaxy clusters and we compared these new data with RM information present in the literature for cooler galaxy clusters.

Hui Li (LANL) in Pratt at 12:30 on 4 August 2010

AGNs and Magnetic Fields in Galaxy Clusters

X-ray and radio observations of galaxy clusters have revealed a wealth of structure in their hot halos associated with extragalactic radio sources. Structures in the form of large scale cavities and weak shocks provide a reliable gauge of the mechanical output of extragalactic radio jets launched by AGNs. The morphology and properties of cavities have given strong constraints on the nature and composition of AGN outflows, especially on large scales. We will present three-dimensional magnetohydrodynamic (MHD) simulations of jets/lobes in the ICM and compare them with our ~70 X-ray cavities as well as individual jet/lobe sources. In addition, we will present cosmological MHD simulations of galaxy cluster formation with AGN jets/lobes feedback and its implications for the origin and energetics of the cluster-wide magnetic fields. We demonstrate that the ICM turbulence is excited and sustained by the frequent mergers during the cluster formation. This turbulence excites a small-scale dynamo process that transports, spreads, and amplifies the fields originated from the radio jet/lobe system. This process could be the primary process of populating the whole cluster with magnetic fields.

Fred Walter (Stony Brook University) in Pratt at 12:30 on 5 August 2010 (Thursday)

Novae! New insights into the recurrent novae from synoptic SMARTS and Swift observations

Novae, thermonuclear explosions on the surfaces of white dwarfs in close binary systems, are the third largest explosions in the universe. Novae are important because they enrich the ISM with low-z elements and because they may be a progenitor of the type Ia supernovae. I will provide an overview of the nova phenomenon, illustrated with results from 6 years of spectro-photometric monitoring of some 50 galactic novae with the SMARTS facilities at Cerro Tololo, Chile. The combination of ground-based observations with X-ray observations from Swift is providing powerful tools to investigate the super-soft phase of "steady" nuclear burning in these system. Multiwavelength high cadence observations are making it clear that the novae are far more complex that we had thought. I will concentrate on observations of the recent novae YY Dor, N LMC 2009, KT Eri, and U Sco. The line profiles and their temporal decay suggest that the accretion disk survives the nova explosion, at least in the recurrent novae.

Yuki Terada (Saitama University, Japan) in Phillips at 13:00 on 13 August 2010 (Friday)

Astro-H: overview of instruments and science

The ASTRO-H mission is the sixth in a series of X-ray satellites lead by ISAS/JAXA Japan. It is the US-Japan collaboration mission, which will be launched in 2014. ASTRO-H will investigate the physics of the high-energy universe in the 0.3 - 600 keV band by performing high-resolution and high-throughput spectroscopy with moderate angular resolution, with the combination of a micro-calorimeter system, soft and hard X-ray imaging optics, and a high-sensitivity soft gamme-ray detector. In this seminar, we will introduce the instruments onboard the Astro-H spacecraft, and survey key science topics of the mission with the current performance estimations, as well as several scientific interests in galactic compact objects.

Martin Elvis (SAO) in Pratt at 12:30 on 24 August 2010 (Tuesday)

Plan B: What does X-ray Astronomy do now?

The Decadal review demonstrates that Astrophysics has hit the Funding Wall; no successor suite of missions to the Great Observatories will be built. Notably no large X-ray mission will be started. In this new Post-Decadal era what should be the path forward for X-ray (and other) Astronomy? This talk is an update of a talk I gave this past July at the '50 Years of Space Science at Leicester' meeting, where I asked "What if IXO is not #1?". There is an answer.

Alessandro Paggi (University of Rome) in Pratt at 12:30 on 1 September 2010

Swift and XMM-Newton observations of BL Lacs

I present an extensive X-ray analysis of a sample of BL Lacs observed with XMM-Newton and Swift. High frequency peaked BL Lacs (HBLs) show in their spectral energy distributions (SEDs) a lower energy peak in the X-ray band, interpreted as synchrotron emission. Their X-ray spectra are generally curved and well described in terms of a log-parabolic shape. Here I present an X-ray analysis of a sample of HBLs not detect at TeV energies, and show their spectral behavior in comparison with that of TeV BL Lacs to investigate the distributions of their spectral parameters. I provide a physical interpretation of both synchrotron peak energies and spectral curvature distributions in terms of systematic and stochastic acceleration mechanisms; based on our analysis, I outline a possible criterion to select the best HBLs candidates for the future TeV observations. On the other hand, X-ray observations of Low frequency peaked BL Lacs (LBLs) show power-law spectra interpreted as inverse Compton emission in the widely entertained synchrotron-self Compton scenario. I also present analysis of simultaneous optical to X-ray observations of all LBLs in the Swift archive; I investigate the distribution of their spectral parameters, and study their variability properties and possible connections with HBL population.

Katja Poppenhaeger (University of Hamburg) in Pratt at 12:30 on 8 September 2010

Star-Planet-Interactions

Star-Planet-Interactions (SPI) have been puzzling the scientific community during the last years: in analogy to binary star systems, one expects that massive, close-in planets can enhance their host star's activity via tidal or magnetic interaction. There are some indications for magnetic interaction in chromospheric observations as measured by CaII H and K line fluxes. In X-rays however, observational results are ambivalent at best. I will review how much reliable observational information we have on SPI and present results derived from the so far most extensive sample of planet hosting stars in X-rays. From this sample, we can show that no overall trend exists for giant planets to cause increased stellar activity which cannot be explained by selection effects. I will also discuss results of a search for SPI signatures for the star upsilon Andromedae, one of the prime candidates for SPI, at both X-ray and optical wavelengths.

Jonathan Trump (UC Santa Cruz) in Pratt at 12:30 on 29 September 2010

Accretion Rate and AGN Unification

I will present results from the COSMOS X-ray AGN survey, which has spectroscopic follow-up from the Magellan/IMACS and MMT/Hectospec instruments and multiwavelength SEDs from Chandra, XMM, GALEX, Subaru, Spitzer, and VLA. These observations show that at low accretion rates (L/Ledd<0.01), the accretion onto an AGN changes from a thin disk to an advection dominated flow. Consequently the ratios of IR, optical, and UV luminosities to X-ray luminosity decrease, as the disk temperature cools and the disk wind no longer creates a broad line region or a dusty "torus". The onset of a RIAF also means that low accretion rate AGNs have stronger radio jets, with the associated synchrotron emission observed as continuum polarization in their optical light. Meanwhile broad-line AGNs (and obscured narrow-line sources, with hidden broad lines) exist only at high accretion rates (L/Ledd>0.01). I will place these ideas in the framework of an AGN unified model based on both accretion rate and obscuration.

No HEAD Lunch Talk () in Pratt at 12:30 on 13 October 2010

Postdoctoral Science Symposium

No HEAD Lunch Talk () in Pratt at 12:30 on 20 October 2010

Einstein Fellows Symposium

Laura Lopez (UC Santa Cruz) in Pratt at 12:30 on 27 October 2010

Dissecting Supernova Remnants Observed with Chandra

Supernova remnants (SNRs) are a complex class of sources, and their heterogeneous nature has hindered the characterization of their general observational properties. To overcome this challenge, I have developed several statistical tools to quantify the small- and large-scale morphological properties of extended objects, including SNRs. In this talk, I will present results from the application of these methods to the Chandra X-ray images of 24 Galactic and Large Magellanic Cloud SNRs. I will show how the techniques can be used to type SNRs (even if they have weak line emission), to constrain explosion mechanisms, and to study chemical mixing of the SN ejecta. Additionally, I will present new results from using the methods to glean particle acceleration properties from X-ray and radio images of young SNRs.

Teddy Cheung (NRL) in Pratt at 12:30 on 3 November 2010

Two Surprises from the Fermi Large Area Telescope

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, observes the high-energy (>100 MeV) gamma-ray sky with unprecedented sensitivity and resolution. Its default mode of observations over its now 2+ years of operation is to scan the entire sky every ~3 hrs, making it especially well-suited to detect and characterize transient and variable sources. Amongst the handful of transients in the Galactic plane detected so far was the surprise discovery of GeV emission from a nova, a source type previously unexpected as a high-energy gamma-ray emitter. The continuous accumulation of all-sky exposure from the LAT scanning observations also allows for the study of persistent sources. This led naturally to yet another unexpected result -- the detection of the giant radio lobes (angular size ~10 degree) of a nearby radio galaxy imaged in gamma-rays. The results of these two surprises from the Fermi-LAT will be presented.

Andrew Goulding (CfA) in Pratt at 12:30 on 10 November 2010

Towards a complete census of AGN activity in nearby galaxies

It is now well established that almost all massive galaxies in the nearby Universe host a central supermassive black hole, and that these black holes have grown through mass accretion events, so-called AGN phases. However, the majority of SMBH growth is hidden from our line-of-sight by high levels of gas/dust. In this talk I will discuss recent results combining X-ray, mid-infrared and optical spectroscopy in order to probe two orders of magnitude deeper in the L_X--z plane than using current X-ray instruments alone, and provide new constraints on the fraction of obscured AGN activity in the nearby Universe. Using these multi-wavelength analyses, we investigate the intrinsic properties and demographics of local obscured AGNs, and conservatively infer the space density of Compton-thick AGNs at z~0.1 in wide-field X-ray and optical surveys.

Stephanie Juneau (Steward Observatory) in Pratt at 12:30 on 17 November 2010

Linking Starbursts, Major Mergers, and Absorbed AGNs at Redshift~0.7

I present a detailed study of the occurrence of star formation, active galactic nuclei (AGNs), and galaxy mergers in a sample of 70-micron selected galaxies from the Far-Infrared Deep Extragalactic Legacy survey (FIDEL). Deep multiwavelength observations reveal a complex connection between starburst, AGN, dust obscuration, and gas outflows. For galaxies with measurable emission lines, the fraction of 70-micron galaxies with any level of AGN activity may be as high as 40-45%, i.e., more common than previously thought. This difference may be due to 70-micron galaxies hosting AGNs that are heavily absorbed (Compton-thick). I present evidence that these systems are ideal test-beds for galaxy merger scenarios in which gas-rich galaxies merge, go through a deeply-embedded ULIRG phase before emerging as an X-ray and optically identified AGN. I compile galaxies at different evolutionary stages allowing us to witness this process at redshift ~0.7. Lastly, I highlight the potential of future multi-wavelength studies to constrain such evolutionary scenarios.

Shijun Lei (Univ. Georgia ) in Pratt at 12:30 on 24 November 2010

Galactic Diffuse Hot Gas: Shadowing observations and Joint Analyses

Million degree gas constitutes most of the interstellar space in our Galaxy but remains the least well known component of the interstellar medium. I introduce in this talk two of my studies of the Galactic diffuse hot gas using shadowing observation and joint analysis methods. In the first work we analyze a pair of Suzaku shadowing observations to determine the X-ray spectrum of the Galaxy's gaseous halo. Our soft X-ray measurements, together with the OVI and CIV observations for the same direction, indicate the existence of hot halo gas covering a wide temperature range of ~10^5-10^7 K. We construct a broken power-law differential emission measure model for the hot gas and discuss the possible origin of the hot gas. We find that a simple model in which hot gas accretes onto the Galactic halo and cools radioactively cannot explain both the observed UV and X-ray portions of our broken power-law model. However, the UV and X-ray intensities and our broken power-law model can be well explained by hot gas produced by supernova explosions supplemented by a smooth source of X-rays. In the second work we construct a sample of 19 directions with both OVII emission intensity and absorption equivalent width measurements made from XMM-Newton archival data. Both the OVII emission and absorption strengths are significantly enhanced toward the inner region of the Galaxy, where the Galactic center soft X-ray enhancement (GCSXE) is seen in the ROSAT 3/4 keV map. We find a tight correlation between the OVII absorption equivalent width and the OVII emission intensity at the 97.9% confidence level for these 19 directions, strongly suggesting that the OVII emission and absorption are largely co-spatial. Our joint analyses of the OVII emission and absorption show that the hot gas on the directions off the GCSXE are in good agreement with a thick disk model. While for the hot gas associated with the GCSXE, our results support its Galactic center/bulge origin.

Malgosia Sobolewska (CfA) in Pratt at 12:30 on 1 December 2010

Simulated spectral states of AGN and observational predictions

Active galactic nuclei (AGN) and galactic black hole binaries (GBHs) represent two classes of accreting black holes. They both contain an accretion disc emitting a thermal radiation, and a non-thermal X-ray emitting 'corona'. GBHs exhibit state transitions and spectral states are characterized by different luminosity levels and shapes of the spectral energy distribution (SED). If AGN transitioned in a similar way, the characteristic timescales of such transitions would exceed ~105 years. Thus a probability to observe an individual AGN at different spectral states is very low. In this paper we follow a spectral evolution of a GBH GRO~J1655-40 and then apply its SED evolution pattern to a simulated population of AGN under a reasonable assumption that a large sample of AGN should contain a mixture of sources in different spectral states. We model the X-ray spectra of GRO J1655-40 with the eqpair model and then scale the best-fitting models with the black hole mass to simulate the AGN spectra. We compare the simulated and observed AGN SEDs to determine the spectral states of observed Type 1 AGN, LINER and NLS1 populations. We conclude that bright Type 1 AGN and NLS1 galaxies are in a spectral state similar to the soft spectral state of GBHs, while the spectral state of LINERs may correspond to the hard spectral state of GBHs. We find that taking into account a spread in the black hole masses over several orders of magnitude, as in the observed AGN samples, leads to a correlation between the X-ray loudness, alpha_ox, and a monochromatic luminosity at 2500A. We predict that the alpha_ox correlates positively with the Eddington luminosity ratio down to a critical value of lambda_crit = L/L_E ~ 0.01, and that this correlation changes its sign for the accretion rates below lambda_crit.

Michael McDonald (Astronomy Department - University of Maryland) in Pratt at 12:30 on 8 December 2010

On the Origin of the Extended Halpha Filaments in Cooling Flow Clusters.

We present results from a survey of cooling flow clusters and groups covering nearly three orders of magnitude in mass, and 1-2 orders of magnitude in temperature and mass deposition rate, aimed at explaining the presence and morphology of warm, ionized gas in the cool cores of galaxy groups and clusters. Using the Maryland-Magellan tunable filter on the Baade 6.5-m telescope at Magellan we have taken a census of these mysterious Halpha filaments with unprecedented depth and resolution. These data have been supplemented with new and archival X-Ray (Chandra), UV (HST, GALEX, XMM-OM), near-IR (2MASS) and radio (VLA) observations. Armed with the most detailed picture of the warm, ionized gas in cooling flow clusters to date, we investigate the possible mechanisms for producing the observed morphologies (buoyant bubbles, runaway cooling, interaction with satellites, etc) as well as possible ionization mechanisms (young stars, heat conduction from the ICM, collisional heating from cosmic rays, etc). Additionally, we determine the effect of environment on the formation of ionized filaments by considering the correlation of Halpha filaments with the global mass, temperature and gas fraction of the system. Our results offer exciting new constraints, both quantitative and qualitative, for the latest models of cooling flow clusters.

Doug Finkbeiner (CfA) in Pratt at 12:30 on 15 December 2010

Giant Gamma-ray Bubbles in the Inner Galaxy: AGN Activity or Bipolar Galactic Wind?

I will discuss our recent paper on the giant gamma-ray bubbles in the inner Galaxy observed by the Fermi Gamma-ray Space Telescope (http://arxiv.org/abs/1005.5480) and show that these are associated with the structures we have called the "Fermi haze" and "WMAP haze" in the past. The bubbleshave sharp edges, suggesting a transient event caused by a huge energy injection in the Galactic center in the last 1-10 Myr, e.g. a BH accretion event or a nuclear starburst. I will argue that these sharp-edged bubbles have nothing to do with a Galactic WIMP annihilation signal, and that they significantly complicate any effort to find such a signal in the inner Galaxy. Moreover, given the presence of such transients it is not even feasible to set useful limits on the electronic signal from dark matter annihilation in the inner Galaxy.

Shea Brown (CSIRO - Australian Telescope National Facility) in Pratt at 12:30 on 22 December 2010

Non-Thermal Probes of Cluster Energization

Non-thermal radio emission has proven to be a promising probe of hierarchical cluster assembly, illuminating shocks and turbulence resulting from minor mergers/accretion. I will present new observations revealing potential shocks in the intra-cluster medium of merging clusters, and highlight the potential of upcoming radio telescopes to constrain models of cosmic-ray acceleration.

Adam Foster (CfA) in Pratt at 12:30 on 19 January 2011

AtomDB 2.0 : Atomic data for X-ray Astronomy

We describe the latest release of AtomDB (version 2.0), a database of atomic data and a plasma modeling code with a focus on X-ray astronomy. This release includes several major updates to the fundamental atomic structure and transition data held within AtomDB, incorporating new ionization balance data, state-selective recombination data, and updated collisional excitation data for many ions, including the iron L-shell ions from Fe^{+16} to Fe^{+23}. We also describe some of the effects that these changes have on calculated emission and diagnostic line ratios, such as changes in the temperature-dependent G-ratio for He-like ions of up to 33%.

Lukasz Stawarz (ISAS, Japan) in Pratt at 12:30 on 26 January 2011

Radio Galaxies, Seyferts, and Young Radio Sources in Gamma-rays

Blazars are established extragalactic sources of gamma-rays. A number of Active Galactic Nuclei (AGN) belonging to this class has been discovered in the past by EGRET, by modern Cherenkov telescopes (such as HESS, Magic, and VERITAS), and is being recently monitored by the Fermi satellite. In addition to blazars, however, there is a growing population of non-blazar AGN detected at high and very high energy gamma-rays, and in particular of the GeV-emitting Radio Galaxies observed by Fermi/LAT. In this talk I will summarize the Fermi results regarding nearby Radio Galaxies obtained during the two years of the LAT operation. I will also discuss the prospects for detecting some other classes of non-blazar AGN in a near future, such as Seyfert Galaxies and Young Radio Sources.

Jason Curtis (Penn State) in Pratt at 10:00 on 31 January 2011 (Monday)

Ruprecht 147 - The Nearest Old Cluster

Benchmark clusters are fundamental to stellar astrophysics, and there are no older, closer clusters than Ruprecht 147 (R147). The K and M dwarfs of R147 will be the only old, single, cool dwarfs with known ages bright enough to admit close spectroscopic study. The only other clusters older than 1 billion years are more than twice as distant, and the only closer clusters are less than half as old. A useful age - activity relation for old clusters requires benchmarks like R147, which to date have not been available. The four published catalogs of cluster properties and membership lists are in wild disagreement. We demonstrate that the most recent list has completely misidentified R147. Our group's past work has already more than doubled the number of known members, and we identify new members with radial velocities measured from high-resolution spectra obtained with MMT/Hectochelle. We fit Padova isochrones to the cluster CMD to derive distance, age and reddening. We show that the published distance of 175 pc is likely to near by 100 pc. We are working to publish and disseminate online our catalog of cluster and stellar properties, offering the community several new avenues for research in stellar astrophysics.

Jeannette Gladstone (University of Alberta - Canada) in Pratt at 12:30 on 09 February 2011

Attempts to constrain the mass of black holes in ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) have luminosities exceeding that of the Eddington limit for spherical accretion onto stellar-mass black holes (~3-20 M(solar)). Such high luminosities have provoked debate over the true nature of these sources. Some argue that such objects are intermediate-mass black holes (IMBHs) residing in standard accretion states, while others propose that these are stellar-mass compact objects residing in a (new) extreme, super-Eddington accretion state. This is an exciting time to be in this field, as it is only now, with access to large international facilities, that we are approaching a conclusion to this debate ... or are we?

Robin Barnard (CfA) in Pratt at 12:30 on 16 February 2011

X-ray Identification of New Black Hole Binaries in M31 Globular Clusters

Until recently, no stellar mass black hole X-ray binaries were associated with globular clusters, even though the high stellar densities are highly favorable. This sparked a great deal of interest in the theoretical modeling community, with many models finding ways to eject the black hole from the cluster. However, other models allow ~1 black hole binary in each cluster; binaries formed by exchange would be transient X-ray sources, while binaries formed by capture would be persistent. However, the most secure method for identifying black hole X-ray binaries requires identifying the companion star, so that the mass function may be determined from the radial velocity curve. This is extremely difficult in regions of high stellar density such as globular clusters. A few years ago, I invented a method for identifying black hole candidates in X-ray binaries from their X-ray properties alone. In summary, this method relies on the fact that all X-ray binaries (black hole or neutron star) exhibit remarkably similar behavior at low accretion rates, but markedly different behavior at luminosities greater than ~10% of the Eddington limit; since black holes are more massive than neutron stars, they can exhibit low state behavior at higher luminosities. This method is only sensitive to black hole binaries that exhibit low accretion rate behavior at high luminosities. Prior to this work I found two black hole candidates in M31 globular clusters. I will report on my analysis of 115 Chandra observations of the central region of M31; this data is extremely well suited to X-ray identification of black hole binaries, since each observation contains hundreds of X-ray sources, giving ~40,000 chances to find a black hole! I used these observations along with archival XMM-Newton data to identify 4 new black hole candidates in M31 globular clusters, 1 transient and 3 persistent; the probability that these persistent sources are coincident background galaxies is ~3E-22. I will discuss the implications of these findings.

No HEAD talk () in Pratt at 12:30 on 23 February 2011

Davide Burlon (MPE - Garching) in Pratt at 12:30 on 2 March 2011

Three-year BAT survey of AGN: reconciling theory and observation?

It is well accepted that unabsorbed as well as absorbed AGN are needed to explain the nature and the shape of the Cosmic X-ray background, even if the fraction of highly absorbed objects (dubbed Compton-thick sources) substantially still escapes detection. We derive and analyze the absorption distribution using a complete sample of AGN detected by Swift-BAT in the first three years of the survey. The fraction of Compton-thick AGN represents only 4.6% of the total AGN population detected by Swift-BAT. However, we show that once corrected for the bias against the detection of very absorbed sources the real intrinsic fraction of Compton-thick AGN is 20(+9-6)%. We proved for the first time (also in the BAT band) that the anti-correlation of the fraction of absorbed AGN and luminosity it tightly connected to the different behavior of the luminosity functions (XLFs) of absorbed and unabsorbed AGN. This points towards a difference between the two subsamples of objects with absorbed AGN being, on average, intrinsically less luminous than unobscured ones. Moreover the XLFs show that the fraction of obscured AGN might also decrease at very low luminosity. This can be successfully interpreted in the framework of a disk cloud outflow scenario as the disappearance of the obscuring region below a critical luminosity. Our results are discussed in the framework of population synthesis models and the origin of the Cosmic X-ray Background.

Daniel Castro (CfA) in Pratt at 12:30 on 16 March 2011

SNRs in the Sedov-Taylor Phase: How Does Efficient Particle Acceleration Impact Their Evolution?

We investigate the effects of the efficient production of cosmic rays on the evolution of supernova remnants (SNRs) in the adiabatic Sedov-Taylor phase. We model the SNR by coupling the hydrodynamic evolution with nonlinear diffusive shock acceleration (DSA), and track self-consistently the ionization state of the shock-heated plasma. Using a plasma emissivity code and the results of the model, we predict the thermal X-ray emission and combine it with the non-thermal component in order to obtain the complete spectrum in this energy range. Hence, we study how the interpretation of thermal X-ray observations is affected by the efficiency of the DSA process, and find that, compared to test particle cases, the efficient DSA example yields a smaller shock radius and speed, a larger compression ratio, and lower intensity X-ray thermal emission. We also find that a model where the shock is not assumed to produce CRs can fit the X-ray observational properties of an example with efficient particle acceleration, with a different set of input parameters, and in particular a much lower explosion energy. Additionally, we model the broadband non-thermal emission, and investigate what signatures result from the acceleration of particles.

Peter Capak (Caltech) in Pratt at 12:30 on 23 March 2011

Large Scale Structure and Extreme Star formation at z>4

Theory expects and statistical observations confirm that large scale structure has a significant roll in the formation and evolution of the first galaxies. However, the influence of large scale structure at high redshift (z>4) remains largely un-quantified due to the spacial extent of structures at these redshifts and the faint fluxes of the galaxies that populate them. Using imaging from the COSMOS survey combined with deep spectroscopy from Keck covering a 0.5x1 degree region we begin to quantify the roll of large scale structure. We show the typical structure extends over 10's of arc-minutes and that Quasars and Extreme-Starbursts (SMGs) appear to preferentially populate these structures. We also link the members of these proto-clusters to their likely decedents at z~2.

Cecilia Garraffo (CfA) in Pratt at 12:30 on 13 April 2011

Modified Gravity in Higher Dimensions: The Black Hole Scenario

Black holes are very interesting objects both for their astrophysical properties and for being good labs to test ideas of fundamental physics. Since many years physicist have been puzzled by the questions/paradoxes this objects seem to introduce. Good examples of these are the information loss paradox, given by their thermodynamical properties, and the loss of predictability implied by the existence of space-time singularity. In this talk I will discuss how some of this problems can be addressed by introducing modified gravity models in extra dimensions, which include higher-curvature corrections to general relativity. I will focus my attention on a particular theory of this kind, known as Lovelock theory of gravity, which is the natural extension of Einstein theory to higher dimensions and exhibits special features. I will also describe some new instabilities that appear in this theory.

Mario Giuseppe Guarcello (CfA) in Pratt at 12:30 on 27 April 2011

Star Formation in Massive Clusters: the outstanding cases of the Eagle Nebula and Cygnus OB2

Massive stars hosted in young clusters play a key role in the star formation process and the evolution of the young stars in the parental cloud, for instance inducing the photoevaporation of nearby circumstellar disks, dispersing the surrounding cloud, triggering the star formation process. These processes have been studied in several star forming regions, in order to understand their effects on the whole stellar population of the hosting cluster. Given their content of massive stars and their proximity to the Sun, the Eagle Nebula (M16), at 1750 pc, and the Cygnus OB2 cluster, at 1450 pc, represent ideal targets to study the effects of massive stars on the star formation process. The Eagle Nebula (M16) is a massive star forming region with several episodes of recent or even ongoing star formation. The nebula hosts in its center the young open cluster NGC 6611, with more than 50 OB stars. The environment in CygOB2, in the vaste molecular Cygnus-X cloud, is more extreme, since the presence of about 120 O stars and more than 2000 B stars. However, in the outer regions of both nebulae the environment changes drastically, with star formation sites characterized by few massive stars and low UV fluxes, offering different environments where stars form and their circumstellar disks evolve. I will present the results of the multiband study of M16 about the disks survival times in star-forming regions with a content of few tens of OB stars, and the possibility that OB stars trigger the formation of new stars in the whole parental cloud, and not only in their vicinity. Then, I will show how the multiband analysis of the stellar population of CygOB2, related to the deep 1.08 Msec Chandra Cygnus OB2 Legacy Survey, will shed more light on the effect of massive stars on the formation of new stars and on the evolution of circumstellar disks in their proximity.

Elena D'Onghia (CfA) in Pratt at 12:30 on 4 May 2011

Collective Origin of Spiral Structure in Disk Galaxies

It is now nearly 50 years since spiral structure in galaxies was recognized as likely originating from the action of density waves propagating through a differentially rotating disk. However, the origin of these features remains controversial. For example, whereas observations show that spiral arms are density waves, N-body experiments performed to date have not yielded long-lived spiral structure, as predicted by the stationary density wave theory. We use high resolution numerical simulations to explore the idea that spiral arms might be seeded by density inhomogeneities orbiting in the disk itself. These perturbations can be identified with fluctuations in the distribution of gas in the interstellar medium of galaxies, such as giant molecular clouds. Our simulations show that when sufficient numbers of these perturbers are present, they collectively amplify to yield large-scale spiral patterns that resemble the spiral arms in flocculent and intermediate spiral galaxies. By combining our N-body experiments with simple analytic arguments, we develop a theory for spiral structure formation based on the "collective" effect of swing amplification. Our model makes numerous testable predictions, making it possible to finally confront the theory that spiral arms are stochastic with observations.

Visiting Student Special (CfA) in Pratt at 12:30 on 11 May 2011

Five talks by visiting students in the SAO/Southampton exchange program

Tom Armstrong, (adviser: Joseph Hora) Investigating Infrared Dark Clouds and Star Formation in the Massive Cygnus-X Complex. We have created an unbiased catalogue of the infrared dark clouds (IRDCs) in the star formation regions Cygnus-X, using IRAC and MIPS data from Spitzer. Finding a total of 1234 cloud clumps we calculated their properties and those of their cores, along with finding embedded objects and determining the YSO clustering around some of the more massive cloud complexes. Our results enable us to examine the clouds in much greater detail than in previous studies and provide evidence for IRDCs as the sites of young cluster formation along with demonstrating the effectiveness of using extinction mapping to trace cloud properties. Adam Kent (adviser: Matt Ashby) A Near-Infrared Investigation of Nearby Galaxy Bulge to Total Luminosity Ratios Two-dimensional decompositions of 39 MMIRS Ks band mosaics of nearby bright galaxies into bulge and disk components with GALFIT are used to test two assumptions used to calculate Ks bulge magnitudes for nearby bright galaxies in Devereux et al. (2009). The bulge magnitudes of ellipticals are not significantly brighter than those found by Marconi and Hunt (2003) and the bulge magnitudes of Sb-Sbc spiral galaxies are as found by Graham and Worley (2008). The bulge magnitudes of lenticular galaxies are 1.42 +/- 0.90 mag brighter than those found by Graham and Worley, with a 1sigma correction range to 2MASS total magnitude between 1.20 and 2.80 mag. Sam Park (adviser: Michael Garcia) An X-ray/UV/Optical Study of Cen X-4 and A0620-00 We extend previous studies of the SED of Cen X-4 into the far-UV, and find a previously unknown strong turnover in the UV. This allows us to establish a robust temperature estimate for the UV emission, which strongly suggests that it originates in the 'hot spot' where the accretion stream intersects the accretion disk. Interestingly the mass accretion rate inferred from the hot spot luminosity is ~10**4 times higher than that inferred from the neutron star accretion luminosity. We also present a new data set and analysis of the optical ellipsoidal variation in order to determine the binary inclination. Electra Panagaulia, (adviser: Paul Nulsen) Content of the Radio Lobes of Fornax A The analysis of the X-ray emission from the lobes of Fornax A based on XMM-Newton observations will be presented. Inverse Compton emission from the western lobe was detected and imaged, as was a thermal filament that seems to sit near the radio lobe. The relativistic electron and magnetic pressures in the western radio lobe were found to be similar, and the radio lobe appears to be confined by the hot extended atmosphere of the host galaxy, NGC 1316. Erik van der Veen (adviser: Jeremy Drake) A radial velocity survey of the massive star forming region Cygnus OB2. Cygnus OB2 is our nearest massive star-forming region at only 1.45kpc, making it an important and fascinating young stellar cluster to study. An extensive set of new spectra allowed us to determine radial velocities and effective temperatures of its low- and intermediate-mass members. Using these, we are characterizing the association in order to improve our understanding of the dynamics and survival timescales of such systems.

Karri Koljonen (Aalto University/Metsahovi Radio Observatory ) in Pratt at 15:00 on 16 May 2011 (Monday)

The anatomy of major radio/X-ray flares in the enigmatic microquasar Cygnus X-3

Cygnus X-3 exhibits major radio flares/jet ejection events like no other microquasar with radio flux densities up to 20 Jy. During these major flares Cygnus X-3 displays a variety of phenomena across the electromagnetic spectrum including specific temporal properties such as gamma-ray flares observed before the onset of major radio flare to quasi-periodic oscillations observed during the major radio flare decay. Here we try to form a more unified picture of the nature of these jet ejection events by dissecting the multi-wavelength observations with a view to examining the emission processes from the components of the system: the disk, the corona and the jet.

Fabrizio Nicastro (CfA) in Pratt at 12:30 on 18 May 2011

WHIM-EX: a X-Ray NASA Explorer to Solve the MIssing Baryons Problem

The "Wheat and the Chessboad" problem (http://en.wikipedia.org/wiki/Wheat_and_chessboard_problem) shows that exponential growth cannot go on forever.In space astronomy this means our missions cannot keep on being twice as expensive as the last one. At some point we must "hit the funding wall". We just did. An exciting opportunity for space-astronomy, is offered by the revived NASA-Explorer program foreseen for the next decade. NASA Explorers are small, light, relatively cheap (~$250M), quick-to--launch and focused, one-main-science-driver, missions. In principle, a number of Explorers can compensate for the lack of big multi-purpose observatories (e.g. the original IXO design could be replaced by 3 or 4 well-designed Explorers focused on distinct science drivers). Here I present one of these concepts, WHIM-EX, which is mainly designed to solve one of the most controversial problem of modern astrophysics, that of the 'Missing Baryons', but that is also perfectly suited to deeply investigate into a much wider scientific context: from AGN/IGM and IGM/galaxy feedback, to Doppler-maps of active X-ray star coronae, to the outskirts of galaxy clusters, etc. WHIM-Ex is one of 15 candidates for a full EX mission in the current NASA Explorer call (selection foreseen in fall 2011), and is a high spectral resolution (R=4000), large throughput (A>250 cm2), soft X-ray (0.1-1 keV) spectrometer, based on an innovative optical design that optimizes efficiency, feasibility and costs, by integrating the grating spectrometers into the optical modules, employing sub-aperturing, and exploiting four-reflection optics by pairs of 296 identical parabola-hyperbola-flattening-grating modules. WHIM-EX will detect >100 Warm-Hot Intergalactic Medium filaments within the first year of operation (~ 15 Ms, i.e. 70% of the total time available during the first year (for a visibility efficiency of 66%, due to the low-Earth orbit of WHIM-ex). WHIM-EX will measure the Cosmological Mass Density of baryons in the Universe with a precision better than 10%, comparable to the other components of Omega_b. A progressively increasing fraction of GO time will be reserved to the community, after the first year of operation. The minimum mission life-time is of 3 years (with launch in 2017, if selected), but a 2-year extension is quite possible.

Ettore Flaccomio (Osservatorio Astronomico di Palermo) in Pratt at 12:30 on 15 June 2011

Coronae and circumstellar accretion in CTTS: new lessons from X-ray time-variability studies

The magnetic fields of young accreting low mass stars (Classical T Tauri Stars, CTTS) play an instrumental role in regulating the transfer of mass and angular momentum in star-disk systems, while also confining and heating hot coronae. The bright X-ray emission from coronae and accretion shocks can in turn heat and ionize circumstellar disks, thus affecting their properties and evolution. Understanding the X-ray emission from young stars is therefore relevant for the overall understanding of early stellar and disk evolution as well as for planet formation. In low mass Pre Main Sequence (PMS) stars, X-rays are abundantly produced in hot magnetically confined coronae. An additional soft component is generated in accretion shocks at the foot-points of magnetically channelled accretion streams. Neither of these two processes is well understood. For example, we do not understand why the observed coronal X-ray emission of accreting PMS stars is on average lower than that from non and-accreting ones of the same mass and also spans a wider range of luminosities. As for the X-ray emission from the accretion shock, the observed flux is generally about one order of magnitude lower than expected from the dissipation of gravitational energy in the accretion shock. One of the defining characteristics of CTTSs is their extreme variability in virtually all spectral bands, most likely due to the highly dynamical nature of accretion and magnetic activity, coupled with stellar rotation. The precise physical mechanisms that drive variability are, however, not clear. Studies of rotationally induced or intrinsic variability should therefore help derive a coherent picture of the physical processes occurring during the CTTS stage. In the X-ray band strong variability is, however, common to all magnetically active stars, regardless of their evolutionary status. Indeed, until recently, no striking peculiarity has been reported for the X-ray variability of accreting stars with respect to non accreting ones. I will present three examples of how X-ray variability studies can help us clarify the mechanisms responsible for the observed coronal and accretion shock X-ray emission, especially when combined with suitable simultaneous optical observations.

No Lunch Talk (Peer Review) in Pratt at 12:30 on 22 June 2011

TBA

Raffaele D'Abrusco (CfA) in Pratt at 12:30 on 6 July 2011

The exploration of multi-wavelength astronomical datasets: an original approach to knowledge discovery through Unsupervised Clustering techniques and its application to a Chandra Source Catalog multi-wavelength dataset

It is common knowledge that a complete understanding of all astronomical sources requires a global multi-wavelength approach and that, at the same time, the availability of large surveys of the sky in different spectral regions has propelled the aggregation of massive and complex datasets. The traditional approach to data analysis that involves well informed testing of different models cannot make justice of the richness of the these new datasets and, in some sense, of the intrinsically peculiar type of knowledge therein contained. Knowledge Discovery (KD) techniques, while relatively new to astronomy, have been successfully used in several other disciplines, from finance to genomics, for the determination of patterns in complex and large datasets. In this talk I shall describe an original method for the characterization of the multi-dimensional astronomical sources, based on KD unsupervised clustering algorithms that are used to determine the spontaneous aggregations of sources in the high-dimensional space generated by their observables (features space). Then, a data-driven criterion is applied to pick the most interesting clusterings in terms of astronomical properties of the sample and its aggregation. I will discuss the application of this method to a sample of optically selected AGNs with X-ray observations in the Chandra Source Catalog and other multi-wavelength data, whose results are the establishment of diagnostics for an objective classifications of these sources based on their multiwavelength properties and the exploration of unknown/hidden correlations in the features space that could help shed light on the understanding of AGNs. I will finally outline the future developments of my work.

Jeremy Lim (University of Hong Kong) in Pratt at 12:30 on 15 July 2011 (Friday)

The Enigmatic Nebula at the Center of the Perseus Cluster

The optically-luminous nebula associated with the central giant elliptical galaxy of the Perseus cluster, NGC 1275, has been an enigma ever since its discovery more than 50 years ago. Today, we know that such luminous nebulae are commonly associated with the central galaxies of cool-core groups/clusters but not otherwise, providing circumstantial evidence for a link with (residual) X-rays cooling flows. I review current understanding of the composition, excitation, origin, and possible fate of the nebula associated with NGC 1275, emphasizing observational tests of models for the nature of this nebula. The nebula is multiphase, comprising molecular, atomic, and ionized gas with temperatures ranging from ~100 K to ~106 K. The bulk of the nebula is not photoionized; instead, the nebula is most likely collisionally excited by energetic particles. Our observations, however, argue against the only population of energetic particles so far observed in NGC 1275 as the source of the excitation. The inner region of the nebula may be deposited by a residual X-ray cooling flow, whereas the outer region may have been structured by nebular material dragged outwards by rising bubbles. I show for the first time the velocity field of the entire optical nebula in NGC 1275, and compare the measured velocity field against models for the flow pattern behind rising bubble. Finally, I discuss the role of the nebula in forming stars and fueling the AGN in NGC 1275.

Josh Grindlay and Scott Wolk (HEAD Hardware Talks) in Phillips Auditorium at 12:30 on 20 July 2011

1)Two small wide-field survey missions: REXIS and MIRAX-HXI and 2) The proposed Explorer WHIMeX

1) Josh Grindlay: We proposed last year a small (~3kg !) wide-field (~40deg) coded aperture imaging instrument to be the "student experiment" on the OSIRIS-REx mission, which NASA recently selected as the next asteroid sample return mission (2016 launch; 2020 encounter phase). REXIS (Regolith X-ray Imaging Spectrometer) is a ~30cm cube package that images via a coded mask solar fluorescent x-rays (0.3-7 keV) with a 2 x 2 array of Suzaku-type CCDs (from MIT) to make spectral line maps of the asteroid during both the survey phase and encounter phase. For the recent Explorer 2011 call, we submitted a Mission of Opportunity to provide the imaging CZT detector planes for an array of 2 x 2 coded aperture telescopes for the MIRAX experiment on the Lattes satellite to be launched by Brazil in 2016. Our HXI (Hard X-ray Imager) would enable 5-200 keV imaging with 5arcmin and ~1.5 keV resolution from a near-equatorial scanning (1 RPO) satellite and is designed to provide a deep survey of the Galactic Center and Bulge and Southern Plane, as well as high latitude sky for studies of BH and NS accreting binaries, a general high energy TDA survey, and studies of Short GRBs during the time-frame of Advanced LIGO for EM-GW studies of GRBs. Both instruments and their science prospects will be described, briefly. 2) Scott Wolk for the WHIMeX team: Theoretical studies of cosmological structure formation predict that a large fraction of primordial baryons have not accreted onto galaxies. Instead, they are distributed between the galaxies in a complex intergalactic medium (IGM) structured in filaments and voids. Ranging in temperature from 10,000 K to several million degrees, this gas has a distribution of ions determined by collisional ionization and photoionization by EUV and X-ray backgrounds. Ultraviolet spectral surveys (Hubble, FUSE) have detected the hot IGM in resonant absorption lines of trace species (H I, O VI, N V, Ne VIII). However, a significant fraction (40-50 percent) of the baryons has eluded detection. They may reside in million-degree gas produced by cosmological shocks, galactic winds, and virialized circumgalactic gas. Searching for this hot, metal-contaminated gas will require high-throughput X-ray spectrographs, to detect narrow (50-100 km/s) absorption lines from the key ions (C V, N VI, O VII, O VIII, Ne IX) that dominate million-degree plasmas. With the further deferral of the International X-ray Observatory, there is a need for low cost approaches to high resolution x-ray spectroscopy. WhimEx is a mission concept that will be proposed in response to the current Explorer AO. WhimEx combines the teams and the technology that were being developed for the IXO reflection grating spectrometer. With WhimEx, astronomers will be able to probe the WHIM, AGN outflows and many other classes of x-ray sources.

Suzanne Romaine and Michael Garcia (HEAD Hardware Talks) in Pratt at 12:30 on 27 July 2011

(1) Coatings for Hard X-ray Optics and (2) What's new with Micro-Channel Plate Optics?

1) TBA 2)Channel Plate, or 'Lobster Eye' optics were first described as x-ray optics in 1979 by Angel. I will talk about recent advances which have allowed them to be selected for several approved missions. Those advances include 'radial packing' which allows Wolter-1 like optics, manufacturing improvements which allow ~5 arc-min PSF and less than 1 arc-min over small areas, and high-z coatings allowing improved reflectivity. These optics can be ~30x lighter than the thin glass optics studied for IXO, and could provide up to 5m^2 of area up to 5keV for a mission such as the Extreme Physics Explorer.

Francesco Massaro (CfA) in Phillips at 14:30 on 3 August 2011

Large Scale Extragalactic Jets in the Chandra Era

We report initial results from a systematic investigation of the properties of large-scale AGN jets detected in X-rays. We have uniformly analyzed archival Chandra data for more than 100 such sources and measure fluxes in three X-ray bands to estimate spectra. We discuss the sample, the reduction methods, and present first results for the ratio of X-ray to radio flux and the spectral analysis for jet knots and hotspots. Utilizing archival VLA and MERLIN data, we examine the X-ray and radio properties for the jet knots and hotspots in the sample which includes quasars and low- and high-power (FR1 and FR2) radio galaxies. As two different processes have been proposed for the X-ray emission mechanism -- synchrotron and inverse Compton -- we discuss on a possible new classification scheme for extragalactic jets based on our data. From our comparison of their radio-to-X-ray properties, several aspects on their nature became unexpectedly unclear, as different emission processes seems to provide no differences in the observable quantities.

Justin Kasper (CfA) in Pratt at 12:30 on 9 August 2011 (Tuesday)

Solar Probe Plus: A status update on the first mission to the atmosphere of a star

Arguably the most significant open question in heliophysics is the identification of the physical processes responsible for sustaining the solar corona at millions of degrees and for heating the solar wind as it expands into interplanetary space. The ultimate source of this energy is the convective motion of the surface of the photosphere and its embedded magnetic field, but the mechanisms by which the large scale and low frequency motion of the field is able to dissipate sufficient heat in the corona and solar wind have not been conclusively identified. The primary science objective of the NASA Solar Probe Plus mission is to determine the structure and dynamics of the Sun's coronal magnetic field and to understand how the corona and solar wind are heated and accelerated and how energetic particles are produced and evolve. To accomplish this, the spacecraft carries a broad payload of in situ and remote sensing instruments and uses a sequence of Venus gravitational assists to dive within 8.5 solar radii of the surface of the Sun, making it the first spacecraft to enter the sub-Alfvenic solar corona. Instrument selections for Solar Probe Plus were made in 2010, and included the Solar Wind Electrons Alphas and Protons (SWEAP) Investigation, a suite of instruments led by the Smithsonian Astrophysical Observatory. SWEAP uses instruments behind the spacecraft heat shield and a Sun-viewing Faraday Cup to ensure continuous and comprehensive sampling of the corona and wind throughout each encounter. As our October Mission Confirmation Review approaches, I will review Solar Probe Science, the status of the spacecraft, the scientific payload including SWEAP, and the link between our objectives and general questions in astrophysical plasmas.

Manami Sasaki (Tuebingen) in Pratt at 12:30 on 15 August 2011 (Monday)

Shock-heated Plasma in the Interstellar Medium

Supernova remnants (SNRs), interstellar bubbles, and superbubbles are generated either by stellar winds, by one or multiple stellar explosions, or by a combination of these and are driven by the expansion of strong shock waves propagating into the interstellar medium (ISM). The evolution of SNRs and bubbles can be studied best in soft X-ray line and continuum emission, since these plasmas are very hot (10^6 - 10^7 K). I will give a short overview of the studies of the hot ISM in the Large Magellanic Cloud with ROSAT, XMM-Newton, and Chandra and present our recent results. In addition I will discuss the prospects of ISM research with the eROSITA telescope on board "Spectrum Roentgen Gamma" (SRG) satellite scheduled for launch in 2013.

Sudip Bhattacharyya (Tata Institute of Fundamental Research, Mumbai) in Pratt at 12:30 on 17 August 2011

Thermonuclear X-ray Bursts from Neutron Stars

A class of neutron stars accrete matter from companion stars. This matter accumulates on the surface of the neutron star, and when the ignition condition is met, it rapidly burns via thermonuclear runaway processes. This burning, which is truly a multi-disciplinary field consisting of astrophysics, nuclear physics, fluid dynamics and gravitation, can be observed in X-ray wavelengths as an intense burst. Timing and spectral studies of these bursts could be used to probe some aspects of extreme environments, ranging from super-dense matter to very strong gravity. I will talk about various aspects and importance of these bursts from the astronomical points of view.

Reinout van Weeren (Leiden Observatory) in Pratt at 12:30 on 31 August 2011

Radio relics: unique probes of galaxy cluster mergers

Radio relics are enormous Mpc-sized synchrotron emitting sources found in the outskirts of massive merging galaxy clusters. It is has been proposed that these so-called relics trace cluster merger shock waves in which particles are accelerated to extreme energies. In this talk I will present the latest observational results on several unique new relics that provide new insights into the physical properties of extremely dilute cosmic plasmas, magnetic fields, and particle acceleration in shocks. In addition, I will present first high angular resolution observations of several clusters below 50 MHz from the new pan-European LOFAR radio telescope that is currently being commissioned. With its enormous sensitivity at the lowest frequencies it will be the breakthrough instrument for observations of diffuse radio emission in clusters.

Sylvain Chaty (University Paris Diderot & CEA Saclay ) in Pratt at 14:00 on 2 September 2011 (Friday)

Nature, formation and evolution of High Mass X-ray Binaries

I will describe the nature, formation and evolution of the 3 kinds of high mass X-ray binaries: HMXBs hosting Be star, accreting the wind from supergiants, and accreting through Roche lobe filling supergiants. A wealth of new observations, from the high-energy side (mainly INTEGRAL satellite), completed by multi-wavelength observations (mainly optical/near-infrared/mid-infrared from ESO), has shown that a new population of supergiant HMXBs has been recently revealed. New observations even suggest the existence of evolutionary links between Be and stellar wind accreting supergiant X-ray binaries. I will describe observational facts about these different categories of HMXBs, discuss the different models of accretion in these sources (e.g. transitory disk vs clumpy wind), show the evidences of a link between these different kinds of HMXBs, and finally include comparisons with population synthesis models.

Jianfeng Wu (Penn State) in Pratt at 14:30 on 13 September 2011 (Tuesday)

X-ray Insights into the Nature of Weak-Line Quasars

The Sloan Digital Sky Survey has discovered > 100 quasars with weak or undetectable emission lines in their optical/UV spectra at a wide range of redshift. A substantial fraction of these weak-line quasars (WLQs) also have unusual X-ray properties. I will report X-ray observations of a notable class of WLQs selected to have unusual UV emission-line properties (weak and blueshifted high-ionization lines; strong UV Fe emission) similar to those of the remarkable low-redshift quasar PHL 1811. All of the radio-quiet PHL 1811 analogs, without exception, are notably X-ray weak by a mean factor of ~13. These sources lack broad absorption lines and have blue UV/optical continua, suggesting they are intrinsically X-ray weak. However, their average X-ray spectrum appears to be harder than those of typical quasars, which may indicate the presence of heavy intrinsic X-ray absorption. Correlations between relative X-ray brightness and UV emission-line properties suggest that PHL 1811 analogs may have extreme wind-dominated broad emission-line regions. Observationally, PHL 1811 analogs appear to be a subset (~30%) of WLQs. The existence of a subset of quasars in which high-ionization "shielding gas" covers most of the BELR, but little more than the BELR, could potentially unify the PHL 1811 analogs and WLQs. I will also report recent X-ray results on low-redshift WLQs which support this unification model.

Ralf Heilmann (HEAD Hardware Talks) in Pratt at 12:30 on 14 September 2011

Critical Angle Transmission Gratings for High-Resolution Soft X-Ray Spectroscopy

High-resolution spectroscopy at energies below 1 keV covers the lines of C, N, O, Ne and Fe ions, and is central to studies of the Interstellar Medium, the Warm Hot Intergalactic Medium, warm absorption and outflows in Active Galactic Nuclei, coronal emission from stars, etc. Dispersive grating spectrographs currently offer the only path towards resolving power R > 3000. We have developed critical-angle transmission gratings that combine the advantages of blazed reflection gratings (high efficiency, use of higher orders) with those of conventional transmission gratings (low mass and size, relaxed alignment and figure tolerances, transparent at higher energies, polarization insensitive). Prototypes show diffraction efficiencies in good agreement with theory. We are currently working on increasing the size of the gratings and on minimizing the hierarchical support structures.

Elke Rodiger (Bremen) in Pratt at 12:30 on 21 September 2011

Gas sloshing, cold front formation and the merger history of galaxy clusters: combining observations and simulations

Galaxy clusters are sites of a variety of dynamical processes, e.g. the impact of active galactic nuclei or minor and major mergers. Each process leaves characteristic traces in the intra-cluster medium (ICM), which can be studied with today's high quality X-ray observations. The wealth of structure seen in the ICM in turn requires high resolution 3D simulations to be fully understood. By combining and comparing theoretical and observational work, our group aims at deciphering the dynamical state and history of galaxy clusters. I will present our results regarding minor merger induced gas sloshing. I will demonstrate that gas sloshing leaves characteristic traces in addition to the well-known cold fronts, and that sloshing can explain the observed features in detail. Disentangling all properties of the disturbing secondary cluster is, however, somewhat hindered by the fact that the sloshing inside the primary cluster is mainly governed by its own potential. As a trade-off, this property might be utilized as an alternative means to study the potential of the primary cluster.

Luisa Ostorero (Universita` di Torino) in Pratt at 12:30 on 28 September 2011

Emission and absorption in young radio sources: a multiwavelength perspective

Compact and young radio sources, also known for their GHz-Peaked Spectrum (GPS), are currently considered the key objects to study the early phase of the life of powerful radio-loud Active Galactic Nuclei. We developed the first dynamical-radiative model that describes the evolution of their structure and their emission and absorption properties, from the radio to the X-ray band. I will show how this model can reproduce the broad-band spectra of a sample of X-ray emitting GPS galaxies. I will also present the first results of the multiwavelength studies we have been conducting to test the model's predictions in terms of gamma-ray emission of young radio sources and physical properties of the circumnuclear absorbers.

Ken Pounds (University of Leicester) in Phillips Auditorium at 14:00 on 29 September 2011 (Thursday)

AGN Outflows and Feedback

Though initially disputed, the evidence for high velocity winds in non-BAL AGN has been consolidated in the past few years. Paradoxically when the wind energy in one of the primary candidates, PG1211+143, was quantified it became necessary to consider coupling to the host galaxy to be inefficient. Recent observations of a multi-velocity outflow in the narrow line Seyfert galaxy NGC 4051 now suggest that much of the wind energy is lost after shocking with the ISM. Total momentum of the shocked flow is conserved, however, and could over time develop sufficient thrust to terminate further growth.

Almus Kenter (HEAD Hardware and Advance Projects) in Pratt at 12:30 on 5 October 2011

CMOS Detectors for Astronomy

SAO has been involved in a multi-year program to develop CMOS imaging detectors for astronomy. The primary emphasis (and funding) for this program has been to optimize these detectors for soft x-ray applications. Historically, CMOS imaging detectors have been relegated to consumer applications because of poor fill factor, high read, and high fixed pattern noise. Recent developments have solved many of these performance issues and the new detectors combine the speed of CMOS with the low noise of CCDs. I will present a review of the technology with background and comparison to CCDs. The latest, imminent and future SAO efforts will be described.

Lisa Winter (JILA) in Pratt at 12:30 on 12 October 2011

Star Formation and Outflows in a Sample of Local Active Galaxies

While feedback from the central supermassive black hole likely affects the host galaxy evolution in the distant universe, through suppressing star formation, we can not directly observe these processes at work. We can, however, easily observe the host galaxy and AGN properties of nearby sources. To understand the outflow and host galaxy properties in a local sample of AGN, we present our results from optical and X-ray spectroscopic follow-ups of a sample of 50 Seyfert 1s detected in the very hard X-rays (14-195 keV) with the Swift Burst Alert Telescope. Due to the high energy selection, this survey is largely unbiased to the gas and dust which obscures softer bands. We find that outflows are detected in a majority of the sample and may be present in all local Seyfert 1s. We investigate how these outflows affect their host galaxies through searching for correlations between star formation rate and both accretion rate and outflow strength.

Ashley Pagnotta (Louisiana State University) in Phillips Auditorium at 14:30 on 14 October 2011 (Friday)

On the Progenitors of Local Type Ia Supernovae

Although the basic mechanism responsible for Type Ia supernovae appears to be well understood (thermonuclear explosion of a carbon-oxygen white dwarf that has reached the Chandrasekhar mass limit), the identity of the progenitor system(s) remains a mystery. With implications from stellar evolution to frontline cosmology, it is critical to attack this problem from every possible angle. We present results from our study of three known historical Ia supernovae in the Large Magellanic Cloud (LMC) which allow us to begin to eliminate possible progenitor candidates for at least the local population. We used archival Hubble Space Telescope images of SNR 0509-67.5, SNR 0509-68.7, and SNR 0519-69.0 to determine the site of each explosion and then search the surrounding area for potential ex-companion stars left behind. The search was carried out within an error ellipse that accounts for measurement error on the geometric center of the remnant, the orbital velocity of the pre-supernova binary system, and kicks from the actual explosion. For SNR 0509-67.5, the error ellipse is empty to the HST 5-sigma limiting magnitude of V=26.9. Using an LMC distance modulus of 18.5, this implies that any single degenerate ex-companion must be fainter than M_V=+8.4 (corresponding approximately to a K9 main sequence star), which eliminates all currently-published single-degenerate models, and leaves us with the conclusion that this system had a double-degenerate progenitor. For SNR 0509-68.7 and SNR 0519-69.0, we can eliminate the possibility of red giant and subgiant ex-companions. It has been shown that the two confident galactic Ia supernovae (Tycho's SN 1572 and SN 1006) also do not have red giant ex-companion stars. Combined with our three systems, this eliminates the symbiotic progenitor channel for all of the nearby Ia supernovae.

Wystan Benbow (HEAD Hardware and Advance Projects) in Pratt at 12:30 on 19 October 2011

Status and Plans for the Next-generation VHE Gamma-ray Observatory

Over the past decade, tremendous progress has been made in the field of very high energy (VHE; E > 100 GeV) gamma-ray astronomy . More than 130 VHE gamma-ray sources, from more than a dozen source classes, are now known to exist. The current sensitivity and angular resolution is also such that detailed temporal, spectral and morphological studies can be performed. A next-generation VHE gamma-ray observatory, the Cherenkov Telescope Array (CTA), was endorsed by the Astro-2010 decadal survey and is currently in the R&D phase. Compared to current VHE observatories, CTA will have an order of magnitude increase in sensitivity and a factor of 3 increase in angular resolution. Observations with CTA will likely begin later this decade. The status and plans for CTA will be discussed.

Brandon Kelly (CfA) in Pratt at 12:30 on 26 October 2011

Modeling AGN Variability

Current and future time-domain surveys will provide new insights into the physics of many astronomical objects, providing complementary and orthogonal information to spectroscopy. For AGN, variability probes the structure and physics of the accretion disk. In particular, the scaling of variability properties with black hole mass constrains accretion flow solutions, and provides insight into the similarities and differences between supermassive and stellar-mass black holes. In addition, variability provides an important criteria for selecting samples of AGN. Unfortunately, AGN lightcurves often suffer from a number of problems, including irregular and sparse sampling as well as additional variability due to measurement noise. These issues significantly complicate the analysis and quantification of AGN variability. In this talk I will discuss recently developed methods for modeling AGN variability in the time domain which provide reliable and powerful ways of quantifying AGN variability. In addition, I will present recent results obtained from applying our methods to optical and X-ray lightcurves of AGN, including scaling relationships between AGN variability properties and black hole mass or Eddington ratio. Finally, I will discuss ongoing and future work on extending and improving our variability modeling. Although in this talk I focus on AGN, the methods I will discuss are also applicable to studies of variability of accretion flows around other compact objects and protostars.

Daniel Patnaude (CfA) in Pratt at 12:30 on 2 November 2011

Stellar Forensics: X-ray Observations of Supernova Remnants Provide Clues to How a Star Lives and Dies

The X-ray emission from supernovae and their remnants reveals a great deal about the progenitor's immediate circumstellar environment, the supernova explosion mechanism, and quite possibly the structure of the progenitor prior to the explosion. Additionally, by studying the X-ray emission from the strong shocks found in SNRs, we can learn about the evolution of the ejecta, the mass loss history of the progenitor, and how particles are accelerated to the highest energies. In this talk I will discuss some recent results from Chandra observations of young Galactic and extra-Galactic SNRs. First I will discuss what multi-epoch observations of Cassiopeia A teach us about particle acceleration in strong shocks. Additionally, I will show how the SNR reverse shock probes the ejecta and provides clues concerning the progenitor structure. Then I will discuss what the bulk properties of the X-ray emission from Kepler's SNR are telling us about the progenitor system of this unusual Type Ia SNR. Finally, I will present the latest data on SN 1979C, a young Type IIL SN in M100. I will discuss implications for its constant X-ray emission and compare these data to models for X-ray emission from SNe, pulsar wind nebulae, and accreting sources.

Edison Liang (Rice University) in Pratt at 15:00 on 7 November 2011 (Monday)

From picosecond to Gigaparsec: High Energy Astrophysics Experiments using Ultra-intense Lasers

The proliferation of short-pulse (picoseconds), ultra-intense (Petawatt) lasers provides unprecedented opportunities for studying high-energy astrophysics phenomena in the laboratory setting. From pulsar winds to blazar jets, from microquasars to gamma-ray bursts, relativistic plasmas have heretofore been observed only remotely through their radiation, or studied on computers. Now relativistic plasmas created by ultra-intense lasers can be used to simulate many aspects of these exotic phenomena, and astrophysicists can begin to test and calibrate their simulation codes using laboratory experiments. In this talk I will review how the science of high-energy astrophysics intersects the capabilities of ultra-intense lasers, and explore various plasma astrophysics processes that may be studied experimentally in the coming decade.

Xiaojie Xu (UMass, Amherst) in Pratt at 12:30 on 9 November 2011

Demography of Stellar X-ray Sources in the Galactic Bulge and Globular Clusters

We have studied the accumulated X-ray spectra and luminosity functions (LFs)of X-ray sources in four Galactic globular clusters (GCs, including 47 Tuc, NGC 6266, NGC 6397 and omega-Cen) and a field toward Galactic Bulge (GB). These stellar systems have deep observations with Chandra X-ray Observatory, allowing for statistical analysis with good source counting statistics. We conduct the analysis down to the detection thresholds of existing Chandra observations. The spectra can be characterized with a two temperature optically thin thermal plasma. While the spectrum of the GB sources shows a significant 6.7 keV line, those of the GCs do not, apparently due to their low Fe abundances. In the analysis of LFs, by accounting for the detection incompleteness and Eddington bias as well as the background AGN contamination. The LF of the GB sources is consistent with that of the field, whereas those of the GCs appear substantially flatter. This flattening of LFs is correlated with the increasing encounter rates, suggesting a dynamical origin for many of the X-ray sources in the GCs. We further find that the number of relatively X-ray bright/faint sources per stellar mass in the GCs is greater/smaller than in the field. We will discuss the possible scenarios as well.

Mathieu Servillat (CfA) in Pratt at 12:30 on 16 November 2011

Rare objects in Globular Clusters: from dwarf novae to intermediate mass black holes

Globular clusters are old, dense stellar systems which harbor an excess of X-ray sources compared to the number of X-ray sources per stellar mass in the Galactic plane. Those are mainly close binaries, often hosting a compact object, which are connected to the dynamical evolution of the cluster. Some globular clusters are also believed to host a central black hole of intermediate mass (10^2-10^5 solar masses) which could be the missing link between stellar-mass and supermassive black holes. First I will discuss the specificities of cataclysmic variables in globular clusters through the multi-wavelength study of X-ray sources in several Galactic globular clusters. This study highlights the need for multi-wavelength and variability surveys of the Galactic plane to better characterize the faint and quiet end of the reference population. This is one of the aim of the Chandra Multi-wavelength Plane Survey (ChaMPlane). Also, the 100 yr coverage of the DASCH project (digitizing photographic plates at Harvard) could bring new insights to the field. In a second part, I will focus on the study of the ultra-luminous X-ray source ESO 243-49 HLX-1 with >10^42 erg/s, the best intermediate mass black hole candidate currently known. The source was observed in two main, well defined X-ray spectral states that are consistent with the thermal and the hard states, reminiscent of Galactic stellar-mass black hole binaries. Both beamed emission and super-accretion are thus unlikely, leading to a constraint of the mass of >9000 Msun. I will then discuss the newly obtained spectral energy distribution with HST and Swift XRT which suggests the presence of 10^7-10^8 yr stellar population of 10^6 Msun surrounding the black hole.

Nirupam Roy (NRAO) in Pratt at 12:30 on 23 November 2011

Small scale structures and turbulence in the atomic ISM

The observed scale-free intensity fluctuations from pc to AU scale is believed to be the signature of turbulence in the interstellar medium. In this talk, I will present results from a variety of observational techniques that are used to study the small scale structures of the Galactic atomic ISM. In this context, I will discuss the importance of the small scale physical processes in the ISM, and the implications of these observational results for our current understanding of the multiphase nature and the turbulence in the ISM.

Phil Charles (South African Astronomical Observatory) in Phillips Auditorium at 14:00 on 29 November 2011 (Tuesday)

The Completion of SALT and First Science Results

Following the repair of the SALT optics in late 2010, the telescope re-entered the commissioning phase with its principal scientific instruments, SALTICAM (imager) and RSS (multi-function spectrograph). I will describe their current status and that of the telescope, together with an update on its performance, which is now much closer to specification, thereby finally demonstrating the potential of this new paradigm in very large telescope design.

Richard Plotkin (University of Amsterdam) in Pratt at 12:30 on 30 November 2011

Black Hole Unification Across the Mass Scale

Radiative and mechanical outflows from accreting black holes (i.e., winds/jets) interact with their large-scale environments, with important implications for many fields of astrophysics. Nature provides a very diverse set of black holes to study, from stellar mass black holes in binary systems to supermassive black holes driving Active Galactic Nuclei (AGN). In this talk I show how statistical studies comparing black holes across the entire mass scale can afford a more holistic view of black hole accretion. In particular, the fundamental plane of black hole activity - a relation connecting hard state black holes and their supermassive analogs - suggests that black holes regulate their output similarly across the entire mass scale. I will show how the fundamental plane can be exploited as an effective tool to answer the hotly debated question: what is the dominant X-ray emission process in weakly accreting black holes? Along the way I use low-luminosity blazars (i.e., BL Lacs) to highlight some important systematic challenges to fitting the fundamental plane. I also briefly show that, at odds with simple AGN unification, BL Lac dusty tori may be intrinsically weak or altogether missing. I will conclude by discussing how this result may be expected from black hole mass scaling, and I will also discuss implications on the `nature vs. nurture' debate for radio galaxies and AGN unification in general.

Jiangtao Li (UMass, Amherst) in Pratt at 14:30 on 6 December 2011 (Tuesday)

Chandra Survey of galactic coronae around nearby edge-on disk galaxies

The X-ray emitting coronae in nearby galaxies are expected to be produced either by accretion from the IGM or by various galactic feedbacks. It is known that the total hot gas luminosity of these galaxies correlates with stellar mass for early-type galaxies and with SFR for active star forming galaxies. However, such relations always have large scatter, indicating various other processes must be involved in regulating the coronal properties. We conduct a systematical analysis of the Chandra data of 53 nearby edge-on disk galaxies. The data are reduced in a uniform manner. Various coronal properties, such as the luminosity, temperature, emission measure, vertical and horizontal extension, and other derived parameters, are characterized for most of the sample galaxies. For some galaxies with high enough counting statistics, we also study the thermal and chemical states of the coronal gas. We then compare these hot gas properties to other galactic properties to further study the role of different processes in producing and/or maintaining the coronae. The soft X-ray luminosity of the coronae generally correlates well with the SFR or the total SN (core collapse and Type Ia) energy input rate over more than 3 orders of magnitude in Lx. But the X-ray radiation efficiency only has a median value of 0.5%, and significantly correlates with the gravitation-to-stellar mass ratio. Including this ratio in the fit could significantly improve the SFR-Lx or E_SN-Lx correlations. We also find evidence for the effects of gravitation, environment, and cold-hot gas interaction in regulating the coronal properties.

Petri Savolainen (CfA) in Pratt at 12:30 on 7 December 2011

A year of X-ray and radio observations on Cygnus X-3

I present results from my modeling of Swift and RXTE X-ray spectra of Cygnus X-3, taken during April 2010 - April 2011, with some multiwavelength context. The model includes a Comptonized disk blackbody, interstellar and local absorption, various line features, and an occasional very soft (~ 50 eV) component. During the first 9 months, the spectrum remained relatively hard as the source moved between quiescence and minor flaring. The last quarter saw a decay in the hard X-rays, punctuated by a curious soft X-ray / gamma-ray flare, leading to a month of radio / hard X-ray quenching. As expected, Cygnus X-3 emerged from this period with a major radio / hard X-ray flare, followed by a decay back to quiescence.

Beike Hiemstra (Kapteyn Institute) in Phillips Auditorium at 12:30 on 9 December 2011 (Friday)

Snapshots of low-mass X-ray binaries: reflection signatures and spectral states

The emission from low-mass X-ray binaries is known to vary a lot, exhibiting different spectral and timing properties which characterizes the different spectral states. According to the standard accretion disc scenario the disc is cold and truncated at large radii in the hard state, whereas the disc temperature increases and its inner radius moves inwards in the transition to the soft state, towards a disc extending down to the last stable orbit in the soft state. My talk will be a triptych in which I walk through the different spectral states of low-mass X-ray binaries thereby addressing some of the (sometimes peculiar) characteristics seen in the individual states. It will include: (i) the discussion on whether the accretion disc in the hard state of black hole candidates is truncated or not, (ii) the results on a very strong (one of the strongest ever seen) and broad Fe emission line in the intermediate state of a black hole candidate, and (iii) the investigation on how the corona and the neutron star surface contribute to the Fe line and reflection continuum in different spectral states.

Francesco Massaro (Stanford University) in Pratt at 12:30 on 14 December 2011

Unidentified gamma-ray sources: a WISE method to hunt gamma-ray blazars

One of the main scientific objectives of the recent Fermi mission is unveiling the nature of the unidentified gamma-ray sources (UGSs). Despite the large improvements of Fermi in the gamma-ray source localization with respect to the past gamma-ray missions, about 1/3 of the gamma-ray objects detected still do not have a low energy counterpart associated. Recently, we discovered that blazars, the rarest and the most gamma-ray detected class of Active Galactic Nuclei (AGNs), can be recognized and separated from other extragalactic sources dominated by thermal emission using the IR colors. Consequently, we built an association method for the gamma-ray sources able to recognize if there is a blazar candidate within the positional error region of a generic gamma-ray source. Here I will present a parametrization of the region in the IR color parameter space occupied by the gamma-ray emitting blazars. Then, adopting this parametrization to verify if the gamma-ray AGNs of the uncertain type detected by Fermi are consistent with being blazar candidates. Finally, applying this tool to the unidentified gamma-ray sources I will show, for the first time, that we are also able to provide a candidate counterpart for 184 out of 313 sources analyzed.

Manasvita Joshi ( Boston University ) in Pratt at 12:30 on 18 January 2012

Time-dependent Multi-zone Radiation Transfer Model For Blazar Jets

Blazars form the most extreme class of Active Galactic Nuclei (AGN) known to mankind. They are highly variable at all wavelengths, on time scales of months, to a few days, to even less than an hour in some cases. Simultaneous multiwavelength studies of blazars are extremely important to understand the nature of physical processes taking place in such objects. Here, I will discuss my work on internal shock model to explore the time-dependent evolution of radiation in a blazar jet using a multi-zone radiation feedback scheme. I will talk about the role of intrinsic parameters and the interplay between synchrotron and inverse Compton radiation processes responsible for producing the resultant spectral energy distribution. I will finish with a brief synopsis of my work on multi-waveband spectral analyses of the blazar 3C279, which is currently underway, using the above-mentioned leptonic jet model.

James Aird ( UCSD ) in Pratt at 12:30 on 25 January 2012

The incidence of AGN is independent of host stellar mass

I will present evidence that the incidence of Active Galactic Nuclei (AGN), when defined in terms of Eddington ratio, is independent of the stellar mass of the host galaxy. We use X-ray data from three extragalactic fields (XMM-LSS, COSMOS and ELAIS-S1) to identify AGN within PRIMUS, a low-resolution spectroscopic survey of >120,000 galaxies to z=1.2. We find that the Eddington ratio distribution for moderate-luminosity AGN is a universal function, exhibiting a power-law distribution with a slope of -0.6, over a wide range of galaxy stellar masses. We also find that the AGN fraction strongly increases with redshift to z~1, and is weakly enhanced (factor ~2) in galaxies with blue or green optical colors. AGN activity and star-formation are correlated, but we do not find evidence that the presence of an AGN (and therefore AGN feedback) is related to the quenching of star-formation or the color transformation of galaxies. Indeed, our results show that AGN are found in galaxies of all stellar masses and colors, and are not predominantly in red, passively-evolving, massive galaxies - while a higher fraction of AGN may be observed in such galaxies, this is purely a selection effect related to the underlying Eddington-ratio distribution.

Dharam Vir Lal ( CFA ) in Pratt at 12:30 on 1 February 2012

3C449 : Our understanding of the source from radio and X-ray observations

Groups and clusters commonly contain radio galaxies, which eject large amounts of energy into their external environments. Its influence is to regulate gas cooling and galaxy evolution by an amount, which depends on energy ejected, mixing, and entrainment. We present results from a deep Chandra observations of 3C449, a canonical FR I radio galaxy to study the large scale gas environment in a nearby (z=0.017) 1.5 keV group. We will (i) first present our understanding of the source purely from the radio data, add to it the latest Chandra X-ray data, and (ii) present its implications along with our ongoing effort to understand the nature of this source and the associated gas.

Saeqa (Saku) Vrtilek ( CFA ) in Pratt at 12:30 on 8 February 2012

Putting black holes in their proper place

The diverse behaviors displayed by X-ray binaries make it difficult to determine the nature of the underlying compact objects. In particular, identification of systems containing black holes is currently considered robust only if a dynamical mass ratio is obtained. However, we have recently developed a model-independent means of identifying the central bodies --- neutron star or black hole --- of most accreting binary systems. We find that different categories of object (black holes, pulsars, and non-pulsing neutron stars) occupy distinct regions in a 3-dimensional color-color-intensity (CCI) diagram. Assuming that this clustering effect is due to intrinsic properties of the sources (such as mass accretion rate, binary separation, mass ratio, magnetic field strength, etc.), we suggest possible physics that drives each object to its specific location in the CCI phase space. We also suggest a surface in this space which separates systems that produce jets from those which do not, and the use of CCI for identifying X-ray pulsars prior to establishing a period.

Jifeng Liu ( CFA ) in Pratt at 12:30 on 10 February 2012 (Friday)

Black holes in the Milky Way and nearby galaxies

Black holes are the inevitable prediction of Einstein's gravitational theory, and are expected in the astrophysical settings. Astronomical observations have already revealed, by studying their companions, stellar black holes of a few (tens of) solar masses as end products of massive stars and supermassive massive black holes above several millions of solar masses in the centers of galaxies. In this talk, I will first describe some studies on stellar black holes, including measuring the distorted space-time around the black holes through their spin, and my newly assembled project to find stellar black holes in quiescence combining the power of GALEX, SDSS and China's LAMOST sky surveys. I will then describe the efforts, including my own, to study ultraluminous X-ray sources in search of intermediate mass black holes of a few thousands of solar masses, the ``missing'' link between the stellar black holes and the supermassive black holes.

Andrea Marinucci ( CFA ) in Pratt at 12:30 on 22 February 2012

The X-ray reflector in NGC 4945: a time and space resolved portrait

We present a time, spectral and imaging analysis of the X-ray reflector in NGC 4945, which reveals its geometrical and physical structure with unprecedented detail. NGC4945 hosts one of the brightest AGN in the sky above 10 keV, but it is only visible through its reflected/scattered emission below 10 keV, due to absorption by a column density of 4 E+14 cm^2. A new Suzaku campaign of 5 observations spanning 6 months, together with past XMM-Newton and Chandra observations, show a remarkable constancy (within 10%) of the reflected component. Instead, Swift-BAT reveals strong intrinsic variability on time scales longer than one year. Modeling the circumnuclear gas as a thin cylinder with the axis on the plane of the sky, we show that the reflector is at a distance greater than 30-50 pc, well within the imaging capabilities of Chandra at the distance of NGC 4945 (1'' ~18 pc). Accordingly, the Chandra imaging reveals a resolved, flattened, 150 pc-long clumpy structure, whose spectrum is fully due to cold reflection of the primary AGN emission. The clumpiness may explain the small covering factor derived from the spectral and variability properties.

Mike Eracleous ( Penn State University ) in Pratt at 12:30 on 7 March 2012

THE QUEST FOR THE DYNAMICAL SIGNATURE OF CLOSE SUPERMASSIVE BINARY BLACK HOLES

Supermassive binary black holes (masses of order 1 million to 1 billion solar masses and separations less than 1 pc) are predicted to be an inevitable late stage in the evolution of galaxy mergers. Such binaries have also been invoked as explanations of the formation of the cores of elliptical galaxies following a merger and the mass deficits therein, the apparent precession of radio jets, and the formation of X-shaped radio sources and they are predicted to be prime sources of gravitational waves. Yet, they remain elusive. After a historical introduction, I will describe a systematic search for such objects using the SDSS spectroscopic database and the followup observations of the initial candidates. I will present the first results from this search and discuss critical tests of the methodology as well as plans for future work.

Andy Goulding ( CFA) in Pratt at 12:30 on 21 March 2012

The Evolution of AGN and their Host Galaxies to z~1 in Wide-field Multi-wavelength Surveys

Galaxy properties (i.e., luminosity, color, morphology, star-formation history) are known to evolve strongly with time. The redshift z~0.5-1.5 is believed to be a crucial epoch: (1) galaxies are evolving strongly as a function of stellar mass; (2) AGN activity is prevalent; (3) massive clusters are forming and (4) the red sequence is becoming established. To unambiguously determine the dominant physical processes which are driving the growth and evolution of galaxies and their central black holes at z~1 requires sensitive multi-wavelength wide-field surveys. We have combined the AGN identified in sensitive Chandra ACIS-I X-ray imaging (~3000 sources; Goulding et al. 2012a), Spitzer IRAC infrared photometry (~4800 sources), and FIRST and NVSS radio data (~700 sources) with the Keck/DEIMOS catalog of ~49,600 optical spectroscopic galaxies in the combined 3.2 deg^2 DEEP2 fields. Using this extensive suite of multi-wavelength data, we have identified ~2100 DEEP2 galaxies at z~0.8-1.4, which have signatures of X-ray, IR and/or radio-bright AGN. By comparing the properties of AGN in DEEP2 at z~0.8-1.4 to those of AGN in BOOTES at z~0.3-0.8, we place new direct wavelength-independent constraints on the evolution of AGN hosts. We find that whilst there is clear evidence for mass/luminosity downsizing from z~1 to the present day, there appears to be no evolution in color or morphology at this epoch, regardless of AGN accretion mode. However, from X-ray stacking analyses of IR AGN and star-forming galaxies, we find strong evidence for a large population of gas-rich (star-forming) obscured AGN which are formally undetected in the deep X-ray imaging. Taken together, this provides further indication that dust/gas rich systems may play a crucial role in galaxy evolution and the build-up of the red-sequence.

Robin Barnard ( CFA) in Pratt at 12:30 on 28 March 2012

Playing Black Hole Peek-a-Boo in the Andromeda Galaxy

I present results from a ~12 year Chandra monitoring campaign that covers the central region of M31, looking for X-ray transients. These are likely to be black hole binaries, fed by low mass stars. We followed up 12 promising transients with HST observations, identifying B band counterparts for 4 transients via difference imaging, and 4 sigma upper limits of B ~26--29 for the other 8. We derive orbital periods for these systems from empirical relations between the X-ray and optical luminosities, and use survival analysis techniques to compare our period distribution with that of Galactic low mass black hole binaries.

Jan Egedal ( MIT ) in Pratt at 12:30 on 4 April 2012

Magnetic Reconnection in Plasmas; a Celestial Phenomenon in the Laboratory

Coronal mass ejections from the sun are the most explosive events that occur in our solar system. Closer to home, the aurora borealis is one of the most spectacular, naturally occurring, light show at high latitudes on the earth. Both of these large scale events are driven by magnetic reconnection in plasmas. The spontaneous rearrangement of magnetic field topology provides the For more than fifty years, magnetic reconnection has been a fascinating topic of research in plasma physics. While we do not fully understand the process of reconnection, significant progress has been made in the past decade through detailed analysis of laboratory experiments, and computer simulations. The Versatile Toroidal Facility at MIT is one such experiment dedicated to the study of magnetic reconnection. In this talk I will describe the recent experimental observations which have led to a new theoretical paradigm for magnetic reconnection. Large scale computer simulations support the theoretical and experimental results. The analysis of experimental observations in a laboratory device has led to a comprehensive understanding of data from spacecraft observing celestial reconnection events in the earth's magnetosphere. The theory may also be applicable to reconnection events on the sun.

(1) Samuel Connolly, (2) Charith Peris ( CFA ) in Pratt at 12:30 on 11 April 2012

(1) A shrinking disk around V926Sco? (2) Tomographic images of V691 Cra: Spiral structure in the disk?

(1) We present phase-resolved spectroscopic observations of the low-mass X-ray binary V926Sco (4U 1735-444), collected using the Walter Baade 6.5 m Magellan telescope at the Las Campanas Observatory. The spectrum of V926Sco includes lines of H and He, as well as the Bowen blend of N III and CIII fluorescence. Modulation tomograms using H$\alpha$ and HeII show that the inner disk velocities are significantly lower than those observed in black holes in quiescence. The velocities of this neutron star system are also somewhat lower than those seen in previous observations of V926Sco (made in 2003), suggesting that the inner radius of the disk has expanded since these observations. This is consistent with RXTE/ASM light curves of the object, which show that it has become less bright during this time. In H$\alphs$, we see emission concentrated on one side of the disk similar to that observed in previous observations made in HeII, interpreted as due to an extended disk bulge. We also find that the centre of the disk is offset from the neutron star's center of mass; we can correct for this effect by adjusting the mass ratio towards the lower end of the 0.05 -0.41 range previously suggested. (2) We present Doppler and modulation tomography of the low-mass X-ray binary V691 CrA with data obtained using the 6.5-m Magellan Baade telescope at the Las Campanas Observatory in 2010 and 2011. Although the system is in quiescence during our observations, the disk and hotspot are observed in Ha and He II ($\lambda$4686) in both 2010 and 2011. A clear image of the disk and the hot spot is also seen in He II ($\lambda$5411) using the 2010 data. We observe the secondary in the N III ($\lambda$4640) Bowen Fluorescence emission line consistent with earlier results as well as in Ha. We also detect He I ($\lambda$5876) in absorption from the leading side of the inner face of the companion star. The Ha double peaks are imbedded in a deep trough suggesting the presence of absorbing material in the system.

Tim Roberts ( Durham University (UK) ) in Pratt at 12:30 on 13 April 2012 (Friday)

New adventures in ultraluminous X-ray sources

It is now commonly accepted that the majority of ultraluminous X-ray sources (ULXs) are powered by the accretion of matter onto a stellar remnant black hole, although in some cases these black holes may still be several times larger than those we have observed in our own Galaxy. This means most ULXs are accreting at or above the Eddington limit, opening up a new observational window on the most extreme accretion processes, with potential implications for many areas of astrophysics. In this talk I will concentrate on two topics we have been investigating recently. Firstly, I will look at data from the nearest ULXs, with luminosities around 10^39 erg/s, and discuss what this reveals about the transition between sub- and super-Eddington accretion. Secondly, I will present data from an observational study of a small sample of the most extreme ULXs extracted from the 2XMM catalogue (with observed luminosities in excess of 5 x 10^40 erg/s), and discuss whether their properties are consistent with less luminous ULXs, or whether these brightest objects may instead be powered by accretion onto intermediate-mass black holes.

Eilat Glikman ( Yale ) in Pratt at 12:30 on 18 April 2012

The Reddest Quasars: A Transitional Phase in Quasar/Galaxy Co-Evolution

Quasars are extremely luminous sources, powered by accretion of gas onto a supermassive black hole in the nucleus of some galaxies. Most of the >100,000 quasars identified in the literature have been identified in optical surveys through the "ultraviolet excess" (UVX) method. However, these samples are known to be incomplete and biased because of obscuration and anisotropic radiation. To overcome some of these biases and search for candidate obscured quasars, we matched radio sources from the FIRST 1.4 GHz survey, sensitive to 1 mJy, with the 2MASS near-infrared survey and selected objects with red optical-to-near-infrared colors. We followed up our candidates with optical and/or near-infrared spectroscopy and identified 119 dust-reddened quasars, defined as having at least one broad line and a reddening of E(B-V)> 0.1. The sample spans a wide redshift range, 0.1 < z < 3 and reaches a reddening, E(B-V) < 1.5. When corrected for extinction, red quasars are the most luminous objects at every redshift and the fraction of red quasars increases with luminosity. The properties of red quasars suggest that they are revealing an emergent phase where the heavily obscured quasar is emerging from its dusty environment prior to becoming a "normal" blue quasar. We compute the fraction of quasars that are in this red phase and determine that its duration is 20% as long as the unobscured quasars phase: a few million years.

Vincenzo Cotroneo ( CFA ) in Pratt at 12:30 on 25 April 2012

X-ray adjustable optics at CfA

The present generation of X-ray telescopes emphasizes either high image quality (e.g. Chandra with sub-arc second resolution) or large effective area (e.g. XMM-Newton ), while future observatories under consideration (e.g. Athena, AXSIO ) point to greatly enhance the effective area, maintaining moderate (5-10 arc-seconds) image quality. The key point is that the image quality is directly driven by the surface and figure quality of the mirror. At the same time, the effective area is limited by the mirror wall obstructions and by the number of shells. Thin mirror shells can be densely nested resulting in a high effective area, but are not stiff enough to be realized and maintained with an accurate figure. Thick shells on the opposite, can be accurately shaped to give high angular resolution, but have too high a mass (and unacceptable cost) to be used for the realization of a high collecting area telescope. A possible solution is the realization of deformable thin mirrors, whose intrinsic figure errors can be corrected after manufacturing, to obtain high resolution optics with large effective area. The approach under development at CfA is based on thin piezoelectric films deposited directly on the substrate. This approach offers virtually no obstruction and a high scalability, making it possible the realization of optics with subarsecond resolution and large effective area.

Bozena Czerny ( Copernicus Astronomical Center ) in Pratt at 12:30 on 2 May 2012

Dust origin of the BLR

Joanna Holt ( Universiteit Leiden ) in Pratt at 12:30 on 9 May 2012

CANCELLED

Markos Trichas ( CFA ) in Pratt at 12:30 on 16 May 2012

CANCELLED DUE TO FIRE ALARM/ BRIGADE

Elena Rasia ( Univ of Michigan, Ann Arbor ) in Pratt at 12:30 on 23 May 2012

Lensing and X-ray: masses and concentrations of galaxy clusters.

I will briefly review our recent works on lensing and X-ray estimates of 1) mass and 2) concentration. Our first project is based on 20 simulated massive clusters from which we created Chandra X-ray and Subaru SuprimeCam images using our tools: X-MAS and Skylens. The 'observational' mass derived from these synthetic catalogs is compared with the input value of the simulations. Weak lensing masses resulted underestimated by 5-10% due to presence of substructure and triaxiality. The X-ray masses are more underestimated (25-35%) because of lack of hydrostatic equilibrium and temperature inhomogeneity. I will comment on the significance of these results relating to previous work and observations. The second project investigates the concentration-mass relation and its dependence on the radial range used for the fitting and on the baryonic physics of the Intra Cluster medium (ICM). ~50 objects simulated with four different ICM physics are considered. The results are compared with recent observational efforts.

Thomas Bisbas ( University College London and CFA ) in Pratt at 12:30 on 30 May 2012

3D-PDR: A new three-dimensional radiative transfer and astrochemistry code for treating Photodissociation Regions

In this seminar we will discuss on numerical modelling of photodissociation regions (PDRs). PDRs define the transition zone between an ionized and a dark molecular region; they consist of neutral gas which interacts with far-ultraviolet radiation and are characterized by strong infrared line emission. In particular we will present the very recently implemented code 3D-PDR which is able to simulate three-dimensional PDRs of arbitrary density distribution. Using Smoothed Particle Hydrodynamic simulations we are able to make realistic initial conditions which are used in order to obtain synthetic emission maps showing the chemical structure of PDRs.

Ignazio Pillitteri ( SAO ) in Classroom (A101) at 12:30 on 13 June 2012

The search of Star-Planet Interaction at high energies

More than 750 exo-planets have been discovered to date. Of particular interest are those close-in planets transiting their host stars at short distance (hot Jupiters). They allow estimate of the mass, density, radius, chemical composition of the atmosphere, and thermal mapping of their surfaces. The search for effects of star-planet interaction (SPI) on the host star has given controversial results, with some systems showing evidence of activity variation phased with the planetary motion. We present the results of the search of SPI in X-ray band, focusing in particular on the well studied system of HD 189733. This system with a hot Jupiter at only 0.03 AU from the companion star shows features in X-rays that are well explained by SPI effects. We discovered that the hot Jupiter of HD 189733 has modified the evolution of the angular momentum of the parent star, resulting in a fast rotating old star with enhanced X-ray and chromospheric activity. High X-ray resolution spectroscopy shows that the corona of this star is cold and dense, and similar to those of active PMS stars. From three observation in a time span of 2012 at the secondary transit, we see that the X-ray time variability is enhanced after the planet's eclipse, and likely it is phased with the planet motion.

Constanza Argiroffi ( Palermo ) in Classroom (A101) at 12:30 on 28 June 2012 (Thursday)

Rotationally modulated X-ray emission from accretion shocks in young stars

Young stars are known to be intense X-ray sources, with X-rays mainly emitted by coronal plasmas. However it is still debated the nature of the high density plasma producing soft X-rays in young accreting stars. In this talk I present results obtained from coordinated quasi-simultaneous optical and X-ray observing campaigns of two young accreting stars, V2129 Oph and V4046 Sgr. These observations aimed at looking for phase-resolved X-ray signatures of shock-heated plasma. V2129 Oph and V4046 Sgr were observed with Chandra/HETGS (200 ks) and XMM-Newton/RGS (360 ks), respectively. Their stellar photospheres, magnetic fields, and accretion geometries were constrained by quasi-simultaneous optical monitoring (photometry, spectroscopy, and spectropolarimetry). In both stars soft X-rays, produced by the high density cool plasma component, appear to vary with stellar rotation. In particular, in V4046 Sgr rotational modulation is clearly detected: X-ray emission lines produced by cool plasma display periodic flux variations, with a period that is precisely half the stellar rotational period. These results can be interpreted in terms of X-rays emitted in the accretion shock, therefore from small portions of the stellar surfaces not azimuthally symmetrically distributed with respect to the stellar rotational axis. In this scenario the observed variability is due to the different viewing angles of the accretion shock at different rotational phases. These results strongly support models in which, in young stars, the high-density X-ray-emitting plasma is material heated in accretion shocks located at the base of accretion flows.

Hans Böhringer ( MPE ) in Classroom (A101) at 14:30 on 3 July 2012 (Tuesday)

X-ray Galaxy Clusters as Cosmological and Astrophysical Probes

Galaxy clusters form an integral part of the large-scale structure of the Universe. Therefore a census of clusters can provide quite precise insight into the cosmic large-scale structure and can be used to test cosmological models. X-ray observations provide us with the most detailed information on galaxy cluster structure and on various physical parameters of clusters and they offer a very good means to detect galaxy clusters. Based on X-ray surveys of galaxy clusters and detailed follow-up observations I give a review on what we have learned on the structure of galaxy clusters and its statistics. I also show how the mass or X-ray luminosity function of clusters and their spatial distribution can be used to assess the large-scale structure of the dark matter distribution and how this assessment can be used to test cosmological models. Results from the cluster population detected in the ROSAT All-Sky Survey provide very good constraints on the matter density and the normalization of the power spectrum of the dark matter density fluctuations on large scales. The upcoming eROSITA mission will greatly improve the capabilities to use a large sample of up to 100 000 X-ray detected galaxy clusters for these test of cosmological models.

Lennart van Haaften (Nijmegen University ) in Classroom (A101) at 12:30 on 5 July 2012 (Thursday)

Evolution, observations and populations of ultracompact X-ray binaries

Ultracompact X-ray binaries (UCXBs) are low-mass X-ray binaries with an orbital period below circa 80 minutes. They consist of a neutron star or black hole that accretes hydrogen-poor matter from a (semi-)degenerate companion. Angular momentum loss via gravitational wave radiation drives mass transfer via Roche-lobe overflow. UCXBs are important for the study of hydrogen-deficient accretion and X-ray bursts, they are potentially detectable gravitational wave emitters, and represent an outcome of complex binary evolution including one or two common envelope stages. I discuss the main evolutionary stages of UCXB evolution with emphasis on old, long-period systems. Using binary population synthesis, UCXB tracks and long-term UCXB variability observations, I model the orbital periods, luminosities and donor chemical compositions of the observable UCXB population in the Galactic Bulge. The high number of old systems predicted, combined with the fact that all 13 known UCXBs are relatively young, raises questions about what happens to old UCXBs. They may become disrupted at some point, be in quiescence for most of the time, or simply too faint to see. I investigate possible explanations involving the thermal-viscous disk instability, the magnetic propeller effect and donor evaporation.

Zhongli Zhang ( MPA, Germany ) in Classroom (A101) at 12:30 on 13 July 2012 (Friday)

Study of populations of low-mass X-ray binaries in elliptical galaxies

We studied populations of low-mass X-ray binaries (LMXBs) in elliptical galaxies, focusing on their dependence on the stellar environment. This includes the dependence on the stellar density (globular clusters .vs. field .vs. galactic nuclei), velocity dispersion (globular clusters. vs. galactic nuclei), and the stellar age (young and old ellipticals). LMXBs show different luminosity function shapes in different environments, indicating their different formation histories. We also found LMXBs are more extended than the stellar distribution. The LMXBs in the galaxy outskirt are assumed to have two origins: GC-LMXBs from GCs in the outside and supernova kicked LMXBs from the galaxy inside.

Andrea Prestwich ( CFA ) in Classroom (A101) at 12:30 on 18 July 2012

Do ULX form in metal-poor gas?

Ultra-Luminous X-ray sources (ULX) are X-ray binaries with L$_x$ $>10^{39}$ ergs s$^{-1}$. The most spectacular examples of ULX occur in starburst galaxies and are now understood to be young, luminous High Mass X-ray Binaries. The conditions under which ULX form are poorly understood, but recent evidence suggests they may be more common in low metallicity systems. Here we investigate the hypothesis that ULX form preferentially in low metallicity galaxies by searching for ULX in a sample of Extremely Metal Poor Galaxies (XMPG) observed with the Chandra X-ray Observatory (CXO). XMPG are defined as galaxies with $(O/H)+12<7.65$, or less than 5\% solar. These are the most metal-deficient galaxies known, and a logical place to find ULX if they favor metal poor systems. We compare the number of ULX per unit of star formation (\nulx) in the XMPG sample with \nulx\ in a comparison sample of galaxies with higher metallicities taken from the Sings Infrared Galaxy Sample (SINGS). We find that ULX occur preferentially in the low metallicity sample, but the formal statistical significance of the excess is low (2.7$\sigma$). We discuss the implications of these results for the formation of black holes in low metallicity gas.

Mario Guarcello ( CFA ) in Classroom (A101) at 12:30 on 25 July 2012

X-ray emission from young stellar clusters: a study of NGC6611 and the Eagle Nebula

Mechanisms regulating the origin of X-rays in young stellar objects and the correlation with their evolutionary stage are under debate. Studies of the X-ray properties in young clusters allow us to understand these mechanisms. I will present the X-ray study, based on three Chandra/ACIS-I observations, of the stellar population of the Eagle Nebula (M16) and its central cluster NGC 6611. At 1750 pc from the Sun, it harbors 93 OB stars, together with a population of low-mass stars from embedded protostars to disk-less Class III objects, with age <= 3 Myrs. A total of 1755 X-ray point sources have been detected, among which 1183 candidate cluster members (219 disk-bearing and 964 disk-less) and the 85% of the O stars (11). I will show that the X-ray Luminosity Function of M16 is similar to that of the population of coeval clusters, supporting the universality of the XLF in very young clusters, and that also in NGC 6611, as observed in some other young clusters, the X-ray emission level in disk-bearing stars is lower than that in disk-less objects. The spectral properties of the X-ray emission from the detected O stars are typical of soft emission from shocks propagating in the stellar wind. In only one O star a hard X-ray emission has been detected, but it is only marginally significant contributing less than 5% of the total spectrum.

Francisco Muller-Sanchez ( UCLA ) in Classroom (A101) at 12:30 on 8 August 2012

CANCELLED

Stefano Ettori ( INAF - Astronomical Observatory of Bologna ) in Classroom (A101) at 12:30 on 15 August 2012

Where the wild baryons are: the outskirts of galaxy clusters

Until recently, only about 10% of the total intracluster gas volume had been studied with high precision, leaving a vast region essentially unexplored. But tracing the clusters' properties out to large radii will be important for understanding the processes responsable for theformation of the collapsed structures and for using them as accurate cosmological tools. I will present new results on the physical properties of the outer regions of galaxy clusters as obtained from numerical simulations and X-ray data. I will discuss our studies on the average properties of, and the level of clumpiness and scatter present in, the gas density and temperature profiles. I will describe the observational constraints on the gas density profile at R200 as obtained locally with ROSAT-PSPC and at intermediate redshift with Chandra exposures, and how we can improve these constraints with the next generation of X-ray satellites (if any). I will illustrate a new generalized form of the universal temperature profile and how the distribution of the metals in the ICM depends upon the cluster radius and redshift.

Randall Smith and Jay Bookbinder ( CFA ) in Classroom (A101) at 12:30 on 22 August 2012

Future X-ray Missions: Updates from the PhysPAG and NAC APS meetings

Last week the Physics of the Cosmos Program Assessment Group (PhysPAG) and the X-ray Science Analysis Group (XraySAG; Jay Bookbinder, Chair) held a three-day conference in Washington DC. A few weeks ago the Astrophysics Subcommittee of the NASA Advisory Council Science Committee met as well. At both meetings the recently-appointed Director of the NASA SMD Astrophysics Division Paul Hertz presented his vision for accomplishing the goals of the Astronomy & Astrophysics Decadal Survey and listened to suggestions and comments. Additionally, Jay chaired the first meeting of the X-ray SAG, whose charter is to survey the X-ray field and make findings about the science and technology needs related to future X-ray missions. While acknowledging the tight fiscal and other constraints NASA is working under, Paul Hertz has a "success-oriented" plan for this decade and is looking to the astronomy community to help him achieve it. This presentation/discussion will go over what the possibilities are and what help is needed.

Pasquale Mazzotta (on behalf of the Planck Collaboration) ( CFA ) in Pratt at 12:30 on 5 September 2012

Planck results on the Coma and other nearby massive clusters of galaxies

We present the analysis of Planck data on the Coma Cluster. Thanks to its great sensitivity, Planck is able, for the first time, to detect SZ emission up to $r \approx 3 \times R500$. We find that the Arnuad et al. ``universal'' pressure profile does not fit the Coma at larger radii. This may indicate that at these larger radii either i) the cluster SZ emission is contaminated by unresolved SZ sources along the line of sight or ii) the pressure profile of Coma is higher at r > R500 than the mean pressure profile predicted by the simulations used to constrain the models. This result is consistent with the profile obtained by stacking the data of a sample of nearby massive clusters of galaxies observed by Planck. Finally, we find that the y and radio-synchrotron signals in the Coma clusters are quasi-linearly correlated on Mpc scales with small intrinsic scatter. This implies either that the energy density of cosmic-ray electrons is relatively constant throughout the cluster, or that the magnetic fields fall off much more slowly with radius than previously thought.

Yasu Tanaka ( Hiroshima University ) in Pratt at 12:30 on 12 September 2012

Suzaku deep observation of Centaurus A Southern lobe

The radio source Centaurus A is the closest active galaxy whose distance is 3.7 Mpc. It is surrounded by giant radio lobes extending for ~300 kpc in East-West, and ~600 kpc in the North-South directions, and the large-scale structure has been studied in great detail mainly with radio instruments. In this talk, we present results of Suzaku deep (80 ks each) X-ray observation toward selected two regions in the Southern lobe of the Centaurus A. Because of the large extension of the target on the sky (~5 deg x 9 deg), only a small part of the structure (the radio-brightest part of the Southern lobe) was observed. Faint diffuse X-ray emission extended over the whole FoV was detected over the X-ray background in that region, which was obtained from exposures adjacent to the radio lobe. X-ray features possibly associated with radio filamentary structure were also seen in the Suzaku X-ray image. Currently the association is not conclusive, and hence we expect that future Chandra image will provide definitive answer. We discuss implications of the Suzaku findings.

Katja Poppenhager ( CFA ) in Pratt at 12:30 on 19 September 2012

Using X-ray transits to study exoplanetary atmospheres

Many exoplanets orbit their host stars at close distances, with orbital periods of only a few days. Theoretical models predict that the incident stellar flux can deposit sufficient energy in those planetary atmosphere to lift parts of it out of the planet's gravitational well, causing substantial mass loss. And indeed, mass loss of atomic hydrogen has been observed in UV spectral lines for a handful of planets; however, at the temperatures thought to be present in the planetary outer atmospheres, hydrogen is mostly ionized, so that these measurements can only provide a lower limit to the total mass loss rate. I will present the first X-ray detection of an exoplanetary transit in front of the host star; we find a surprisingly deep X-ray transit with three times the optical transit depth. This can be explained by extended planetary atmosphere layers which are dense enough to be opaque to X-rays, but not at optical wavelengths. We derive a new, significantly higher mass-loss rate for the exoplanet compared to previous estimates.

Sebastian F. Hoenig ( UC Santa Barbara ) in Pratt at 12:30 on 20 September 2012 (Thursday)

The Dusty Heart of Active Galaxies - Dissecting the nucleus with high-angular resolution observations in the IR

Over the last years we have made significant progress in our understanding of the dusty environment around AGN -- commonly referred to as "dust torus", one of the cornerstones of AGN unification. A good part of this progress has been made possible by the high spatial resolution capabilities of the VLT and VLTI in the infrared. We are now able to not only resolve the dusty environment but constrain the distribution of the material that is believed to eventually accrete onto the supermassive black hole. I will give an overview of some of our most recent results involving small samples of AGN as well as individual objects using VLT/VISIR, VLTI/MIDI, and the Keck interferometer together with 3D clumpy torus models.

Adam Foster ( CFA ) in Pratt at 12:30 on 26 September 2012

Non-equilibrium models in AtomDB

Non equilibrium ionization plamas exist in many astrophysical phenomena - one obvious example being in supernova remnants. The models commonly used to model these in XSPEC and Sherpa (nei, vnei) are based on old atomic data from over 10 years ago. The nature of the models makes it difficult to update this data to incorporate new data. We are undertaking an extensive project to update the data in AtomDB and produce new models for modeling NEI plasmas. We report here on current progress and how the data will be useable in XSPEC, and invite beta (alpha?) testers to assist us in finalizing the models.

Krzysztof Nalewajko ( JILA ) in Pratt at 12:30 on 3 October 2012

Studying energy dissipation in relativistic jets via multiwavelength observations of blazars

Blazars are spectacular sources of high-energy radiation associated with their relativistic jets. The exact location of their main emission site and the underlying mechanism of energy dissipation are subjects of a long-standing debate. Recently, vast multiwavelength observational datasets on blazars were collected using many observatories. I will describe the effort of interpreting these data, focusing on three quasar-hosted blazars: 3C 279, PKS 1510-089 and PKS 1222+216. I will argue that at least two different dissipation mechanisms are necessary in order to explain the observed behavior of blazars.

Barbara De Marco ( Centro de Astrobiologia (CSIC-INTA), Madrid ) in Pratt at 12:30 on 17 October 2012

Soft X-ray lags in radio quiet AGN

The study of time lags between X-ray energy bands in AGN represents a powerful tool to unveil the physical origin of the different spectral components observed in time-averaged spectra, and to understand the geometry of the inner regions. The recent discovery of small-amplitude, soft lags (i.e. soft X-ray variations lagging hard X-ray variations) in a number of radio quiet AGN raised the question as to whether they can (all) be ascribed to a reverberation mechanism, possibly involving the inner accretion disc regions. I will discuss results of a systematic analysis of time lags between X-ray energy bands in a large sample of unabsorbed, radio quiet active galactic nuclei (AGN), observed by XMM-Newton. The analysis of X-ray lags is performed in the Fourier-frequency domain, between energy bands where the soft excess and the primary power law dominate the emission. This study led us to find a highly significant correlation between the lags time scales (i.e. characteristic frequency and amplitude) and the black hole mass.

Joo Heon Yoon ( Columbia University ) in Pratt at 12:30 on 24 October 2012

Lyman alpha Absorbers in Different Environments: Filamentary Feeding and Changes in the Circumgalactic Medium.

I present the first systematic survey of multiple background QSOs behind a galaxy cluster with the Cosmic Origins Spectrograph onboard HST to examine Lyman alpha absorbing clouds in an active environment. In total, 43 Lyman alpha absorbers are found in and around the Virgo Cluster with N_HI > 10^13 cm^-2 toward 23 QSO sightlines. We find Lyman alpha absorbers are dominant in the outskirts and mostly coincide with the infalling substructures of the Virgo Cluster. This indicates a connection between the large-scale flow of gas around the cluster and gas in individual galaxies. The covering fraction of Lyman alpha absorbing clouds is unity for N_HI > 10^13 cm^-2 surrounding the cluster. These results are consistent with cosmological simulations. We also probe the known properties of the Lyman alpha absorption lines and their relationship to individual galaxies. We find that environment influences not only galaxies, but also the circumgalactic medium significantly. These results provide important clues to how gas flows in different environments.

Robert Lindner ( Rutgers University ) in Pratt at 12:30 on 31 October 2012

Characterizing the physical properties of massive clusters in the LABOCA/ACT Survey of Clusters at All Redshifts (LASCAR)

I will present results from the LABOCA/ACT Survey of Clusters at All Redshifts (LASCAR) project. LASCAR has used the Large APEX Bolometer Camera (LABOCA) on the Atacama Pathfinder Experiment (APEX) to obtain 19''-resolution 870um imaging of ten of the most massive galaxy clusters that were detected as Sunyaev-Zel'dovich effect (SZE) decrements in a 455 sq degree Atacama Cosmology Telescope (ACT) southern survey. We have detected strong 870um SZE increments in five clusters out to z~1 which we use to study the physical properties of their intracluster media. We have also acquired 2.1GHz imaging from the Australia Telescope Compact Array (ATCA) and use it to disentangle SMGs from SZE fluctuations near the cluster centers. The project benefits from multiwavelength follow-up at optical (NTT, SOAR, VLT, Gemini), X-ray (Chandra), and infrared (Spitzer) wavelengths. Included in our sample is the ''El Gordo'' system, an exceptionally massive Bullet-like cluster merger at z=0.870 with two prominent complexes of radio relics surrounding a central radio halo.

Markos Trichas ( CFA ) in Pratt at 12:30 on 7 November 2012

The Synergy between X-ray and Infrared Surveys: Probing the AGN and Starburst Connection

Combination of X-ray with infrared observations provides the most robust constraints on the star formation of AGN to test competing models for the interplay between galaxy formation and black hole growth. The Herschel Multi-tiered Extragalactic Survey (HerMES), the largest project that will ever be performed with Herschel, is ideally suited to studying star formation over the z=1-3 epoch. Here I will summarize past work done with Spitzer and WISE and I will present the latest results from our Chandra/HerMES survey.

Jonathan Stern ( Technion ) in Pratt at 12:30 on 14 November 2012

Active Galactic Nuclei and their Hosts

We study the properties of low redshift broad line AGN, and their relation to their host galaxies, based on a new sample derived from the SDSS survey. The sample is supplemented by data from the GALEX, ROSAT, and 2MASS surveys. We find the following. The average AGN hosts are regular non emission line galaxies (NEG), which become bluer with increasing AGN luminosity, suggesting a correlation of the AGN luminosity and the host star formation rate. The observed AGN optical-UV emission is subject to some reddening, and the intrinsic emission is blue, consistent with accretion disk model predictions. The narrow emission lines reveal that the covering factor of circumnuclear gas (10s - 100s pc) decreases with increasing AGN luminosity, and the gas metallicity follows the host mass, similar to the mass - metallicity relation of normal galaxies. The metallicity of the broad line region (0.01s - 0.1s pc) also appears to be related to the host mass.

Kari Helgason ( University of Maryland/GSFC) in Pratt at 12:30 on 21 November 2012

How transparent is the universe to very high energy photons?

Propagating gamma-rays can interact with lower energy photons, producing an electron-positron pair. The extragalactic background light therefore supplies opacity for high energy photons (GeV-TeV) connecting gamma-ray astronomy with cosmic background studies. Determining the transparency of the Universe is of fundamental importance for a wide variety of current observatories such as the space-borne Fermi/LAT instrument operating at energies 250 - 300 GeV to ground-based Cherenkov telescopes probing TeV energies. I will present my work on the extragalactic background light and give an overview of the interesting interplay with high energy astrophysics.

Andy Goulding ( CFA ) in Pratt at 12:30 on 28 November 2012

Dust extinction in all Compton-thick AGN

I will present the results of a recent study exploring the origins of dust extinction features in all bona-fide Compton-thick AGN using infrared spectroscopy. Unified AGN models predict that the dusty torus should produce strong silicate absorption features in heavily obscured systems; however, we show that only a minority of nearby Compton-thick AGN have strong Si-absorption, and find that the dominant contribution to the observed IR dust extinction is dust located in the host galaxy (i.e., due to disturbed morphologies; dust-lanes; galaxy inclination angles). I will discuss how these findings impact current AGN torus models along with new implications for the reprocessing of the disk and coronal emission in the general AGN population.

Fabrizio Fiore (INAF- Ossavatorio Astronomico di Roma ) in Pratt at 12:30 on 12 December 2012

Supermassive Black Hole growth and AGN feedbacks through the cosmic times

AGN observations and evolutionary studies can be used to investigate the physics of baryon transformation in galaxies and the cosmological framework. I will present a pilot program to push the search of unobscured and obscured AGN up to z=5-6 and discuss the astrophysical and cosmological perspectives of this line of research. Targeting high-z supermassive black holes (SMBHs), the structures with the fastest (exponential) growth rate, can help investigating the evolution of the Universe at those early epochs, because little differences in the time of expansion of the Universe can be significantly emphasized. By comparing the high-z SMBH mass function to model predictions we might on one side disentangle competing cosmological scenarios, and on the other side put constraints on SMBH seed scenarios, the physics of accretion and AGN triggering mechanisms. I will then discuss interactions and links between the nuclear SMBHs and their host galaxies. The SMBH/AGN/galaxy co-evolution depends on some physical mechanism ('feedback') linking accretion and ejection occurring on sub-parsec scale in galaxy nuclei to the transformations occurring in the rest of the galaxy. I will present searches for "direct" evidence of AGN feedback in bright nearby, high-luminosity, highly obscured QSOs, and discuss the perspectives of extending these studies up tp z=1-3, the golden epoch of AGN and galaxy activity.

Henric Krawczynski ( Washington University in St Louis ) in Pratt at 12:30 on 09 January 2013

The Hard X-ray Polarimeter X-Calibur and X-ray Polarization Observations of Black Holes in X-ray Binaries

The main topic of this talk will be a report on the design, performance, and status of the balloon-borne hard X-ray polarimeter X-Calibur. The polarimeter uses a Compton scattering slab and an assembly of 32 Cadmium Zinc Telluride (CZT) detectors to measure the linear polarization properties of the 20 keV - 70 keV X-ray emission from cosmic sources over. The experiment is scheduled for a one-day balloon flight from Fort Sumner (NM) in Fall 2013. The first flight should allow us to constrain the polarization properties of the X-ray emission from the stellar mass black holes Cyg X-1 and GRS 1915+105, from the Crab Nebula and Pulsar, from the accreting X-ray pulsar Her X-1, and from one Active Galactic Nucleus (e.g. the blazar Mrk 421). For the brighter sources (i.e. the Crab Nebula), the one-day flight will allow us to measure polarization degrees down to a few percent. The talk will include a discussion of the scientific potential of soft and hard X-ray polarimetry with a special emphasis on polarimetric observations of black holes in X-ray binaries.

Giulia Migliori (CFA ) in Pratt at 12:30 on 16 January 2013

The jet high-energy emission in young and compact extragalactic radio sources

Young radio sources represent the first stage in the evolution of extragalactic radio sources and constitute an important fraction of the radio source population. Chandra and XMM-Newton observational campaigns have established them as X-ray loud sources, however the origin of the observed emission is still under debate, whether it is mainly contributed by the disk-corona system or by the non-thermal emission from the extended structures, namely jet and lobes. Answering this question is important to understand how the radio source will evolve and interacts with its host galaxy. In this talk I will present our study on the jet high-energy emission in young and compact radio quasars. For the first time, we focused on the gamma-ray band, which is expected to be contributed only by the non-thermal component. Predictions for gamma- and X-ray emission are derived using a general jet leptonic model. The results of the simulations are tested using Fermi-LAT and Chandra observations.

Brad Schaeffer (LSU ) in Pratt at 12:30 on 23 January 2013

Supernova Progenitors

The mystery of the identity of progenitor systems for Type Ia supernovae is one of the current big questions in astrophysics, with many possibilities. I will present a solution that (a) the dominant progenitors are certainly double-degenerate systems (i.e., two white dwarfs inspiralling to their death), (b) any additional progenitor classes certainly do not include any red giants or helium stars, and this eliminates most of the remaining possibilities.

Maxim Lyutikov (Perdue ) in Pratt at 12:30 on 30 January 2013

Crab pulsar and the nebula: paradigm shifts?

We discuss growing evidence that pulsar high energy emission is generated via Inverse Compton mechanism. We reproduce the broadband spectrum of Crab pulsar, from UV to very high energy gamma-rays - nearly ten decades in energy, within the framework of the cyclotron-self-Compton model. Secondly, recent observations of flares in the Crab nebula call into question the prevalent model of particle acceleration in relativistic astrophysical environments, the stochastic shock acceleration. Magnetic reconnection is likely to play an important, and perhaps a dominant role.

Floyd Jackson (CFA ) in Pratt at 12:00 on 6 February 2013

The properties of discrete X-ray sources in Star forming galaxies

The first part of my talk will focus on a study of 3 moderate-redshift (z 0.1) X-ray bright (> 10^42 erg s^1) galaxies, all of which display no clear signs of the presence of an AGN in the optical band. Given the high X-ray luminosities of these objects, they must either be the most X-ray luminous starburst galaxies known; or they must harbor a hidden AGN. We use new, pointed observations of the galaxies to determine their detailed X-ray characteristics, and demonstrate that each X-ray source is consistent with an AGN. The most likely explanation for the lack of AGN signatures in the optical spectra of these galaxies is that the AGN emission lines are being diluted by star formation signatures from within the host galaxies. Secondly, I will talk about a study performed on 8 of the brightest X-ray point-sources in the prototypical starburst galaxy M82, using the rich data set afforded to us by a 480 ks Chandra observation of the galaxy. From our investigation, we find that the sources with X-ray luminosities < 10^39 erg s^1 are heterogeneous, but all display X-ray properties that are typically observed in canonically accreting X-ray binaries. A possible bi-modality is seen in accretion states between the more luminous and less luminous sources in this subset of our sample. The majority of these sources show significant long-term variability, with one of the sources being identified as a transient candidate. The three brightest sources in our sample, all of which are known ULXs, display long-term variability and spectral characteristics consistent with previous observations.

Stephanie LaMassa (Yale University) in Classroom (A101) at 12:00 on 13 February 2013

Investigating Obscured AGN in the Local Universe

I will present analysis on two homogeneous samples of Seyfert 2 galaxies that were selected based on intrinsic AGN flux proxies: an [OIII]-selected sample from SDSS and a mid-infrared sample. X-ray analysis has shown that a majority of these sources display evidence of heavy obscuration: systematic under-representation of X-ray flux when normalized by intrinsic flux proxies and large values of the Fe K-alpha equivalent width, a signature of potential Compton-thick absorption. I also compare the levels of assumed obscuration with AGN properties and the relative amount of star formation activity, finding that Compton-thick Sy2s are not unique from their Compton-thin counterparts. Finally, I will show a prescription I have devised to disentangle AGN activity from star formation processes in soft X-rays.

Belinda Wilkes (CFA ) in Pratt at 12:00 on 20 February 2013

Probing star formation and nuclear structure in high-z, 3CRR Radio Sources

A critical problem in understanding active galaxies is separation of intrinsic physical differences from observed differences that are due to orientation. Relativistic motion in powerful radio sources produces a significant level of anisotropic emission at all but the lowest frequencies. Obscuration is also anisotropic and strongly frequency-dependent. Combined, these two effects result in complex selection effects for observations in most wavebands, and there are few ways to select a sample that is sufficiently unbiased to test orientation effects as predicted by unification models. Low-frequency radio emission is one way to select a close-to orientation-unbiased sample, albeit limited to the minority of AGN with strong radio emission. I will report Chandra, Spitzer and Herschel observations of a complete sample of high-redshift 3CRR radio sources (1<z<2) which generally support unification models, provide an estimate of the orientation dependence of the obscuring material and demonstrate that strong star formation occurs concurrently with strong accretion activity for a significant subset of active galaxies at high redshift and luminosity.

Ryan Allured (University of Iowa ) in Pratt at 12:30 on 28 February 2013 (Thursday)

The Prototype Development and Calibration of the Bragg Reflection Polarimeter on the GEMS Mission

Theoretical predictions of the soft X-ray polarization of black hole binaries indicate a change in both angle and magnitude with energy. The details of this change depend on both the spin and mass of the black hole. The NASA Gravity and Extreme Magnetism Small Explorer mission sought to use this effect to measure the spin of BHBs. The Bragg Reflection Polarimeter (BRP) was the student experiment on this mission, and was in the beginning stages of flight fabrication at the time of the mission's cancellation in May 2012. Prototypes of the multiwire proportional counter and multilayer reflector were developed and tested, meeting nearly all BRP requirements. Monte-Carlo simulations were carried out to estimate the ultimate polarization sensitivity of the BRP, and indicated satisfaction of the BRP science requirement. Finally, a fully polarized, 511 eV beamline was developed and used to calibrate a BRP instrument prototype, validating the sensitivity predictions.

Guido Risality (INAF Arcetri and CFA) in Pratt at 12:00 on 6 March 2013

A rapidly spinning supermassive black hole at the centre of NGC 1365

Active Galactic Nuclei (AGN) are optimal laboratories for tests of general relativity (GR) in the "strong field" regime. In particular their X-ray emission comes from a few gravitational radii from the horizon event of the supermassive black hole, and is strongly affected by GR effects. However, testing GR has been difficult so far because of a strong degeneracy in the X-ray spectra of AGN between the (relativistically distorted) intrinsic emission and the complex absorption/reflection components due to the circumnuclear medium. Here I present the first results of an on-going campaign of simultaneous X-ray observations of NGC 1365 by XMM-Newton and the recently launched hard X-ray telescope NuSTAR. I will show how the availability of high quality broad-band X-ray spectra, and a time-resolved spectral analysis can remove the main systematic uncertainties in GR tests, provide precise measurements of black hole spins, and reveal the structure of the circumnuclear X-ray absorber.

Anne Lofhink (University of Maryland) in Pratt at 12:00 on 20 March 2013

Probing the central engines of active galactic nuclei

AGN influence their surroundings via feedback processes and contribute significantly to the evolution of their host galaxies. While this is well established today, our understanding of these processes is incomplete. A missing crucial piece is an understanding of the processes in the heart of the AGN close to the black hole, where the vast majority of the energy is released. Therefore my talk explores these central regions of several AGN, both radio-quiet and radio-loud, using the tool of X-ray spectroscopy. I will describe some of the problems (and their solutions) we encountered when trying to study the fundamental parameters, such as black hole spin, which shape these central regions. For the Seyfert 1 galaxy Fairall 9, we find that the obtained accretion disk parameters are dependent on the spectral decomposition. To get a unique decomposition, we need to understand the soft X-ray excess seen in many AGN but not yet understood to date. In order to enhance our understanding of this soft X-ray excess we study its variability in the Seyfert 1 galaxy Mrk 841. This analysis hints at the soft excess being a separate spectral component such as Comptonization, which seems to follow a clear variability pattern. Our studies of the radio-loud AGN 3C120 were supplemented by UV/optical and radio monitoring. They confirm the current idea of jet formation, which links the ejection of a new jet knot to a disturbance/disruption of the inner parts of the accretion disk.

Sophia Dai (CFA) in Pratt at 12:00 on 27 March 2013

Star Formation in Dust-Rich Quasars, or Not?

With the Herschel Space Telescope, we have discovered a population of dust-rich broad-emission-line quasars (DBQs) at 0.5 < z < 3.5. They open a unique window to study the star formation environment in the peak of black hole accretion phase, quasars. I will provide a brief summary of my recent work on the SED studies of this population, and talk about their implications in constraining the black hole accretion rate and star formation rate (SFR) in such systems. I will conclude with the potential of Chandra X-ray observations for the DBQ population.

Matt Miller ( University of Michigan ) in Pratt at 12:00 on 3 April 2013

The Structure of the Milky Way's Hot Gas Halo

The Milky Way's million degree gaseous halo contains a considerable amount of mass that, depending on its structural properties, can be a significant mass component. In order to analyze the structure of the Galactic halo, we use XMM-Newton RGS archival data and measure OVII K-alpha absorption-line strengths towards 26 AGN, LMC X-3 and two Galactic sources (4U 1820-30 and X1735-444). These absorption lines allow us to place constraints on the density profile of the halo gas and estimate the baryonic mass contained in the halo. Assuming a universal baryon fraction of 0.171 (baryons to dark matter), we find the mass of the halo gas accounts for 10 - 50% of the missing baryons in the Milky Way. I will also discuss our model in the context of several Milky Way observables, including ram pressure stripping in dwarf spheroidal galaxies, the observed X-ray emission measure in the 0.5 - 2 keV band, the Milky Way's star formation rate, spatial and thermal properties of cooler gas, and the observed Fermi bubbles towards the Galactic center. Although the metallicity of the halo gas is a large uncertainty in our analysis, we are able to place a lower limit on the halo gas between the Sun and the LMC. We find the metallicity is greater than 0.2 solar based on the pulsar dispersion measure towards the LMC.

Leon Golub (CFA) in Pratt at 12:00 on 17 April 2013

A New View of the Solar corona From Hi-C

The Hi-C sounding rocket was launched at White Sands missile Range on Jul 11, 2012, yielding ~5 minutes of solar fine pointing. The instrument provided the highest resolution EUV observations ever obtained, resolving structures down to 0.2 arcsec (140 km FWHM) at a temperature of 1.5 MK in the Fe XII emission line at 193A. Notable among the new coronal structure and dynamics observed are highly braided coronal loops that simplify during the brief rocket flight, ubiquitous flows along fine structure at the local sound speed, possible detection of current sheet fragmentation within a flaring loop, and several other new types of features not seen before.

Susmita Chakravorty (CFA) in Pratt at 12:00 on 8 May 2013

Thermodynamic stability for AGN and BHB winds

Thermodynamic stability is an important notion to understand theproperties of photoionized gas and predict allowed (and hence observable) ranges of ionization states of the gas. This range is a function of the ionizing continuum seen by the outflowing gas. We use this thermodynamic prescription to highlight the importance of individual components, like the radiation from the accretion disk and soft excess, in shaping the physical properties of the AGN winds. We apply our physical insights to understand the high resolution HST and Chandra HETGS spectra of the quasar IRAS13349+2438. We further extend the same stability analysis to winds in X-ray binaries. We attempt to explain the observed phenomenon that windsare present in certain states of the binary and absent in others.

Jeremy Drake (CfA) in Pratt at 01:30 on 9 May 2013 (Thursday)

TBA

Dimitrios Giannos (Purdue University) in Pratt at 12:00 on 15 May 2013

Jets from stellar tidal disruptions in galactic centers

The tidal disruption of a star by a supermassive black hole provides us with a rare glimpse of these otherwise dormant beasts. It has long been predicted that the disruption will be accompanied by a thermal `flare', powered by the accretion of bound stellar debris. Recently, we explored the observational consequences if a fraction of the accretion power is channeled into an ultra-relativistic outflow. The high-energy transient Sw 1644+57 provides strong support to the presence of powerful relativistic jets during tidal disruption events. I will discuss the rich behavior of Sw 1644+57 in the radio and X-rays, focusing on the information we gain on the circum-nuclear medium and the jet physics by modeling this event.