Structure and Spectroscopy of Black Hole Accretion Disks

The warped spacetime near black holes is one of the most exotic observable environments in the Universe. X-ray spectra from active galaxies obtained with the current generation of X-ray observatories reveal line emission that is modified by both special relativistic and general relativistic effects. The interpretation is that we are witnessing X-ray irradiated matter orbiting in an accretion disk around a supermassive black hole, as it prepares to cross the event horizon. This interpretation, however, is based upon highly schematized models of accretion disk structure. This report describes a project to design a detailed computer model of accretion disk atmospheres, with the goal of elucidating the high radiation density environments associated with mass flows in the curved spacetime near gravitationally collapsed objects. We have evolved the capability to generate realistic theoretical X-ray line spectra of accretion disks, thereby providing the means for a workable exploration of the behavior of matter in the strong-field limit of gravitation

Orbital modulation of X-ray emission lines in Cygnus X-3

We address the problem where the X-ray emission lines are formed and investigate orbital dynamics using Chandra HETG observations, photoionizing calculations and numerical wind-particle simulations.The observed Si XIV (6.185 A) and S XVI (4.733 A) line profiles at four orbital phases were fitted with P Cygni-type profiles consisting of an emission and a blue-shifted absorption component. In the models, the emission originates in the photoionized wind of the WR companion illuminated by a hybrid source: the X-ray radiation of the compact star and the photospheric EUV-radiation from the WR star. The emission component exhibits maximum blue-shift at phase 0.5 (when the compact star is in front), while the velocity of the absorption component is constant (around -900 km/s). The simulated FeXXVI Ly alpha line (1.78 A) from the wind is weak compared to the observed one. We suggest that it originates in the vicinity of the compact star, with a maximum blue shift at phase 0.25 (compact star approaching). By combining the mass function derived with that from the infrared HeI absorption (arising from the WR companion), we constrain the masses and inclination of the system. Both a neutron star at large inclination (over 60 degrees) and a black hole at small inclination are possible solutions.Comment: 8 pages, accepted 19/04/2009 for publication in Astronomy and Astrophysic

X-Ray Spectral Diagnostics Of Neon Photoionization Experiments On The Z-Machine

We report on an initial spectroscopic study of low-density, x-ray photoionized neon with x-ray spectroscopy. These experiments, carried out on the Z-machine at Sandia, are optimized to produce a gradient-free, collisionless plasma, and to explore issues related to the rapid x-ray photoionization of relatively cold, low-density plasmas. The initial experiments used time-integrated absorption spectroscopy, backlit by the pinch radiation, to determine the ionization balance in the gas cell. Future experiments will use time-resolved spectroscopy in both absorption and emission. The emission spectra are expected to be similar to those seen from photoionized astrophysical sources, such as x-ray binaries. Indeed, in addition to addressing basic plasma and atomic physics issues, these experiments are designed to help the astrophysical community better understand the new, high-resolution spectra being produced by the Chandra and XMM-Newton telescopes, and to benchmark spectral synthesis codes

Reprocessing of Soft X-ray Emission Lines in Black Hole Accretion Disks

By means of a Monte Carlo code that accounts for Compton scattering and photoabsorption followed by recombination, we have investigated the radiation transfer of Ly alpha, He alpha, and recombination continua photons of H- and He-like C, N, O, and Ne produced in the photoionized atmosphere of a relativistic black hole accretion disk. We find that photoelectric opacity causes significant attenuation of photons with energies above the O VIII K-edge; that the conversion efficiencies of these photons into lower-energy lines and recombination continua are high; and that accounting for this reprocessing significantly (by factors of 21% to 105%) increases the flux of the Ly alpha and He alpha emission lines of H- and He-like C and O escaping the disk atmosphere.Comment: 4 pages including 4 encapsulated postscript figures; LaTeX format, uses aastex.cls and emulateapj5.sty; accepted on 2004 January 13 for publication in The Astrophysical Journa

High-Resolution Chandra Spectroscopy Of Tau Scorpii: A Narrow-Line X-Ray Spectrum From A Hot Star

Long known to be an unusual early-type star by virtue of its hard and strong X-ray emission, tau Scorpii poses a severe challenge to the standard picture of O-star wind-shock X-ray emission. The Chandra HETGS spectrum now provides significant direct evidence that this B0.2 star does not fit this standard wind-shock framework. The many emission lines detected with the Chandra gratings are significantly narrower than what would be expected from a star with the known wind properties of tau Sco, although they are broader than the corresponding lines seen in late-type coronal sources. While line ratios are consistent with the hot plasma on this star being within a few stellar radii of the photosphere, from at least one He-like complex there is evidence that the X-ray emitting plasma is located more than a stellar radius above the photosphere. The Chandra spectrum of Sco is harder and more variable than those of other hot stars, with the exception of the young magnetized O star theta(1) Ori C. We discuss these new results in the context of wind, coronal, and hybrid wind-magnetic models of hot-star X-ray emission