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Chandra Fellows Symposium 2001

Harvard-Smithsonian Center for Astrophysics

October 15, 2001

Titles and Abstracts of Talks

Eric Agol

"Quasar Microlensing: Polarization and Iron Lines as a Probe of Accretion Disk Physics"

Accretion disk models for quasars require strong magnetic fields for transport of angular momentum, allowing gas to accrete and creating magnetic activity which may result in X-ray emission. One consequence of near-equipartition fields is that thermal emission from the accretion disk should be unpolarized due to strong Faraday depolarization in the accretion disk atmosphere. We discuss a means to measure the intrinsic disk polarization using microlensing of a quasar to remove contamination of polarized light created exterior to the disk. X-ray emission near a disk can produce fluorescence from the disk, which if microlensed by a caustic, creates a distinctive signature. We describe predictions for the microlensing of a relativistic disk emission line by a caustic, and discuss recent Chandra observations of the lens MG 0414+0534 which show evidence for microlensing of a narrow iron line, which might be the outer edges of a relativistically broadened line.
Plot Animation

Elizabeth L. Blanton
University of Virginia

"The Interaction of Radio Sources and X-ray-Emitting Gas in Cluster Cooling Flows"

Recent Chandra observations of cooling flow clusters containing central radio sources reveal an anti-correlation between radio and X-ray emission. Abell 2052 is one such cluster that exhibits this morphology. The cD galaxy at the center of Abell 2052 is host to the powerful radio source 3C 317. "Holes" in the X-ray emission are coincident with the radio lobes which are surrounded by bright "shells" of X-ray emission. Heating by central radio sources has been proposed as one solution to the "missing gas" in cooling flows -- there is a lack of gas detected in the X-ray at temperatures at or below approximately 1 keV. However, the gas surrounding the radio source in Abell 2052 is cool. The data are consistent with the radio source displacing and compressing, and at the same time being confined by, the X-ray gas. The compression of the X-ray shells appears to have been relatively gentle and, at most, slightly transonic. The pressure in the X-ray gas (the shells and surrounding cooler gas) is approximately an order of magnitude higher than the minimum pressure derived for the radio source, suggesting that an additional source of pressure is needed to support the radio plasma. The compression of the X-ray shells has speeded up the cooling of the shells, and optical emission line filaments are found coincident with the brightest regions of the shells.

Markus Boettcher
Rice University

"Blazar Broadband Modeling and Observations"

I review our recent work on multi-frequency, multi-epoch modeling of blazars (i.e. flat-spectrum radio quasar and BL Lac objects), in particular our recent work on the quasar 3C279 and BL Lacertae. We will see that radio-selected BL Lac objects (such as BL Lacertae itself) are particularly interesting targets of X-ray observations because they may offer a unique opportunity to probe the high-energy portion of the relativistic electron population in extragalactic jets in cases where external photon fields are playing a non-negligible role for the radiation physics and electron dynamics. In the near future, it will be possible to coordinate such X-ray observations with high-energy gamma-ray observations in the multi-GeV regime by ground-based air Cherenkov telescopes and the soon-to-be-launched GLAST satellite. I will discuss how such combined spectral and variability measurements of radio-selected BL Lac observations will help to constrain physical parameters of this class of blazars.
In view of the exciting prospects of spectral and variability measurements of radio-selected BL Lac objects, we have carried out a compre- hensive multiwavelength observing campaign of BL Lacertae in the fall of 2000, coordinating radio, optical, X-ray, and gamma-ray observations. The first results of this campaign are now becoming available and will be presented and discussed in this talk. Radio-selected BL Lac objects are an interesting class of blazars, with physical properties intermediate between radio quasars and high- frequency-peaked (or X-ray selected) BL Lac objects. The different sub-classes of blazars present an apparently continuous sequence in their physical properties, the origin of which is not understood at this time. We have recently proposed an evolutionary scenario, which allows us to unify all blazar sub-classes in terms of gradual depletion of the circumnuclear region of quasars, which may even provide a link to the class of radio-quiet quasars (at the high-density end of the sequence) and non-active giant elliptical galaxies (on the low-density end of the sequence).

Ming Feng Gu

"A Program for Calculating Atomic Properties and Its Application in X-ray Spectroscopy"

The Flexible Atomic Code (FAC) is an integrated software package for the calculation of various atomic properties, such as radiative transition rates, cross sections of electron impact excitation and ionization, photoionization, autoionization, and their inverse processes, radiative and dielectronic recombination. The program is fully relativistic based on the Dirac equation. The atomic structure is solved in the configuration mixing approximation with the basis states constructed using the Dirac-Fock-Slater method. The continuum processes are treated in the distorted-wave approximation, which is valid for highly ionized ions frequently encountered in the plasma modeling for X-ray spectroscopy. Special attention is paid to the convergence of partial wave summation and the large distance contributions to the continuum radial integrals. The implementation of the factorization-interpolation procedure increases the efficiency of the program considerably, especially for large scale calculations. A flexible scripting user interface makes it easy to specify the input and schedule the computation, and therefore, allows typical calculations to be carried out with minimal understanding of theoretical atomic physics. The program should be useful in providing atomic data for the plasma modeling under a variety of physical conditions. In this talk, I will briefly discuss the theoretical background and the structure of the program. I will also present some simple example calculations to illustrate its practical usage, and compare the results with previously published work.

Jeremy Heyl

"So What Is the Weather Like on Aquila X-1?"

During a Type-I burst, the turbulent deflagation front may excite waves in the neutron star atmosphere with frequencies of about one cycle per second. On the Earth the analogs to these waves are the Rossby waves which distort the path of the jet stream. On a neutron star, these waves are observed as highly coherent flux oscillations during a Type-I burst. The frequencies of these waves change as the upper layers of the neutron star cool which accounts for the small variation in the observed QPO frequencies. In principle several modes could be excited but the fundamental buoyant r-mode exhibits significantly larger variability for a given excitation than all of the other modes.

Jimmy Irwin
University of Michigan

"The Chandra View of the Stellar and Gaseous X-ray Components of Early-type Galaxies"

The sub-arcsecond spatial resolution of the Chandra X-ray Observatory has paved the way for the study of the X-ray binary population of nearby galaxies, which in early-type systems are primarily low-mass X-ray binaries (LMXBs). Here, I present some initial results on the spectral characteristics and luminosity distribution function of the LMXBs in a sample of early-type galaxies. For the galaxies studied so far, there is either a break or cut-off in the luminosity function at a luminosity of ~2e38 ergs/s, which is approximately the Eddington luminosity of a 1.4 solar mass accreting neutron star. If this break is a universal feature of galaxies, it might be used as a distance estimator. At lower X-ray luminosities, the LMXB population is remarkably similar from galaxy to galaxy, and I discuss a method of using this similarity to further constrain the distances to these galaxies. Finally, I discuss the implications of the tendency of LMXBs to be preferentially located within globular clusters, and also the importance of hot X-ray-emitting gas in these galaxies.

Li-Xin Li
Harvard University

"Disk Powered by a Black Hole" When a Kerr black hole is connected to a disk rotating around it by a magnetic field, the rotational energy of the Kerr black hole provides an energy source for the radiation of the disk in addition to disk accretion. If the black hole rotates faster than the disk, energy and angular momentum are extracted from the black hole and transfered to the disk. The energy deposited into the disk is eventually radiated away by the disk, which will increase the efficiency of the disk. Interestingly, a disk magnetically coupled to a rapidly rotating black hole can radiate without accretion. For such a non-accretion disk the radial radiation profile is very different from that of a standard accretion disk: the emissivity index is significantly bigger, most radiation comes from a region which is closer to the center of disk. Possible relevance of the model to observations will be briefly discussed.

Andisheh Mahdavi
University of Hawaii

"The Relationship Between X-ray Luminosity and Velocity Dispersion for Galaxies and Clusters of Galaxies"

We demonstrate that individual elliptical galaxies and clusters of galaxies form a continuous X-ray luminosity---velocity dispersion (Lx - Sigma) relation. Our samples of 280 clusters and 57 galaxies have Lx proportional to Sigma4.4 and Lx proportional to Sigma10, respectively. This unified Lx - Sigma relation spans 8 orders of magnitude in Lx and is fully consistent with the observed and theoretical luminosity---temperature scaling laws. Our results support the notion that galaxies and clusters of galaxies are the luminous tracers of similar dark matter halos.

Kristen Menou
Princeton University

"The Merger History of Supermassive Black Holes in Galaxies"

The ubiquity of supermassive black holes (SMBHs) at the centers of nearby luminous galaxies can arise from the multiple mergers experienced by dark matter halos in hierarchical structure formation models, even if only a small fraction of these galaxies harbor SMBHs at high redshifts. We illustrate this possibility using cosmological Monte Carlo simulations of the merger history of dark matter halos and their associated SMBHs. In our most extreme models, in order to populate nearly every bright galaxy with a SMBH at z=0, only a few percent of the halos with virial temperatures above 104 K are required to harbor a SMBH at high redshift. This possibility must be included in studies of the luminosity function and the clustering properties of quasars. We predict the number of SMBH merger events that are detectable by the gravitational wave experiment LISA, as a function of redshift, out to z=5. Although the event rates can be significantly reduced in scenarios with rare SMBHs, a minimum of ~10 detectable merger events per year is predicted if SMBH binaries coalesce efficiently. The observed distribution of events with redshift could yield valuable information on the SMBH formation process. If SMBH binaries do not coalesce, we find that at least several SMBH slingshot ejections probably occurred from z=5 to the present in each galaxy more massive than ~1011 Msun at z=0. Although our results are sensitive to the minimum cooling mass assumed for the formation of SMBHs, we expect the qualitative predictions of our models to be robust.

Erik Reese
University of California at Berkeley

"Determining the Cosmic Distance Scale with Galaxy Clusters"

Analysis of Sunyaev-Zel'dovich effect (SZE) and X-ray data from a galaxy cluster provides enough information to determine the distance to the cluster. We determine distances to a sample of 18 galaxy clusters from a maximum likelihood joint analysis of our own 30 GHz interferometric SZE observations and publicly available X-ray data. We quantify the statistical and systematic uncertainties inherent to these direct distance measurements, and we determine constraints on the Hubble parameter for various cosmologies. These distances imply a Hubble constant of 60 +4/-4 +14/-19 km s-1 Mpc-1 for the currently favored Omega_M = 0.3, Omega_Lambda = 0.7 cosmology, where the uncertainties correspond to statistical followed by systematic at 68% confidence. With a sample of 18 clusters, systematic uncertainties clearly dominate. The systematics are observationally approachable and will be addressed in the coming years through the current generation of X-ray satellites (Chandra & XMM-Newton) and available radio observatories (OVRO, BIMA, & VLA). Analysis of high redshift clusters detected in future SZE and X-ray surveys will allow a determination of the geometry of the universe from SZE determined distances, providing a distance ladder independent check on recent constraints on the cosmology of the universe.
(pdf version)

Masao Sako

"Inflowing and Outflowing Regions in AGN"

High resolution soft X-ray spectra of Seyfert galaxies obtained with the grating spectrometers onboard Chandra and XMM-Newton are providing us with a detailed view of the circumnuclear regions surrounding the supermassive black hole. In Seyfert 1 galaxies, where the central continuum radiation source is viewed directly, accurate velocity and column density measurements of the outflowing gas along the line of sight are now possible, thereby allowing us to infer the properties of the intrinsic radiation spectrum produced in the vicinity of the black hole. The X-ray spectra of Seyfert 2 galaxies, on the other hand, are dominated by discrete line emission, presumably formed in the same outflowing gas that imprints absorption features in Seyfert 1 spectra. I will show some recent results obtained with Chandra and XMM-Newton, and discuss a few examples where the newly acquired X-ray spectra are challenging our conventional understanding of the physical conditions in these systems.

Dave Strickland
Johns Hopkins University

"Early Results from a High Resolution X-ray Survey of Violent Galactic Outflows"

With an order-of-magnitude increase in spatial resolution over any previous or other current X-ray instrumentation, the Chandra X-ray Observatory is revolutionizing our understanding of the energetic gas in starburst-driven galactic outflows. I will present early results, both imaging and spectroscopic, from an ongoing survey of local edge-on L*-luminosity starburst galaxies using Chandra, discussing some of our work on the first 6 galaxies in the sample: NGC 253, M82, NGC 3079, NGC 3628, NGC 4631 and NGC 4945.

Jacco Vink
Columbia University

"Evidence for Non-thermal Bremsstrahlung from the Supernova Remnant MSH14-63/RCW 86"

I will present the results of an analysis of XMM-Newton observation of the supernova remnant MSH 14-63 (also known as RCW 86 and G315.4-2.3). The X-ray emission from this object is peculiar (as already observed by ASCA), in that there are very distinct hard X-ray and soft X-ray emitting regions. The lack of line emission from the hard X-ray emitting regions has been attributed to X-ray synchrotron emission, but the detection of Fe K emission at 6.4 keV, and the lack of spatial correlation between hard X-ray and radio emission is evidence against a dominant X-ray synchrotron component. Instead I will argue that the hard X-ray continuum is best explained by non-thermal bremsstrahlung from a supra-thermal tail to an otherwise cool electron gas. The existence of low electron temperatures, required to explain the absence of line emission and to explain the low charge state of iron, is supported by low temperatures found in other parts of the remnant, which are as low as 0.2 keV in some regions.

Rudy Wijnands

"A Chandra Observation of the Neutron Star X-ray Transient KS 1731-2600 in Quiescence:
Too cold a Neutron Star?"

The neutron-star X-ray transient KS 1731-260 was discovered in August of 1989 and until recently, the source was persistently bright in X-rays. In January of 2001, the source suddenly returned to quiescence again after more than a decade of actively accreting. I present the results of a Chandra/ACIS-S observation of KS 1731-260 performed just a few weeks after this transition. The source was detected in quiescence at a luminosity and a blackbody temperature which are very similar to those obtained for the normal short-duration neutron-star X-ray transients. If the prolonged accretion in KS 1731-260 had heated the neutron star to a higher temperature than those obtained for the neutron stars in ordinary transients, then a higher luminosity and temperature would be expected (assuming that KS 1731-260 is very similar to those other systems in all other aspects, like age and recurrence time of the outbursts). I will discuss these results in the context of the quiescent X-ray emission mechanisms (such as cooling models, residual accretion in quiescence) in neutron-star X-ray transients.


Last modified: 08/24/11

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