Current schedule (overview)

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

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 6", 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