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Chandra Fellows Symposium 1999

Harvard-Smithsonian Center for Astrophysics

October 25, 1999

Titles and Abstracts of Talks

Amy Barger
Institute for Astronomy, University of Hawaii

"Unveiling AGN and Star Formation Activity in Dusty Galaxies"

Star formation and AGN activity in dust-enshrouded distant galaxies are largely hidden from optical observations since the bulk of the light is reradiated by dust at submillimeter wavelengths. A first view of the optically dark side of the Universe has recently been obtained with the James Clerk Maxwell Telescope on Mauna Kea using the revolutionary SCUBA camera. A new population of extremely luminous dusty galaxies is revealed whose nature, redshift distribution, correlation with the micro-Jansky radio population, and significance will be discussed.

Markus Bottcher
Rice University

"Afterglows and iron lines or What does Chandra have to do with Gamma-Ray Bursts?"

The X-ray afterglows of gamma-ray bursts (GRBs) contain vital information about the structure and composition of the environment of cosmological GRBs. Until now, only two GRB afterglows (GRB 970508 and GRB 970828) have shown marginal evidence for Fe fluorescence lines in their X-ray afterglows. The theoretical understanding of these line features (if they are real) is still in its very early stages. I will describe recent results as well as the status of work in progress on the time dependent radiation transport in the vicinity of cosmological GRBs. Investigated scenarios include isotropic GRB explosions and environments, and the interaction of isotropic and beamed GRB and afterglow blast waves with an iron-enriched torus of dense material around the central source. I will also shortly report on recent analytical results of early ~few hours after the GRB) afterglows from relativistic blast waves in general radiative regimes, which could so far only be handled with time-consuming numerical simulations. The importance of high-sensitivity X-ray observations of early GRB afterglows with good spectral resolution is stressed.

David Buote
Lick Observatory, UC Santa Cruz

"The Metal Abundances of the Hot ISM in the Centers of Elliptical Galaxies and Galaxy Groups."

We summarize our recent work on the re-analysis of the ASCA spectra accumulated within the central ~5 arcminutes of the brightest elliptical galaxies and galaxy groups. The ASCA spectra generally require at least two temperature components in the hot ISM in addition to a possible component arising from discrete sources. Unlike isothermal models, the multitemperature models typically yield metallicities of ~1 solar that are inconsistent with the very sub-solar values previously reported for many of these systems when an isothermal gas is assumed. We discuss the implications of these results for the star formation history of these systems.

Tiziana Di Matteo
Harvard University

"Quiescent Supermassive black holes in Elliptical Galaxies"

We report the detection of hard, power-law emission components in the X-ray spectra of six nearby, giant elliptical galaxies observed with the ASCA satellite, which exhibit strong dynamical evidence for supermassive black holes in their nuclei. The power-law photon indices are in the range $\Gamma = 0.6-1.5$ (mean value 1.2) and intrinsic 1-10 keV luminosities of $2 \times 10^{40}-2 \times 10^{42}$ erg s-1 implying the identification of a new class of accreting X-ray source, with X-ray spectra significantly harder than those of binary X-ray sources, Seyfert nuclei or low luminosity AGN. I will discuss important implications for the study of the quiescent supermassive black holes in the centre of ellipticals. I will describe how the broad band spectral energy distributions for these galaxies, be explained by low-radiative efficiency accretion flows. In particular, I will show that the observed suppression of the synchrotron components in the radio band (excluding the case of M87) and the systematically hard X-ray spectra, which are interpreted as thermal bremsstrahlung emission, support the conjecture that significant mass outflow is a natural consequence of systems accreting at low-radiative efficiencies.

Ann Esin
California Institute of Technology

"X-ray Irradiation of Outer Disks in Black Hole X-Ray Transients"

It is generally believed that optical emission from low-mass X-ray binaries is produced by X-ray irradiation of the outer accretion disk. Though the overall amplitude and the exact spectrum of the observed optical emission depend strongly on the (largely unknown) radial profile of the outer disk, they also carry information about the properties of the irradiating flux. X-ray transients which are believed to contain accreting black holes are particularly interesting objects for study, since during their outbursts these systems display several spectral states, characterized by very different X-ray spectra and possibly different angular distribution of the emitted photons. A combined modeling of X-ray and optical emission in such a system in different spectral states provides an opportunity of separating the effects due to the outer disk properties from those due to the characteristics of the X-ray emitting region. We present the results of such modeling for two very similar black hole X-ray transients, A0620-00 and GRS 1124-68. We find that the optical light curve of A0620-00 observed during its outburst in 1976 can be reproduced very well using the model for X-ray emission proposed by Esin et al. (1997). Assuming that the shape of the outer disk remains constant during the outburst, this result provides an important confirmation that the angular distribution of the X-ray flux changes from being mostly disk-like in the more luminous `soft' states to nearly isotropic in the less luminous `hard' states. We obtain a consistent result in modeling optical emission from GRS 1124-68. An interesting conclusion which emerged from comparing the two systems is the fact that though GRS 1124-68 has a larger disk, the irradiation appears to be a factor of 3 weaker in this system.

Jimmy Irwin
University of Michigan

"The Contribution of Low Mass X-ray Binaries to the X-ray Emission of Early-type Galaxies" The X-ray emission from at least the bright early-type galaxies is known to emanate from hot gas at a temperature of 0.5-1.0 keV. In addition to this gaseous component, stellar sources (most notably low mass X-ray binaries, LMXBs) must contribute to the X-ray emission at some level, although what this level is remains to be determined. Galaxies with the lowest optical-to-X-ray luminosity ratios offer the best opportunity to determine the stellar contribution. These X-ray faint galaxies are believed to have had most if not all of their hot gas removed by environmental or evolutionary effects, leaving only the stellar X-ray component behind. Pre-Chandra telescopes have had insufficient spatial resolution to resolve the X-ray emission into discrete point sources, leaving the question of the nature of the X-ray emission in these galaxies unanswered. Knowledge of the magnitude of the stellar component is crucial so that it can be separated from the hot gas component so that accurate mass estimates can be derived based on the assumption of hydrostatic equilibrium of the hot gas in the gravitational well of the galaxy.

Spectral studies of X-ray faint elliptical and S0 galaxies have found that two spectral components are needed to describe the X-ray emission in these galaxies: a soft (0.2 keV) component and a hard (5-10 keV) component. The latter is often attributed to LMXBs, although recent work has indicated that LMXBs might also be responsible for the soft component. This is supported by the fact that LMXBs in the bulge of M31 are seen to emit very soft emission in addition to a hard component. In fact, the X-ray spectrum of the bulge of M31 (whose emission is nearly all from point sources with little if any from hot gas) is identical to that of the X-ray faint early-type galaxy NGC4382, bolstering the notion that the X-ray emission in X-ray faint early-type galaxies is primarily from stellar sources.

In this talk, I will review the current state of our knowledge of X-ray emission from X-ray faint early-type galaxies, and present new work involving the ROSAT PSPC + ASCA spectrum of the X-ray faint galaxy NGC4697. The impact of future Chandra observations of these galaxies will also be discussed.

Kristen Menou
Princeton University

"Identifying Black Holes in X-ray Binaries"

Recently, a test for distinguishing between the presence of a neutron star (NS) or a black hole (BH) primary in X-ray binaries has been proposed, which goes beyond the standard detection of X-ray pulses or X-ray bursts (for NSs), or the determination of a primary mass exceeding the NS mass limit (for BHs).

Namely, in those binaries where accretion occurs via an Advection-Dominated Accretion Flow, the presence of a hard surface (in the NS case) from which the advected energy is reradiated, versus the presence of an event horizon (in the BH case) through which this energy is lost, should be apparent as a luminosity difference between the two classes of systems.

I will review the current observational and theoretical status of this test, emphasizing the importance of future observations, in particular with the Chandra X-ray Observatory, for validating (or not) this technique as a reliable indicator of the presence of event horizons in X-ray binaries.

Joseph J. Mohr
University of Chicago

"Measuring Absolute Distances to High Redshift Galaxy Clusters"

The hot intracluster medium (ICM) can be studied by its X-ray emission and by the effect it has on the cosmic microwave background, the so-called Sunyaev-Zel'dovich Effect (SZE). Combining X-ray observations with radio observations of the SZE allows one to solve for the intrinsic size and angular diameter distance of a galaxy cluster. These SZE+X-ray distances are important to cosmology, because they are distance ladder independent, and because they do not require an assumption that high and low redshift galaxy clusters are identical.

I will describe first results of an ongoing attempt to use archival X-ray images, published ICM temperatures and interferometric SZE data obtained by the Carlstrom SZE Imaging Collaboration to measure distances to a large sample of clusters. I will then use mock observations of an ensemble of numerical cluster simulations to explore the expected uncertainties and possible systematics in these distances. Finally, I will discuss the expected improvements in the SZE+X-ray distance estimates that will come with the spectral imaging data delivered by Chandra.

Edward Moran
U.C. Berkeley

"Faint Radio Sources and the Origin of the Hard X-ray Background"

The ASCA and BeppoSAX observatories have greatly improved our understanding of the hard X-ray properties of nearby starburst and Seyfert galaxies. Unfortunately, the sensitivities of these instruments are insufficient to probe either population directly at intermediate and high redshifts, where the majority of the cosmic X-ray background (XRB) originates. As a result, models for the XRB must typically rely on highly uncertain assumptions about the evolution of these sources with cosmic time. Given that X-ray galaxies of all types produce radio emission in conjunction with their particular brand of activity, the faint radio source population should provide an observational constraint that reduces the need for such assumptions. Indeed, existing deep radio surveys, which extend to the microjansky level, contain both starburst and Seyfert galaxies at cosmological distances. Optical identification programs, however, have revealed that the majority of sub-mJy radio sources are associated with star-forming galaxies rather than AGNs. Thus, by combining hard X-ray and centimeter radio data for nearby starburst galaxies with the measured log N - log S relation for sub-mJy radio sources, I have estimated the starburst contribution to the hard XRB, finding that these objects produce as much as 25\% of the background at 5 keV. Similar analysis applied to Seyfert galaxies indicates that they can easily account for the remainder of the XRB in this band. The faint radio source statistics suggest that star-forming galaxies will represent a significant fraction of the number counts in very deep Chandra images.

Eliot Quataert
Insititue for Advanced Studies

"The Cooling Flow to Accretion flow Transition"

I discuss the structure of cooling flows on small scales, taking into account the presence of a central supermassive black hole in the host galaxy. If angular momentum is negligible, cooling flows undergo a transition to nearly adiabatic Bondi accretion near the point in the galaxy where the gravity of the black hole begins to dominate the gravity of the galaxy. This transition is observable in the Virgo cluster with the excellent angular resolution of the Chandra X-ray Observatory; it is characterized by a decrease in the importance of mass drop out in the cooling flow and an increase in the temperature of the X-ray emitting plasma (with decreasing radius). The qualitative changes to this picture introduced by the presence of angular momentum in the inflowing material are briefly discussed.

Rudy Wijnands

"The Very Short-lived September 1999 Outburst of V4641 Sgr."

On 15 September 1999, the variable star and known X-ray transient V4641 Sgr (SAX J1819.3-2525) exhibited a very bright (12 Crab) but very short-lived (< 10 hours) X-ray outburst. Near the end of the outburst, a short observation was taken with the proportional counter array on board the Rossi X-ray Timing Explorer. The X-ray luminosity and the rapid X-ray variability as observed during this observation showed that the X-rays were powered by accretion onto a compact object, most likely a black-hole (although a neutron star primary cannot be excluded). The close proximity of V4641 Sgr to the Earth (0.5-1.0 kpc) suggests that many more of V4641 Sgr like objects are present in our galaxy. Most of these objects will go unnoticed because their X-ray outbursts are very short-lived events. A considerable number of the black-holes in our galaxy could be present in V4641 Sgr like objects.

Last modified: 08/24/11

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