About Chandra Archive Proposer Instruments & Calibration Newsletters Data Analysis HelpDesk Calibration Database NASA Archives & Centers Chandra Science Links

Chandra Fellows Symposium 2002

Harvard-Smithsonian Center for Astrophysics

October 7, 2002

Titles and Abstracts of Talks

Eric Agol
Cal Tech

Microlensing and occultation of Main Sequence stars by white dwarf and planetary companions

I will prove a gravitational lensing theorem: the magnification of a large source of uniform brightness by a foreground point-mass lens is 1+pi*(2*R_E^2+R_L^2)/A, where A is the area of the source and R_E and R_L are the Einstein radius and size of the lens. Remarkably, this result is independent of the shape of the source or position of the lens (except near the edges). I will then use this theorem to compute transit lightcurves in binaries for which the foreground star or planet has a size and Einstein radius much smaller than the background star. I will then discuss some generalizations of the theorem and observational applications.

Elizabeth L. Blanton
University of Virginia

Extended Radio Sources in Clusters of Galaxies

Chandra observations of cooling flow clusters have revealed a dramatic interaction between cluster central radio sources and the X-ray-emitting intracluster medium (ICM). Radio sources blow "bubbles" in the hot ICM, which in turn, confines the radio lobes. These bubbles then rise buoyantly outward in the clusters. The radio sources may contribute to the heating of the cooling gas, but probably not by strong shocks. I will present new results from Chandra observations of Abell 2052 and Abell 262 which show this type of radio source / ICM interaction. In addition, I will describe the results of using radio sources that have been distorted by interaction with the ICM as signposts for distant (z~1) clusters.

M. F. Gu

Dielectronic Recombination and Resonance Excitation Effects on Iron L-shell Line Emission in Collisionally Ionized and Photoionized Plasmas

Systematic studies of the role of dielectronic recombination (DR) and resonance excitation (RE) in the soft X-ray line formation of iron L-shell ions are carried out for both collisionally ionized and photoionized plasmas, using a newly developed, relativistic, multi-configurational atomic code. It is found that in collsionally ionized plasmas, the combined effects of DR and RE enhance the 2p-3s line intensities from Fe XVII--XX by as much as a factor 2 at the temperature of maximum fractional abundances for the respective ions. Such enhancement, though not enough to completely explain the observed 2p-3s/2p-3d line ratios from various astrophysical sources, is a vast improvement over the previous theoretical models neglecting these processes. In low temperature photoionized plasmas, the Delta n = 0 DR resonances are found to make significant contributions to the line emission along with the radiative recombination (RR) followed by cascades. Moreover, DR usually populates a different set of lines from RR. Therefore, its inclusion changes the calculated line ratios dramatically.

Jeremy Heyl

"Low Mass X-ray Binaries May Be Important LIGO Sources After All"

Abstract: Rossby waves (r-modes) in the cores of rapidly rotating neutron stars may grow because of the emission of gravitational radiation, preventing accretion from spinning up the star further. Whether and how r-modes affect the spin evolution of low-mass X-ray binaries depends strongly on the nature of the r-mode instability and how the r-modes saturate. I will describe the r-modes and explain why they may become unstable in LMXBs. Then using recent results which describe how the r-modes saturate, I will outline the spin-evolution of LMXBs, calculate how many LXMBs could be detected by LIGO, and speculate how these gravitational-wave sources might appear in photons.

Ann Hornschemeier
Johns Hopkins University

The Stellar Content of the Chandra Deep Field-North

This talk will focus on non-AGN science that may be carried out using deep high galactic latitude Chandra X-ray surveys. For example, approximately three percent of the X-ray sources identified in the 0.5-2 keV band in the Chandra Deep Field-North survey are coronally active late-type stars within the Galaxy. We are thus able to probe fairly normal low-mass stars on evolutionary timescales over which stellar magnetic activity is poorly constrained (billions of years). Additionally, these X-ray detected stars are a useful probe of Galactic structure and of the evolved stellar X-ray Luminosity Function.

Julia C. Lee

Probing Black Hole Systems with Chandra : The HETGS view of the microquasar GRS 1915+105

High resolution X-ray spectroscopy provides a powerful tool for advancing our understanding of the physical environment of systems which harbor black holes. Superior resolution and wide band coverage (as presently possible with the Chandra HETGS) will enable the necessary measurements to determine the line-of-sight velocity and velocity dispersion of the material, as well as the column density and ionization parameter of the accretion disk environment and any disk wind. To give flavor for the myriad and variety of spectral features which can be imprinted on a single spectrum, I will present the results of a 30 ks HETGS observation of the bright Galactic micro-quasar GRS 1915+105 in the low hard state. These spectra reveal neutral K edges, ionized resonance lines and possible XAFS. I will discuss the implications of these detections with regard to the source environment (i.e. temperature, ionization state and Hydrogen column) and geometry. The importance of variability studies for adding to the complete picture will also be discussed.

Li-Xin Li

The Giant X-Ray Flare of NGC 5905: Tidal Disruption of a Star, a Brown Dwarf, or a Planet?

We model the 1990 giant X-ray flare of the quiescent galaxy NGC 5905 as the tidal disruption of a star by a supermassive black hole. From the observed rapid decline of the luminosity, over a timescale of a few years, we argue that the flare was powered by the fallback of debris rather than subsequent accretion via a thin disk. The fallback model allows constraints to be set on the black hole mass and the mass of debris. The latter must be very much less than a solar mass to explain the very low luminosity of the flare. The observations can be explained either as the partial stripping of the outer layers of a low-mass main sequence star or as the disruption of a brown dwarf or a giant planet. We find that the X-ray emission in the flare must have originated within a small patch rather than over the entire torus of circularized material surrounding the black hole. We suggest that the patch corresponds to the ``bright spot'' where the stream of returning debris impacts the torus. Interestingly, although the peak luminosity of the flare was highly sub-Eddington, the peak flux from the bright spot was close to the Eddington limit. We speculate on the implications of this result for observations of other flare events.

Andisheh Mahdavi
University of Hawaii

A Different Dark Matter Potential for Modeling Cluster Atmospheres

Chandra has shown us that the distribution of the X-ray emitting medium in clusters of galaxies is seldom regular and homogeneous. Nevertheless, azimuthally averaged models of clusters remain both popular and useful in describing the gross properties of the medium. These models often rely on an underlying dark matter potential with either a constant density core (the "beta-model") or a cuspy core derived from N-body simulations (the NFW profile). As a replacement for these I will suggest a generalized potential with a single adjustable parameter sensitive to the slope of the matter density. This approach allows rough constraints on the shape of the dark matter potential even in situations where temperature resolution is sparse.

Eric Pfahl

Low-Luminosity X-ray Sources in the Galactic Bulge

Recent Chandra observations of the Galactic center region have revealed a large number of previously unknown point sources. Selected results of these observations will be discussed. I argue that many, perhaps the majority, of the 3-8 keV sources detected by Chandra are neutron stars accreting from the winds of main-sequence stellar companions with masses from 3 to 20 Msun. Binary population synthesis calculations support this basic hypothesis. Further observations at X-ray and IR wavelengths are required to determine the true nature of these point sources.

Erik Reese

Exploring the X-ray Isophotal Size-Temperature Relation in Galaxy Clusters

Clusters of galaxies appear to be regular objects, illustrated by scaling relations of global observable properties. Nearby galaxy clusters and simulations of galaxy clusters show a tight correlation between the X-ray isophotal size of a galaxy cluster and its X-ray temperature. With a simple model of self-similar redshift evolution of clusters, we expect that the X-ray isophotal size-temperature (ST) relation should not evolve with redshift. This makes the ST relation a potentially power tool for measuring cluster distances, thereby constraining cosmological parameters. We present results for a sample of 18 galaxy clusters and then combine our results with those of a previous study to construct a 26 cluster sample. We find our sample to be consistent with the standard evolution in ST although the new sample hints at departures from the standard. In addition, we use the ST relation and our evolution model to determine angular diameter distances to the galaxy clusters and present constraints on cosmology.

Masao Sako

Line Scattering and the Bowen Fluorescence Mechanism in Optically Thick, Highly Ionized Media

I will discuss how radiative transfer effects can alter the emergent line spectrum from an optically thick, highly ionized medium. One example of resonance line overlap, in which the O VIII Ly-alpha (n = 2 -> 1) line scatters with the N VII Ly-zeta (n = 7 -> 1) line, is investigated in detail. It is found that, depending on the temperature and optical depth, a large fraction of the O VIII line can scatter into N VII, which can subsequently increase the intensities of the N VII lines by suppressing the O VIII line intensity. I solve the radiative transfer equation to calculate the destruction/escape probabilities for each of these lines as a function of optical depth as well as destruction probabilities due to underlying photoelectric opacity. Other important overlapping lines and continua in the X-ray band are also identified. These calculations demonstrate that global spectral modeling that does not account for these effects yields unreliable line intensities, and, consequently, abundance estimates, especially when the optical depth through the medium is large.

Anatoly Spitkovsky

Formation of relativistic outflows

Pulsar winds powering synchrotron emission in supernova remnants are prototypical examples of relativistic outflows from magnetized rotators. Although much is observationally known about the global characteristics of such winds, how they form in detail is an unsolved problem. I will describe the "ab-initio" numerical modeling of pulsar magnetosphere using relativistic "particle-in-cell" method. For aligned rotators plasma pulled from the surface of the star forms a compact charge-separated electrosphere which in general is not in corotation with the star. I will show that in 3D this configuration is unstable to a nonaxisymmetric diocotron instability. The instability results in efficient plasma transport across the magnetic field lines and drives the magnetosphere to the corotating Goldreich-Julian solution within several rotation periods.

Dave Strickland
Johns Hopkins

Hot Gas in the Halos of Star-forming Disk Galaxies: Quantifying Supernova Feedback at Work

Abstract: The hot phase of the ISM in star-forming galaxies is a product of the feedback of energy and mass from massive stars, back into the ISM they formed from. Understanding the complex gas-dynamical processes in feedback is vital for constructing true physical models of galaxy formation and evolution. I will briefly discuss the problems besetting purely theoretical approaches, before demonstrating a few examples of how Chandra observations, with their unparalleled ability to separate diffuse emission from unrelated X-ray point sources, finally allows a quantitative investigation of feedback processes on the galactic scale.

Licia Verde

Can Clusters of Galaxies be used as "Standard Candles"?

An extensive sample of galaxy clusters will be available in the coming years, detected through their Sunyaev-Zeldovich effect (SZE). We use a semi-analytic model to study the scientific yield of combining SZE data with X-ray and optical follow-up observations. If clusters at a given redshift z_o can be identified with virialized, spherical halos, they populate a well-defined "fundamental plane" (FP) in the parameter space of the three observables: x-ray temperature (T), total Sunyaev-Zeldovich flux decrement (DS), and angular size (theta). The location and orientation of the FP, as well as its redshift-evolution, are sensitive to both the internal evolution of clusters, and to the underlying cosmological parameters. We show that if clusters are not standard candles (e.g. due to feedback, or energy injection), then this can be inferred from the FP. Likewise, we study the dependence of the FP on the cosmological parameters h, sigma_8, and Omega_0, and quantify constraints on these parameters.

Jacco Vink

XMM-Newton RGS Spectra of SN1006 and RCW 86

Although dispersive spectrometers are mainly designed for spectroscopy of unresolved sources, in a number of cases they can be used to obtain high resolution spectra of extended objects. In the case of supernova remnants the Magellanic Cloud remnants are a case in point. Here, however, I would like to present new results based on XMM RGS data of the large Galactic supernova remnants SN1006 and RCW 86. Although the objects are very large, by pointing at small (< 1') "knots" (SN1006) and narrow features spectra have been obtained with a much better spectral resolution than CCD spectra. For SN1006 I will focus on the issue of the ion/electron temperature relaxation and present upper limits on the ion temperature. For RCW 86 the high resolution spectrum is used to derive constraints on the state of the plasma in relation to the morphology of this remnant (probably the result of a cavity explosion) and the unusual hard X-ray continuum, which is accompanied by Fe K emission at 6.4 keV.

Last modified: 08/24/11

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory.
60 Garden Street, Cambridge, MA 02138 USA.    Email: cxcweb@head.cfa.harvard.edu
Smithsonian Institution, Copyright © 1998-2004. All rights reserved.