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Chandra Fellows Symposium 2000

Harvard-Smithsonian Center for Astrophysics

October 23, 2000

Titles and Abstracts of Talks

Eric Agol
Cal Tech

"The Eddington Ratio in Seyfert Galaxies"

Using the correlation between black hole mass and velocity dispersion of the host galaxy bulge in nearby galaxies, we estimate the masses of a dozen or so Seyfert galaxies. Due to uncertainties in the velocity dispersion measurements, the inferred masses are typically accurate to only ~0.5 dex. We compute the Eddington ratio, L/LEdd, assuming the emission is isotropic. We find that the Eddington ratio is between L/LEdd ~10-3-0.3 for Seyferts. Two narrow-line Seyfert 1s have L/LEdd ~10-2, not L ~ LEdd as is frequently claimed in the literature. The radio galaxy 3C 120 appears to be super-Eddington at one epoch, indicating that it may be beamed.

Amy Barger
University of Hawaii

"The Nature of the Hard X-ray Background Sources"

With recent Chandra observations of the SSA13 field, at least 75% of the hard X-ray background is now resolved into discrete sources. We have made deep optical, near-infrared, submillimeter, and radio observations of the SSA13 X-ray sources. In this talk I will discuss the properties of the X-ray sources and the implications for the growth of massive black holes.

Markus Boettcher
Rice University

"Coupled Fokker-Planck/Monte-Carlo Simulations of X-ray Spectra and Variability of Accreting Compact Objects"

Our time-dependent Monte-Carlo radiation transport code for hot, ionized plasmas has been coupled to an implicit Fokker-Planck code to self-consistently follow electron and radiation spectra in non-stationary situations. Effects of Compton scattering, Coulomb scattering, elastic lepton- lepton scattering, second-order Fermi acceleration, and bremsstrahlung and cyclotron/synchrotron emission and ab- sorption are taken into account. I will briefly review the features of this code and then discuss applications to stationary situations relevant to accretion onto weakly magnetized neutron stars, and simulations of flaring scenarios in accretion-disk/corona models.

David Buote
University of California at Santa Cruz

"Oxygen Absorption in Cooling Flows"

We summarize the discovery of oxygen absorption and warm (105 - 106 K) gas in cooling flows. Special attention is given to new results for M87 for which we find the strongest evidence to date for ionized oxygen absorption in these systems. We briefly discuss implications for observations of cooling flows with Chandra and XMM.

Tiziano Di Matteo

"Accretion onto Nearby Supermassive Black Holes and Chandra Constraints on the Dominant Cluster Galaxy NGC 6166"

Chandra observations of low-luminositiy supermassive black holes in nearby elliptical galaxies provide the tightest limits on both their nuclear luminosities and on their Bondi accretion rates. We examine Chandra constraints on NGC 6166, the dominant galaxy in the cluster Abell 2199, which hosts a ~ 109 solar mass black hole. We measure a nuclear X-ray luminosity Lx,1 keV ~ 1040 erg/s and show that the density and temperature profiles of the hot interstellar medium imply a Bondi accretion rate of ~3 × 10-3 solar masses per year. This accretion rate would predict a nuclear luminosity of ~1043 erg/s for a canonical accretion radiative efficiency of 10%. Advection dominated accretion flow models require Bondi accretion rates of ~10 to explain the central X-ray source luminosity. Mass loss or convection, which may be important in such flows, would imply even lower X-ray luminosities. Unless the Bondi estimate is inappropriate, the nuclear power output from NGC 6166 is likely to be dominated by jet emission. We discuss how jets may provide a feedback mechanism for limiting the gas infall onto the central black holes.

Ann Esin
Cal Tech

"Evidence for Truncated Thin Disk in Low State of New X-Ray Nova XTE J1118+480"

Based on recent multiwavelength observations of the new X-ray nova XTE J1118+480, we can place strong constraints on the geometry of the accretion flow in which a low/hard state spectrum, characteristic of an accreting black hole binary, is produced. We argue that the absence of any soft blackbody-like component in the X-ray data implies the existence of an extended hot optically-thin region, with the optically-thick cool disk truncated at some radius R. We show that such a model can indeed reproduce the main features of the observed spectrum: the relatively high optical to X-ray ratio, the sharp downturn in the far UV band and the hard X-ray spectrum. The absence of the disk blackbody component also underscores the requirement that the seed photons for thermal Comptonization be produced locally in the hot flow, e.g. via synchrotron radiation. We attribute the observed spectral break at ~2 keV to absorption in a warm, partially ionized gas.

Ming Feng Gu

"Experimental Studies of X-ray Line Formation In Iron L-shell Ions: Resonant Processes and Direct Excitation"

The unprecedented spectral resolution of the newly launched satellite observatories, Chandra and XMM-Newton, is capable of revealing very complicated discrete spectroscopic features in the entire soft X-ray band (0.1-10 keV). Among these features, line emission from iron L-shell ions, Fe XVII through Fe XXIV at about 1 keV is of particular importance due to the high emissivity and rich diagnostic potential of these charge states. Quantitative analyses of such spectroscopic data require a thorough understanding of the physical processes responsible for the line emission. Using the electron beam ion trap at Lawrence Livermore National Laboratory, we investigate experimentally various line formation processes that are responsible for iron L-shell line emission. The cross sections of these processes are measured for relatively strong 3 to 2 and 4 to 2 lines in Fe XXI-XXIV over a range of collision energies extending from well below the direct excitation thresholds to about 4 times the ionization potential of the ions, including the contributions of resonant processes.

The comparison with theoretical results shows that the distorted wave approximation is reasonably accurate in the energy range free of resonant processes. For the transitions investigated in our experiment, resonant excitation is shown to have minor effects on the line emission rate coefficients for thermal plasmas in collisional ionization equilibrium. The dielectronic recombination satellites can, however, enhance the rate coefficients by about 5-15 % at temperatures where the ion abundance peaks in collisional ionization equilibrium. For plasmas that have ionization temperatures much larger than electron kinetic temperatures, these resonant processes play an increasingly important role.

Jeremy Heyl

"Neutron Star Cooling for High B Fields"

XMM-Newton and Chandra are giving us a detailed picture of the radiation from isolated neutron stars. The thermal radiation from these objects may potentially constrain our understanding of nuclear physics in a realm inaccessible to terrestrial experiments. To translate the observed fluxes from neutron stars into constraints, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. I describe models of the thermal structure of the envelopes of neutron stars with magnetic fields up to 1016 G and the effect of a decaying magnetic field.

Jimmy Irwin
University of Michigan

"First Chandra Results From X-ray Faint Early-type Galaxies"

I will present results from Chandra observations of two nearby early-type systems that have unusually low X-ray-to-optical luminosity ratios: the elliptical galaxy NGC4697 and the bulge-dominated Sa galaxy NGC1291. The superior spatial resolution of Chandra is allowing us for the first time to separate cleanly the gaseous X-ray component from the stellar X-ray component (primarily from low mass X-ray binaries). The X-ray emission from both galaxies appears quite similar. Roughly half of the X-ray emission is resolved into point sources, and the presence of a low temperature (~0.3 keV) interstellar medium is detected. Several supersoft sources were found in both galaxies. I will also present the luminosity distribution function of the point sources found in each galaxy, and describe how it might be used as a distance estimator.

Kristen Menou
Princeton University

"Viscosity Mechanisms in Accretion Disks"

The self-sustained turbulence which develops in magnetized accretion disks is suppressed in the weakly-ionized, quiescent disks of close binary stars. Because accretion still proceeds during quiescence, another viscosity mechanism operates in these systems. An anticorrelation of the recurrence times of SU UMa dwarf novae with their mass ratio supports spiral waves or shock-waves tidally induced by the companion star as the main process responsible for accretion in the quiescent disks. Other weakly-ionized gaseous disks in systems lacking a massive companion have to rely on yet another transport mechanism or they could be essentially passive.

Edward Moran
University of California, Berkeley

"Seyfert 2 Galaxies: Unification and the X-ray Background"

It is frequently proposed that the majority of the cosmic X-ray background (XRB) is produced by heavily obscured AGNs. If this scenario is correct, the integrated 1 - 10 keV spectrum of such objects, which are expected to have Seyfert 2 optical classifications, should be as flat as (or flatter than) the spectrum of the XRB itself. While observations have shown that the intrinsically steep X-ray spectra of a number of Seyfert 2s are flattened due to transmission through dense material, it remains to be seen if this applies to all Seyfert 2 galaxies, and if their spectra, which display considerable diversity, combine to yield a spectrum whose shape is similar to that of the XRB. To investigate this issue, I have carried out an ASCA survey of a distance-limited sample of 30 nearby type 2 Seyfert galaxies. The majority of the objects in the sample are detected. Using the full dataset, I have constructed an unbiased, luminosity-weighted composite Seyfert 2 X-ray spectrum, which I compare to the spectrum of the XRB measured by ASCA. In addition, I have recently completed an optical spectropolarimetry survey of this entire sample of Seyfert 2s, which has revealed evidence for polarized broad emission lines in 11 of the objects. Using these results, I have compiled hard X-ray detection statistics and composite X-ray spectra separately for Seyfert 2s with and without evidence for polarized broad emission lines, which provides valuable insight into the issue of Seyfert unification.

Eliot Quataert
Institute for Advanced Study

"Ionization, Magneto-rotational and Gravitational Instabilities in AGN Disks"

I discuss the combined role of the thermal ionization, magneto-rotational and gravitational instabilities in thin accretion disks around supermassive black holes. I show that in the portions of the disk unstable to the ionization instability, the gas remains well coupled to the magnetic field even on the cold, neutral branch of the thermal limit cycle. This suggests that the ionization instability is not a significant source of large amplitude time-dependent accretion in AGN. I show, however, that for M >~ 10-2 solar masses per year, the gravitationally unstable and magneto-rotationally unstable regions of the accretion disk overlap; for lower accretion rates they may not. Some low-luminosity AGN, e.g. NGC~4258, may thus be in a transient phase in which mass is building up in a non-accreting gravitationally and magneto-rotationally stable ``dead zone.''

Dave Strickland
Johns Hopkins University

"The Cosmological Impact of Starburst-driven Superwinds"

Massive stars return large amounts of energy and mass into the ISM, in the form of metal-enriched stellar winds and supernova ejecta. If the SN rate per unit volume is high enough, the collective mechanical power can drive global outflows of gas from galaxies --- these are galactic superwinds, many of which are seen in local starburst galaxies.

I will discuss the possible cosmological implications of superwinds, which can be summarized as mass and metal loss from galaxies, along with very significant enrichment and heating of the IGM. Quantitatively understanding the role of superwinds is difficult, both observationally and theoretically. I will discuss the problems (placing an emphasis on understanding the X-ray emission from these objects), and show recent work (including Chandra results) that give a new insights into starburst-driven outflows. If time permits, I will also talk about what observational and theoretical work necessary to make significant advances in our understanding of the feedback between massive stars and the ISM and IGM.

Jacco Vink
Columbia University

"Thermal vs Non-thermal X-ray emission from Young Supernova Remnants"

Supernova shocks are accompanied by interesting physical phenomena. Even the shock itself is remarkable as it is not established by two-particle interactions (which are too slow as a result of the low densities involved), but by collective interactions. Such shocks are called "collisionless shocks". As a consequence standard shock physics may not be completely appropiate to describe the energy distribution of the particles. In particular there may be non-thermal particle distributions giving rise to non-thermal X-ray emission. Non-thermal X-ray emission has been observed in supernova remnants, but at least in a number of cases it may be caused by another, recently discovered phenomon: synchrotron radiation caused by shock accelerated electrons.

In this talk I will emphasize the X-ray observational aspects of non-collisionless shocks.

Rudy Wijnands

"The Neutron Star Spin Frequency in the X-ray Transient and Eclipsing LMXB MXB 1659-298"

I report the discovery of nearly coherent oscillations with a frequency of ~567 Hz during type-I X-ray bursts from the X-ray transient and eclipsing binary X 1659-298. If these oscillations are directly related to the neutron star rotation then the spin period of the neutron star in X 1658-298 is ~1.8 ms. I will discuss the properties of these oscillations in detail and the implications of this discovery for our understanding of burst oscillations in general.

Last modified: 08/24/11

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