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Chandra Fellows Symposium 2009

Harvard-Smithsonian Center for Astrophysics

October 27-28 2009

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Titles and Abstracts of Talks

Title Links to Symposium Presentation

John Fregeau

White Dwarf Kicks in Star Clusters

Recent observations of the white dwarf (WD) populations in the Galactic globular cluster NGC 6397 suggest that WDs receive a kick of a few km/s shortly before they are born. Using our Monte Carlo cluster evolution code, which includes accurate treatments of all relevant physical processes operating in globular clusters, we study the effects of the kicks on their host cluster and on the WD population itself. We find that in clusters whose velocity dispersion is comparable to the kick speed, WD kicks are a significant energy source for the cluster, prolonging the initial cluster core contraction phase significantly so that at late times the cluster core to half-mass radius ratio is a factor of up to ~ 10 larger than in the no-kick case. WD kicks thus represent a possible resolution of the large discrepancy between observed and theoretically predicted values of this key structural parameter. Our modeling also reproduces the observed trend for younger WDs to be more extended in their radial distribution in the cluster than older WDs.

Rodrigo Fernandez
Institute for Advanced Study

Numerical Experiments in Core-Collapse Supernova Hydrodynamics

The explosion mechanism of core-collapse supernovae has defied theorists for almost 50 years. Despite substantial progress, there is still no robust path for exploding stars that form iron cores. An emerging consensus in the theoretical community is that the breaking of spherical symmetry appears to be crucial for success. In this talk I will review recent progress in the field, and the motivation for studying multidimensional hydrodynamic instabilities relevant to the explosion. This work is aimed at understanding these instabilities in terms of fundamental physics, in order to facilitate interpretation of results from more complete core-collapse calculations.

Ed Cackett
University of Michigan

Relativistic Lines and Reflection from the Inner Accretion Disks Around Neutron Stars

A number of neutron star low-mass X-ray binaries have recently been discovered to show broad, asymmetric Fe K emission lines in their X- ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating in the inner part of the accretion disk where strong relativistic effects can broaden emission lines. I will present a comprehensive, systematic analysis of Suzaku and XMM-Newton spectra of 10 neutron star low-mass X-ray binaries, all of which display broad Fe K emission lines. The Fe K lines are well fit by a relativistic line model for a Schwarzschild metric, and imply a narrow range of inner disk radii (6 - 15 GM/c^2) in most cases. This implies that the accretion disk extends close to the neutron star surface over a range of source states. Continuum modeling shows that for the majority of observations, a blackbody component (plausibly associated with the boundary layer) dominates the X-ray emission from 8-20 keV. Thus it appears likely that this spectral component produces the majority of the ionizing flux that illuminates the accretion disk. Therefore, we also fit the spectra with a blurred reflection model, wherein a blackbody component illuminates the disk. This model fits well in most cases, supporting the idea that the boundary layer is illuminating a geometrically thin disk.

Ian Parrish

Heating of Test Particles in Numerical Simulations of MHD Turbulence and the Solar Wind

We study the heating of charged test particles in three-dimensional numerical simulations of weakly compressible magnetohydrodynamic (MHD) turbulence ("Alfvenic turbulence''); these results are relevant to particle heating and acceleration in the solar wind, solar flares, accretion disks onto black holes, and other astrophysics and heliospheric environments. The physics of particle heating depends on whether the gyrofrequency of a particle is comparable to the frequency of a turbulent fluctuation that is resolved on the computational domain. Particles with these frequencies nearly equal undergo strong perpendicular heating (relative to the local magnetic field) and pitch angle scattering. By contrast, particles with large gyrofrequency undergo strong parallel heating. Simulations with a finite resistivity produce additional parallel heating due to parallel electric fields in small-scale current sheets. Many of our results are consistent with linear theory predictions for the particle heating produced by the Alfven and slow magnetosonic waves that make up Alfvenic turbulence. We discuss the implications of our results for solar and astrophysics problems, in particular the thermodynamics of the near-Earth solar wind. We conclude that Alfvenic turbulence produces significant parallel heating via the interaction between particles and magnetic field compressions (``slow waves''). However, on scales above the proton Larmor radius, Alfvenic turbulence does not produce significant perpendicular heating of protons or minor ions. We also will show some initial results of particle heating in turbulence driven by the magnetorotational instability (MRI) and discuss the relevance to accretion disks.

Prateek Sharma

Flares in Sgr A*

Flaring activity across different wavebands, from radio to X- rays, is observed from Sgr A*, the Galactic center black hole. I will describe our recent work on identifying flaring activity in global MHD simulations of hot accretion flows. The flares in numerical simulations arise from magnetic reconnection events, where significant magnetic energy is dissipated as internal energy. Simple model of particle acceleration and flare emission due to accelerated particles (mainly synchrotron) will be compared with the observations of a well- observed multi-wavelength flare. Typical magnetic field strength expected in the inner regions of Sgr A* (~ few 10s of Gauss; constrained by Faraday rotation measurement) is able to produce a synchrotron flare which matches observations.

Bence Kocsis

Gas Driven Massive Black Hole Binaries: Periodic Quasar Variability and the Gravitational Wave Background

Supermassive black hole binaries (SMBHBs) in galactic nuclei are thought to be a common by--product of major galaxy mergers. We use simple disk models for the circumbinary gas and for the binary-disk interaction to follow the orbital decay of SMBHBs through physically distinct regions of the disk, until gravitational waves (GWs) take over their evolution. Prior to the GW--driven phase, the viscous decay is generically in the slow ``secondary--dominated'' regime. In this talk, I will discuss possible observational implications of the gas driven decay. A population of periodically variable active galactic nuclei may be sufficiently common to be detectable during these stages. A statistical sample of these objects can be used to test the decay mechanism for these binaries. I will also demonstrate that the GW background might be significantly modified in the nHz frequency range, relevant for pulsar timing arrays.

Jeremy Schnittman
Johns Hopkins

X-ray Polarization: The Dawn of a New Age in Black Hole Astrophysics

In the coming decade, new space missions will be able to measure X-ray polarization at levels of 1% or better in the ~1-10 keV energy band. This region of the spectrum is critical for observing emission from pulsars, X-ray binaries, supernova remnants, and AGN. In particular, we propose that X-ray polarization may be an ideal tool for determining the nature of black hole accretion disks surrounded by hot coronae. Using a Monte Carlo radiation transport code in full general relativity, we calculate the spectra and polarization features of these black hole systems. At low energies, the signal is dominated by the thermal flux coming directly from the optically thick disk. At higher energy, the thermal seed photons have been inverse-Compton scattered by the corona, often reflecting back off the disk before reaching the observer, giving a distinctive polarization signature. By measuring the degree and angle of this X-ray polarization, we can infer the emission geometry of the accretion flow and in turn may be able to determine the spin of the black hole.

Orly Gnat
Cal Tech

Metal Absorption in Conductively Evaporating Clouds

I will present computations of the metal-absorption properties of thermally conductive interface layers, that may surround evaporating clouds embedded in a hot intergalactic medium. My models explicitly follow the time-dependent ionization equations, and include the effects of photoionization by the present-day metagalactic radiation field on the non-equilibrium ion fractions. I will focus on models for dwarf galaxy-scale clouds, and compare the predictions to the UV absorption column densities observed in local highly-ionized absorbers.

Julie McEnery

A New View of the High-Energy Gamma-ray Sky with the Fermi Gamma-ray Space Telescope

Following its launch in June 2008, high energy gamma-ray observations by the Fermi Gamma-ray Space Telescope have opened a new and important window on a wide variety of phenomena which include the discovery of a population of pulsars pulsing only in gamma-rays, confirmation of X-ray binaries as GeV gamma-ray emitters, detection of emission up to 10s of GeV from gamma-ray bursts, and measurement of the high energy cosmic-ray electron spectrum and the diffuse gamma-ray emission with unprecedented accuracy. Our continuous monitoring of the high-energy gamma-ray sky has uncovered numerous outbursts from blazars and the discovery of as-yet-unidentified transients from the direction of the Galactic plane. In this talk I will describe the current status of the Fermi observatory and review the science highlights from the first year of observations.

Nat Butler

Probes to the Early Universe

I will discuss observational and instrumentation efforts to use Gamma-ray Bursts (GRBs) as tracers of cosmic star formation and as probes to the high redshift (z>6) Universe. At X- and Gamma-ray energies, the uniquely piercing GRB itself sheds light on the rate and nature of GRB production directly, provided the detection process is sufficiently well understood. I will discuss modelling efforts using Swift and other satellite data to constrain the GRB rate density and luminosity function. On the experimental front, I am leading the effort to build a simultaneous 6-color imaging camera called RATIR (http://ratir.org) -- the Reionization and Transients InfraRed camera -- which will be mounted on an existing 1.5m telescope in Baja, CA for 2 years starting in 2010. RATIR will point to GRB satellite locations within minutes and identify z>6 GRBs for larger aperture followup via the attenuation of the GRB light (in the r,i,Z,Y,J, and H bands) by the IGM.

Brian Metzger

Short-Duration Gamma-ray Burst Central Engines

One of the most important discoveries made with Swift is that long and short-duration gamma-ray bursts (GRBs) originate from distinct stellar progenitors. While long GRBs track ongoing star formation and result from the deaths of massive stars, short GRBs have been localized to both early and late-type galaxies, suggesting a more evolved progenitor population. Although the origin of short GRBs remains a mystery, the most popular and well-studied model is accretion following the merger of neutron star binaries. This model is qualitatively consistent with both the demographics of short GRBs and the lack of a bright associated supernova in some cases. Despite these successes, this picture has grown complex with the discovery that short GRBs are often followed by a "tail" of emission (usually soft X-rays) lasting 100 seconds after the burst. Such energetic, late-time emission from the central engine is difficult to explain in standard merger pictures. One proposed explanation is late-time "fall-back" onto the black hole of material that was ejected during the merger into highly-eccentric, marginally-bound orbits. As this matter decompresses from nuclear densities, however, it undergoes rapid-neutron capture (r-process) nucleosynthesis, which can release energy comparable to the orbital binding energy. This implies that the r-process (normally thought unimportant dynamically in astrophysical contexts) has important implications for the quantity and time-dependence of fall-back and, ultimately, the source of flaring and identity of the central engine.

Eran Ofek
Cal Tech

On the Nature of Long-Duration Radio Transients

Recently, a new class of 5-GHz radio transients with durations of order hours to days, lacking any visible-light counterparts, have been detected. I will present new observations and searches for such transients, and discuss their nature.

Vasiliki Pavlidou
Cal Tech

Deciphering the Gamma-ray Background: Starforming Galaxies, AGN and the search for Dark Matter in the GeV Band

The recently launched Fermi Gamma-ray Space Telescope promises a decade of excitement and discovery in the GeV Band. While Fermi represents a dramatic improvement in instrumental capabilities for point source observations in GeV gammas compared to its predecessors, much of GeV science will still be encoded in the unresolved, diffuse background, due to restrictions in point source sensitivity and angular resolution inherent in GeV energies. I will discuss the two guaranteed astrophysical contributions to the isotropic diffuse gamma- ray background (starforming galaxies and gamma-ray--loud AGN), and our efforts to use diffuse GeV observations to understand the high-energy properties of these familiar populations. Finally, I will describe some promising techniques for disentangling unresolved diffuse emission from different source classes and circumventing the difficulties presented by astrophysical backgrounds to hopefully uncover a dark matter annihilation signal which may be hiding among the diffuse GeV photons.

Kevin Schawinski

The Co-Evolution of Black Holes and their Host Galaxies

In order to understand the co-evolution of galaxies and the supermassive black holes at their centers, we need to know which galaxies are more likely to host active black holes and how they differ systematically from their normal counterparts, if they do at all. I present some recent results based on data from the Sloan Digital Sky Survey and the Galaxy Zoo project on the properties of AGN host galaxies in the local Universe. I show that early- and late-type galaxies with active black holes form a very different sub-set of their parent population and that in turn, the role of the AGN phase in the evolution of the host is very different. While we have a relatively good understanding of the role of the AGN phase in early-type galaxies, the triggering and role of AGN in late-type galaxies is less clear.

Ezequiel Treister

The Co-Evolution of Black Holes and their Host Galaxies

We constrain the number density and evolution of Compton-thick (CT) Active Galactic Nuclei (AGN). In the local Universe we use the wide area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on a combination of X-ray and mid-IR parameters. To find CT AGN at high redshifts we study the properties of a sample of 211 objects in the Extended Chandra Deep Field-South selected based on their very high mid-IR to optical flux ratios. We present significant evidence supporting the CT AGN nature of a large fraction of these sources, including a strong stacked X-ray signal. By performing spectral fitting to the rest-frame UV/optical light (dominated by the host galaxy) we found evidence for a significant young stellar population, indicating that these sources are experiencing considerable star-formation. Using this sample we measured the space density of CT AGN at z~2, finding a strong evolution in the number of high-luminosity sources from z=1.5 to 2.5. Such strong evolution was not predicted by any existing AGN luminosity function, but can be accurately explained by a simple prescription in which every new quasar is generated by a major merger of two gas-rich massive galaxies, which is originally heavily obscured and after ~100 Myrs removes most of the surrounding gas and dust to reveal an unobscured quasar.

Bret Lehmer
Johns Hopkins

New Results from the Chandra Deep Protocluster Survey

It is now widely accepted that the growth of galaxies and their central supermasive black holes (SMBHs) are linked. Observations and CDM theories of large scale structures indicate that galaxy growth is accelerated in high-density environments, and the highest density structures underwent their most vigorous star formation at z > 2--3. To study the corresponding growth of SMBHs in such high-density environments, we have conducted a deep ~400 ks Chandra survey covering the SSA22 protocluster at z=3.09: the Chandra Deep Protocluster Survey. The protocluster itself contains a factor of ~6 overdensity in galaxies (i.e., LBGs and LAEs) and is predicted to collapse into a z=0 cluster resembling a rich local cluster (e.g., Coma). I will report on the current status of the Chandra Deep Protocluster Survey and will highlight two recent investigations that make use of the new Chandra data. These investigations include (1) a study of how the growth of galaxies and SMBHs depends on environment in the z=3 Universe, and (2) a comprehensive explorationof the role that AGN and star-formation activity play in the production of enigmatic extended (>30 kpc) Lyman-alpha emitting blobs (LABs) that are found in excess in the protocluster.

Eduardo Rozo

Cosmological Constraints from the SDSS maxBCG Cluster Catalog

The maxBCG cluster catalog is a volume limited cluster catalog of optically selected clusters drawn from the SDSS photometric data. We present the constraints that the observed cluster abundance function places on cosmological parameters, and demonstrate that these results are in excellent agreement and competitive with X-ray cluster abundance measurements. As in X-ray studies, our errors are dominated by systematic uncertainties, and we will discuss ongoing efforts to mitigate these. The lessons learned here should prove to be an invaluable stepping stone for upcoming photometric surveys such as the Dark Energy Survey (DES).

Matt McQuinn

A New Constraint on the Intergalactic HeII Fraction at z~3

I will discuss three different ways to detect and study the reionization of helium in the intergalactic medium. First, I will show that temperature inhomogeneities imprinted by this process may be detectable in measurements of the HI Lyman-alpha forest. Second, I will argue that one can still infer HeII fractions of the order of unity from the HeII Lyman-alpha forest even though this absorption saturates for HeII fractions of 10^-3. Third, I will propose a new and more direct probe of helium reionization -- 8.7 GHz hyperfine absorption from intergalactic ^3HeII.

Jesper Rasmussen
Carnegie Observatory
Witnessing the Formation of a Brightest Cluster Galaxy

The central dominant galaxies in galaxy clusters constitute the most massive and luminous galaxies in the Universe. Despite this, the formation of these brightest cluster galaxies (BCGs) and the impact of this on the surrounding cluster environment remain poorly understood. I will discuss recent multi-wavelength observations of the nearby cluster MZ 10451, in which both processes can be studied in unprecedented detail. The core of this low-mass X-ray cluster harbors two optically bright early-type galaxies in the process of merging. Using Chandra observations of the cluster core, we can determine the nature of the merger and, for the first time, obtain a detailed picture of how such mergers may have affected the thermodynamics of baryons in cluster cores throughout cosmic history. I will discuss the first results of this study, along with some implications for our understanding of BCG formation.

Aurora Simionescu

Metal Transport by Cold Fronts in M87

We used Suzaku's low background and good collecting area to observe a prominent cold front in the outskirts of M87 in the nearby Virgo cluster. Observations of the center of M87 with Chandra and XMM-Newton have already revealed important information about the physical processes in cool-core clusters, and this large-scale feature towards the outskirts presents an optimal target to extend our detailed knowledge of M87. We show accurate temperature and metallicity profiles across this cold front and prove that it is also associated with a discontinuity in the chemical composition. The gas on the inner, bright region of the front is more abundant in Fe than the gas outside the front, while the Mg abundance remains similar between the two regions, indicating a drastic change in the relative contributions by type Ia vs. core-collapse supernovae. We discuss the implications of variations in the Mg/Si/Fe ratios in terms of currently available supernova yield models. We also present preliminary hydrodynamic simulations of gas sloshing in the M87 gravitational potential to deduce details about the formation of the cold front and about the role of the cold front in transporting and distributing metals in the ICM.

Norbert Werner

Small-Scale Turbulence in Giant Elliptical Galaxies

The dense cores of X-ray emitting gaseous halos of large elliptical galaxies with temperatures below about 0.8 keV show two prominent Fe XVII emission features, which provide a sensitive diagnostic tool to measure the effects of resonant scattering. We present here high-resolution spectra of five bright nearby elliptical galaxies, obtained with the Reflection Grating Spectrometers (RGS) on the XMM-Newton satellite. The spectra for the cores of four of the galaxies show the Fe XVII line at 15.01 Angstrom being suppressed by resonant scattering. The data for NGC 4636 in particular allow the effects of resonant scattering to be studied in detail and to prove that the 15.01 Angstrom line is suppressed only in the dense core and not in the surrounding regions. Using deprojected density and temperature profiles for this galaxy obtained with the Chandra satellite, we model the radial intensity profiles of the strongest resonance lines, accounting for the effects of resonant scattering, for different values of the characteristic turbulent velocity. Comparing the model to the data, we are able for the first time to directly deduce upper limits on turbulent velocities. We find that the isotropic turbulent velocities on spatial scales smaller than about 1 kpc are less than 100 km/s and the turbulent pressure support in the galaxy core is smaller than 5% of the thermal pressure at the 90% confidence level, and less than 20% at 95% confidence. Furthermore, I will discuss the prospects of using observations of resonant scattering and turbulent velocity broadening of emission lines with Astro-H and IXO, which will produce a real breakthrough by extending these measurements to higher mass systems and large radii.

Uri Keshet

Cold Fronts and Spiral Flows in Galaxy Clusters

In recent years, spiral patterns were detected in several cool core galaxy clusters (CCCs), through cold fronts and thermodynamic or chemical inhomogeneities. I will show that cold fronts directly gauge the local flow, revealing bulk tangential velocities that constitute a considerable fraction of the sound speed. I will argue that spiral flows are ubiquitous in CCCs, and point out some properties and implications of such flows. For example, spiral inflows could provide a natural, self regulating solution to the cooling problem in CCCs and large galaxies.

Last modified: 10/13/09

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