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Chandra Fellows Symposium 2007

Harvard-Smithsonian Center for Astrophysics

October 10, 2007

Titles and Abstracts of Talks

Masahiro Tsujimoto
 Penn State

An X-ray Imaging Study of the Stellar Population in RCW49

We present the results of a high-resolution X-ray imaging study of the stellar population in the Galactic massive star-forming region RCW49 and its central OB association Westerlund 2. We obtained a 40 ks X-ray image of a 17'x17' field using the Chandra X-ray Observatory and deep NIR images using the Infrared Survey Facility in a concentric 8'3x8'3 region. We detected 468 X-ray sources and identified optical, NIR, and Spitzer Space Telescope MIR counterparts for 379 of them. The unprecedented spatial resolution and sensitivity of the X-ray image, enhanced by optical and infrared imaging data, yielded the following results: (1) The central OB association Westerlund 2 is resolved for the first time in the X-ray band. X-ray emission is detected from all spectroscopically-identified early-type stars in this region. (2) Most (86%) X-ray sources with optical or infrared identifications are cluster members in comparison with a control field in the Galactic Plane. (3) A loose constraint (2--5~kpc) for the distance to RCW49 is derived from the mean X-ray luminosity of T Tauri stars. (4) The cluster X-ray population consists of low-mass pre--main-sequence and early-type stars as obtained from X-ray and NIR photometry. About 30 new OB star candidates are identified. (5) We estimate a cluster radius of 6'--7' based on the X-ray surface number density profiles. (6) A large fraction (90%) of cluster members are identified individually using complimentary X-ray and MIR excess emission. (7) The brightest five X-ray sources, two Wolf-Rayet stars and three O stars, have hard thermal spectra.

Jan-Uwe Ness
 Arizona State University

Understanding the High-resolution X-ray spectra of RS Ophiuchi

The 6th recorded outburst of the recurrent nova RS Ophiuchi occurred in February 2006, and a total of 392 ksec of Swift, 150 ksec of Chandra, and 124 ksec of XMM-Newton were invested to obtain X-ray observations with dense coverage over the evolution (Swift) and with high spectral resolution (gratings of Chandra and XMM-Newton). During the three months of activity, the X-ray spectra have changed dramatically, and I introduce the spectra and discuss approaches to find appropriate spectral models. Of the three phases of evolution, the first one shows the spectrum from a shock between the expanding ejecta and the giant companion with an optically thin collisional plasma. The second phase is dominated by emission from an atmosphere around the white dwarf that emits a continuum spectrum with absorption lines which can only be modeled with radiative transport codes. The last phase represents the cooling surrounding plasma after nuclear burning has ceased, and these spectra can also be modeled with optically thin collisional spectra. The collisional models from the first and last phases yield elemental abundances which can be used to better constrain the atmosphere models, which yield a description of the structure of the outburst.

John Fregeau
 Northwestern University

The link between X-ray binaries and globular cluster dynamics

Although dense stellar systems such as globular clusters have long been known to be overabundant per unit mass in X-ray binaries, it is only recently that theory and observations have begun to converge on a basic understanding of the formation and evolution of such systems in dense stellar environments. The collision number $\Gamma$, derived from observable cluster quantities, offers a rough estimate of the expected number of X-ray sources in a given cluster, yet fails for some core-collapsed clusters. In this talk I will present the results of semi-analytical and numerical calculations of star cluster evolution that address this discrepancy, discuss the information on the recent dynamical history of clusters that can be inferred, and discuss the implications for our understanding of the current dynamical state of all Galactic globular clusters.

Carles Badenes
 Princeton University

No Smoke from the Gun: The Quest for Signatures of Type Ia Supernova Progenitors

The identification of the progenitor systems of Type Ia Supernovae remains one of the most pressing unsolved problems in stellar evolution. Decades of continuing effort have failed to find a clear 'smoking gun' that would single out what kind or kinds of systems contribute to the observed Type Ia SN rate, although some recent works provide very interesting hints. I will address this issue from the point of view of the (mostly X-ray) observations of young Type Ia Supernova Remnants in the Galaxy and the LMC. A comparative study of these objects can provide important clues that might help to solve the Type Ia progenitor mystery.

Jifeng Liu

How multiwavelength studies help to uncover the nature of superluminous supersoft X-ray sources in external galaxies

Observations of the Milky Way and Magellanic Clouds with Einstein and ROSAT suggest that luminous supersoft X-ray sources are white dwarfs that steadily or cyclically burn accreted matter on their surface. Observations of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources that are generally hotter and more luminous than their Galactic and Magellanic counterparts. Some of these have luminosities and temperatures consistent with white dwarfs approaching the Chandrasekhar mass and are promising type Ia supernova progenitors. Others have luminosities exceeding the Eddington luminosity for a white dwarf or a stellar mass black hole, and are possibly intermediate-mass black holes of a few thousand solar masses. Here I show how multiwavelength studies can help to differentiate the two models.

Jeremy D. Schnittman
 Johns Hopkins University

Self-consistent X-ray Spectra from Accreting Black Hole Binaries

We have developed a Monte Carlo ray-tracing code in the Kerr metric to calculate the observed spectra from accreting black hole binaries. Unlike previous efforts, here we track photons from the emission region to a distant observer, propagating through a corona of hot electrons, getting boosted in energy through inverse-Compton scattering. Some of these high-energy photons are scattered back to the disk, producing relativistically broadened fluorescent emission lines. Thus a single integrated model can account for all major features of the X-ray spectrum: the thermal peak, power-law slope, and iron emission line. These self-consistent spectral models will help interpret observations to measure black hole parameters such as mass, spin, accretion rate, and inclination, as well as the corona temperature and optical depth. As a first application of this ray-tracing model, we consider a steady state Novikov-Thorne disk with a non-zero torque at the inner-most stable circular orbit (ISCO), surrounded by a spherically symmetric corona. We show how various features of the X-ray spectra are sensitive to different model parameters and which of these parameters are degenerate. Finally, we discuss an extension to fully 3-dimensional MHD simulations where coronal heating is linked to physical attributes such as the local current density.

Shane Davis

The Impact of Magnetic Stresses and Inhomogeneities on Accretion Disk Spectra

I will review efforts by myself and my collaborators to improve our understanding of the emission from accretion flows in black hole X-ray binaries. Specifically, we are examining the role magnetic stresses and resulting inhomogeneities play in determining the vertical structure and emission from accretion disks, relying heavily on results from magnetohydrodynamic simulations. I will also discus the impact of these results on efforts to estimate black hole spin using the continuum spectrum.

Jonathan McKinney

How to form ultrarelativistic jets

We now understand the basic requirements for astrophysical systems, such as accreting black holes, to generate moderately relativistic outflows. Part of this knowledge comes from analytical and numerical models of relativistic outflows using the relativistic magnetohydrodynamical (MHD) approximation. A more difficult regime to understand is the ultrarelativistic limit, which tasks our fundamental grasp of the requirements to accelerate and collimate jets to the fastest bulk speeds in Nature. I discuss the astrophysical and environmental requirements to form ultrarelativistic jets as viewed from the MHD approximation and discuss insights garnered from theory and simulations.

Belinda Wilkes
 Smithsonian Astrophysical Observatory

Chandra's Role in the Changing Face of AGN

Chandra's broad-band and sensitive X-ray vision has expanded our view of Active Galaxies (AGN) deeper into the heart of the population. The relative lack of bias towards objects which are obscured in the optical and soft-X-ray is proving successful in discovering, along with more normal AGN, unusual ones that make up a significant fraction of the population. As well as the type 2 AGN and quasars expected from pre-Chandra models of the Cosmic X-ray Background (CXRB), the sources include other varieties: X-ray bright, normal galaxies (XBONGS), red, broad-lined AGN and low-luminosity AGN. Numbers of known sources remain sufficiently small that the relative importance of the various types remains to be determined. On-going, large area, medium-depth, multi-wavelength surveys will obtain sufficiently large samples to quantify this over the next few years.

The X-ray properties of the AGN population are now well-determined by existing deep and medium-deep surveys. The number counts (log N vs log S) and X-ray luminosity functions are constrained within several energy bands with the largest uncertainties being at high redshift. Cosmic X-ray Background (CXRB) modeling provides fairly strong constraints on the population as a function of obscuration level and redshift. While the numbers of unobscured and moderately obscured AGN are consistent between models and data, a comparable and largely undiscovered Compton-thick population is still required to reproduce observed CXRB flux levels. Although such sources are hard to find with Chandra because their emission below 10-30 keV is obscured, they are being found at long wavelengths, IR (Spitzer) and radio, or to higher energies, Gamma rays (Integral).

As we explore the full population for the first time, new AGN with various levels of obscuration open up new possibilities for us to study the geometry of their central regions and the material responsible for both obscuration and emission. Obscuration allows galaxy, reflected and scattered components to contribute significantly to the observed emission in these sources. Their observed spectral energy distributions (SEDs) are often very different from those of optically selected AGN. A large number of components are already known to contribute in low redshift, lower luminosity AGN. We find that the observed optical colors of our higher luminosity sources can largely be explained by a combination of these: an unobscured AGN SED, emission and obscuration from the host galaxy, polarised AGN emission in the optical/IR, nuclear obscuration and a combination of direct and reflected emission with variable obscuration and a soft excess in the X-ray. Principal Component Analysis also reveals that accretion rate, which determines the shape of the intrinsic AGN SED, explains about 30% of the variance, with narrow-line Seyfert 1 galaxies, known to have high accretion rates, systematically shifted to higher optical-to-X-ray ratios. This raises the question of how fundamental is orientation-based obscuration, which are the basis of the AGN-galaxy unification models, in determining our view of an AGN. This question is best addressed with low frequency, radio-selected AGN, such as 3CR, where orientation bias is minimised where initial results confirm a complex picture.

Elena Rossi
 Univ. of Colorado

Growing Supermassive Black Holes by z = 6 with "Quasistars

I present in this talk my study of the structure and evolution of ``quasistars," accreting black holes embedded within massive hydrostatic gaseous envelopes. These configurations may model the early growth of supermassive black hole seeds . The accretion rate onto the black hole adjusts so that the luminosity carried by the convective envelope equals the Eddington limit for the total mass (BH+envelope mass). This greatly exceeds the Eddington limit for the black hole mass alone, leading to rapid growth of the black hole. I present analytic models and numerical stellar structure calculations that study the structure and evolution of quasistars. I show that the photospheric temperature of the envelope decreases with time while the black hole mass increases and that it eventually hits a limiting value, analogous to the Hayashi track for red giants and protostars, below which no hydrostatic solution for the convective envelope exists. For metal-free (Population~III) opacities this limiting temperature is approximately 4000 K. After a quasistar reaches this limiting temperature, it is rapidly dispersed by radiation pressure. I find that black hole seeds with masses between 10^3 and 10^4 solar masses could form via this mechanism in less than a few Myr.

Elena Gallo
 UC Santa Barbara

AMUSE-Virgo: on the survival of super-massive black holes in faint spheroids.

I will present the first Chandra results from the AGN Multiwavelength Survey of Early-type galaxies in the Virgo cluster (AMUSE-Virgo). This program targets 100 early-type Virgo galaxies with Chandra ACIS-S and Spitzer MIPS, with the aim to provide an unbiased census of super-massive black hole (SMBH) activity in the local universe. The sample covers over 4 orders of magnitude in black hole mass as estimated from the mass-velocity dispersion relation, large enough that it can be divided in SMBH mass bins to test whether the nuclear activity duty cycle is mass dependent. I will report on the Chandra observations of the first 16 targets, combined with results from archival data of other, more massive, 16 targets. Hard X-ray emission from a position coincident with the galaxy nucleus is detected in 50 per cent of the galaxies, and ascribed to low-level accretion-powered activity from a SMBH. Two of the detected nuclei are hosted in galaxies with absolute B magnitudes fainter than -18, suggesting that supermassive black holes are still being harbored in such faint, low-mass objects.

Jesper Rasmussen
 Carnegie Observatories

Probing galaxy evolution in groups with Chandra and XMM

There is a long-standing argument about which processes are the key ones in modifying the morphology of galaxies in dense environments. Ram pressure stripping of galactic gas is generally assumed to be inefficient in small galaxy groups, mainly due to the low galaxy velocities in groups. In order to test this assumption and constrain the role of ram pressure stripping for galaxy evolution in these systems, we have performed a study of the X-ray emitting intragroup gas within a sample of nine galaxy groups. Eight of these are compact groups in which the member galaxies are surprisingly deficient in neutral hydrogen, thus representing cases where galaxy morphological transformations could be expected to be currently active. Combining the observations with analytical models of galaxies moving through the hot gas within the groups, I will present the first results of this study, and outline the implications for galaxy evolution in groups and clusters of galaxies.

David Sand
 Univ. Arizona

A systematic search for galaxy cluster supernovae at 0.1 < z < 0.2

The role of intracluster stars in the metal enrichment of the intracluster medium is unknown but possibly large since their resulting supernovae will inject metals directly into the ICM. In order to investigate the relative role that host versus hostless supernovae play in ICM metal enrichment, we have undertaken a large survey of ~50 X-ray selected galaxy clusters (0.1 < z < 0.2) using the 1 degree imager on the 2.3m Bok telescope. We have three principal goals: 1) determine the mean fraction of intracluster star light by observing the relative fraction of host versus hostless SN-Ia, 2) determine the overall SNe-Ia rate in our cluster sample sufficiently well to place clear constraints on the SN-Ia 'delay time', which is the time between the formation of a stellar system and the eventual explosion of some of its members as SN-Ia, and 3) combine these two measurements to determine the contribution of intracluster SNe to the global chemical enrichment of clusters. Here we present the initial results of the survey and future plans.

Elena Rasia
 Univ. Michigan

Galaxy Clusters as plasma physics laboratories and cosmological probes

Clusters of Galaxies are, simultaneously, ideal laboratories to study the physics of the IntraCluster Medium and to trace the large scale structure. The huge progress in recent years in simulations and observations allows us to investigate the potential of these clusters to probe both plasma astrophysics and cosmology.

With the availability of our package, X-MAS2, we studied in detail the possible systematic bias in X-ray metallicity measurements as well as the dependence of the M-Y_X relation from the dynamical state of the cluster.

Ian Parrish

The Magnetothermal Instability and its Application to Clusters of Galaxies

In many dilute astrophysical plasmas, the mean free path along magnetic field lines can be very large. As a result, thermal conduction is anisotropic and occurs only along magnetic field lines. In this regime, the condition for convective stability is significantly modified, resulting in the magnetothermal instability (MTI). We use the MHD code Athena combined with an anisotropic thermal conduction module to simulate this instability into the nonlinear regime. We demonstrate in three dimensions that the MTI can drive a magnetic dynamo and reduce large-scale temperature gradients. As an example of the MTI, we show simulations of the intracluster medium of clusters of galaxies. In these simulations we find a depression of the initial temperature gradients and an amplification of the seed magnetic field-both relevant to ICM physics and any attempts to use clusters for precision cosmology.

Last modified: 08/24/11

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