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Chandra Fellows Symposium 2003

Harvard-Smithsonian Center for Astrophysics

October 8, 2003

Titles and Abstracts of Talks

Elizabeth L. Blanton
University of Virginia

Chandra Observations of Radio Sources in Clusters: Impact on the ICM and Probes of High-Z Systems

Recent observations of clusters of galaxies with Chandra have revealed the profound effect that central radio sources have on the intracluster medium (ICM), and in turn, the effect that the ICM has on the radio sources. The radio sources inflate bubbles in the ICM which then rise buoyantly transporting energy and magnetic fields into the cluster. The energy transferred to the ICM from several outbursts of the radio sources is a leading candidate for the solution to the "cooling flow problem" (sufficient quantities of gas are not seen at sufficiently low temperatures), in that the radio sources can heat the gas cooling in the centers of clusters. Results of the analysis of Chandra data of cooling flow clusters with central radio sources will be presented, with emphasis placed on the energy transferred to the ICM from the central objects in Abell 262 and Abell 2052. In addition, since radio sources are confined and distorted by the ICM, we can use radio sources with particular morphologies as tracers of clusters at a range of redshifts. Results of a Chandra observation of a redshift z=0.96 cluster discovered using a radio source as a cluster signpost will be presented.

Taotao Fang

Probing the Soft X-ray Emission from the Warm-Hot Intergalactic Medium

Cosmological simulations reveal the cosmic web structure of the universe, where moderately overdense filaments connect collapsed regions such as groups and clusters of galaxies. These filaments, named "Warm-Hot Intergalactic Medium", or WHIM, are shock-heated to temperatures between 10^5 - 10^7 K and emit X-ray. I will discuss detailed imaging and spectroscopy studies of the soft X-ray emission from the WHIM gas via cosmological simulation. I will also talk about the correlation between this X-ray emission and the large scale structure. Finally I will discuss the detectability with proposed X-ray telescopes.

Sebastian Heinz

Baby beams and jumbo jets: investigating the relationship between microquasars and AGNs

The striking morphological and spectral similarity of jets from black holes of different mass hints at fundamental commonalities in their makeup and dynamics. Starting from the assumption that this similarity is an expression of the scale invariance of the underlying physics, I will show how one can derive very general expressions for the dependence of the jet emission on the mass of the black hole and on the accretion rate in the inner accretion disk. The comparison with a sample of black holes with observed masses and X-ray and radio luminosities confirms these predictions and can be used in principle to infer important information about the state of the inner accretion disk and about the nature of the X-ray emission.

Ann E. Hornschemeier
Johns Hopkins

X-ray Probes of Star Formation Outside the Local Universe

X-rays provide a unique window into star formation processes, providing important information on the evolution of stellar endpoints and hot gas that cannot be obtained at other wavelengths. With Chandra we may study this X-ray emission at cosmologically interesting distances (0.1 < z < 2.0), placing constraints on X-ray emission over substantial evolutionary timescales (> 1 Gigayear). I will report on results from the HST Advanced Camera for Surveys GOODS observations of the two Chandra Deep Fields. This includes physical constraints on the ULX population at appreciable look-back times, allowing us to make a strong link between star formation and ULX sources. I will discuss current efforts to better characterize the broad X-ray spectral properties of star-forming galaxies in the relatively nearby (z<0.1) Universe in a two-fold approach: (1) through a massive cross-correlation of the Chandra archive and the Sloan Digital Sky survey and (2) through upcoming Chandra observations of galaxies in the Coma cluster. This combined approach assembles a complete picture of the X-ray properties of star-forming galaxies.

Peter Jonker

Following a Black Hole Soft X-ray Transient Towards Quiescence in Radio and X-Rays

I will present the results of my recent Chandra and VLA DDT observations of the Galactic Black Hole Candidate (BHC) XTE J1908+094 during its decay towards quiescence. A main goal of these observations is to test the whether the observed radio - X-ray correlation found by Gallo et al. (2003) and Corbel et al. (2003) in two other Galactic BHCs over 3-4 decades in flux persists all the way towards quiescence. Another goal is to determine detailed X-ray spectra at X-ray luminosities which have not or scarcely been studied before the Chandra/XMM era. The radio - X-ray correlation is among other things important for the analysis presented in Fender, Gallo, Jonker (2003). Previous radio and X-ray observations of the BHC A0620-00 in quiescence suggest that the radio - X-ray relation can indeed be extended towards quiescence. Further, I'll show that one can use the normalization of the radio - X-ray correlation to estimate the distance towards the BHC.

Julia Lee

The Chandra HETGS High Resolution View of `Broad' Iron Line Seyferts

Broad Fe K-alpha X-ray emission lines have been observed in Seyfert 1s and 2s by moderate resolution X-ray observatories such as ASCA, BeppoSAX, and RXTE. I will report on the ``narrow'' component of these broad lines in select Seyferts as seen with the Chandra HETGS to illustrate how we can combine the spectroscopic power of the existing X-ray missions to disentangle the broad line profile and determine its origin, be it from the accretion disk and/or circum-source material.

Li-Xin Li

Instability in Accretion Disks and Quasi-Periodic Oscillations in X-Ray Binaries

We consider the interface between an accretion disk and a magnetosphere surrounding the accreting mass. We argue that such an interface can occur not only with a magnetized neutron star but also sometimes with an unmagnetized neutron star or a black hole. The gas at the magnetospheric interface is generally Rayleigh-Taylor unstable and may also be Kelvin-Helmholtz unstable. Because of these instabilities, modes with low azimuthal wavenumbers m are expected to grow to large amplitude. It is proposed that the resulting nonaxisymmetric structures contribute to the high frequency quasi-periodic oscillations that have been seen in neutron-star and black-hole X-ray binaries. The mode oscillation frequencies are calculated to be approximately equal to m \Omega_m, where \Omega_m is the angular velocity of the accreting gas at the magnetospheric radius. Thus, mode frequencies should often be in the approximate ratio 1:2:3, etc. If the pressure of the gas in the disk is not large, then the m=1 mode will be stable. In this case, the mode frequencies should be in the approximate ratio 2:3, etc. There is some observational evidence for such simple frequency ratios.

Andisheh Mahdavi

Cluster Mass Profiles from X-ray and Optical Data

Simulations of structure formation in the universe predict specific shapes for the matter distribution in clusters of galaxies. To test them, it is desireable to measure the mass profiles of real systems. X-ray astronomers do this by hydrostatically modeling the temperature and surface brightness of the intracluster gas. I will discuss methods of combining these models with optical data to obtain joint and more powerful constraints on the matter distribution. Treating the galaxies as test particles in a potential, one can use the distribution of their positions and velocities to supplement the gasdynamical data. While the analysis involves the introduction of parameters that are foreign to gas dynamics, e.g. the galaxy velocity anisotropy, the result is an enhancement of the final constraints, as well as an independent confirmation of the validity of the X-ray methods.

Eric Pfahl

Probing the Spacetime Around Sgr A* with Radio Pulsars

The supermassive black hole at the center of the Milky Way hosts a bound cluster of two dozen or more massive stars. Many massive stars leave neutron-star remnants, which are then detectable as radio pulsars. It is suggested that roughly 100 to 1000 active radio pulsars may presently orbit Sgr A* with periods of less than 100 years. Deep, high-frequency radio observations may discover 1 to 10 such pulsars. Long-term pulsar timing observations would clearly reveal the Newtonian dynamical signature of the black hole, and likely show the effects of weak- and strong-field relativistic gravity, including the influence of frame-dragging due to the spin of the black hole. Observation of a pulsar orbiting Sgr A* would prove to be a unique and powerful laboratory for investigating Sgr A* and its environment, as well as relativistic gravity and the physics of black holes.

Enrico Ramirez-Ruiz
Institute for Advanced Studies

Basic ingredients for the biggest blasts in the cosmos

There is now general agreement that gamma-ray bursts involve extraordinary power outputs, posing a severe theoretical challenge and - irrespective of which detailed model proves correct - involving some fascinating and extreme physical processes. I shall focus on some of the key issues relating the modelling of the trigger and the formation of the ultra relativistic outflow, which have been proven to be a formidable challenge to computational techniques.

Erik Reese

Determining the Cosmic Distance Scale with Galaxy Clusters

Cosmic microwave background (CMB) photons traveling through the hot gas of galaxy clusters experience inverse Compton scattering causing a spectral distortion of the CMB known as the Sunyaev-Zel'dovich effect. Analysis of Sunyaev-Zel'dovich Effect (SZE) and X-ray data provides a method of directly determining distances to galaxy clusters at any redshift. This method is independent of the extra-galactic distance ladder and does not rely on clusters being standard candles or rulers. We present preliminary results of a joint analysis to 30 GHz interferometric SZE and Chandra X-ray observations for a sample of galaxy clusters over a wide redshift range (0.14-0.89). Markov Chain Monte Carlo methods are employed to simultaneously extract both spatial and spectral properties of the cluster gas from the data. Upcoming SZE surveys will provide large catalogs of galaxy clusters, many of which will be at high (z>1) redshift, enabling a determination of the geometry of the universe with this method. Cosmological constraints from SZE/X-ray distances are complementary to those from analysis of cluster number counts from surveys.

Mateusz Ruszkowski
University of Colorado at Boulder

Hot Times for Cooling Flows

I will briefly discuss the cooling flow problem in clusters of galaxies. I will present the ``effervescent heating'' model as an explanation for the minimum temperatures in cooling flow clusters and the density and temperature profiles. The model does not require any excessive distributed mass deposition rates in agreement with Chandra observations. Results of 1D time-dependent ZEUS simulations of this effect will presented. I will also discuss the application of this model to the entropy floor problem and a possible relation between the black hole masses of central AGN and masses of clusters. I will present (2D and 3D) simulations of cooling flow clusters using the adaptive mesh FLASH code and discuss the ``lowest note in the Universe'' recently publicized in the media (CNN, BBC, NY Times, msnbc, Popular Science etc.). The simulations reveal sound waves, cool rims and large lateral spreading (isotropization) of the buoyantly rising bubbles of hot gas in cluster atmospheres. The strong discrepancy between the morphology of X-ray cavities and the radio emission and its implications for assessing the role of AGN heating will also be discussed.

Masao Sako

Statistical Significances of X-ray Emission Line Detections in the Afterglows of Gamma-ray Bursts

We present detailed statistical tests to quantify the true significances of X-ray emission line detections in the afterglows of gamma-ray bursts (GRB). We test a simple hypothesis in which the observed spectra are produced by a power law continuum model modified by photoelectric absorption by neutralmaterial both in our Galaxy and possibly local to the burst. Spectral line-like fluctuations do appear in the spectra of most GRB and Monte Carlo simulations are performed to measure the statistical significances of these features. Results from observations of several GRB, in particular, those with claimed line detections will be discussed.

Anatoly Spitkovsky

Dynamics of Nuclear Burning During Type I X-ray Bursts: Effects of Rotation and Magnetic Fields

I present the results of multidimensional simulations of ignition and propagation of thermonuclear burning in the atmosphere of a rapidly rotating neutron star during type I X-ray burst. Due to the effects of Coriolis force, the burning spreads in geostrophically balanced burning fronts with latitude-dependent speeds. I demonstrate how such fronts propagate on a sphere, and propose that during the ignition the burning is likely to spread from equator to the poles. The variation of Coriolis force with latitude makes burning hot spots drift against the rotation of the star, contributing to the observed frequency evolution of burst oscillations. Finally, I will show how magnetic fields influence the hydrodynamics during nuclear burning and apply this to observations of bursting accreting pulsar SAX J1808.

Martin Weisskopf
Chandra Project Scientist

The Development of the Chandra X-Ray Observatory

The Chandra X-Ray Observatory traces its origins almost to the beginning of the field of X-Ray astronomy. We will present the history of the development of this Great Observatory from the perspective of the Project Scientist.

Last modified: 08/24/11

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