Other
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009phdt.........1h&link_type=abstract
Proquest Dissertations And Theses 2009. Section 0098, Part 0606 125 pages; [Ph.D. dissertation].United States -- Maryland: The
Other
Ultraluminous X-Ray Sources, X-Rays, Atmosphere Calculations, Accretion Disks, Black Holes, Intermediate Mass Black Holes
Scientific paper
We have calculated the structures and the emergent spectra of stationary, geometrically thin accretion disks around intermediate mass black holes (IMBH) in both the Schwarzschild and extreme Kerr metrics. Equations of radiative transfer, hydrostatic equilibrium, energy balance, ionization equilibrium, and statistical equilibrium are solved simultaneously and consistently. The six most astrophysically abundant elements (H, He, C, N, O, and Fe) are included, as well as energy transfer by Comptonization. The observed spectrum as a function of viewing angle is computed, incorporating all general relativistic effects. We find that, in contrast with the predictions of the commonly used multicolor disk (MCD) model, opacity associated with photoionization of heavy elements can significantly alter the spectrum near its peak. These ionization edges can create spectral breaks visible in the spectra of slowly spinning black holes viewed from almost all angles and in the spectra of rapidly spinning black holes seen approximately face-on. For fixed mass and accretion rate relative to Eddington, both the black hole spin and the viewing angle can significantly shift the observed peak energy of the spectrum, particularly for rapid spin viewed edge-on. We present a detailed test of the approximations made in various forms of the MCD model. Linear limb-darkening is confirmed to be a reasonable approximation for the integrated flux but not for many specific frequencies of interest.
To test if our atmosphere calculation would shed a new light in modeling observations of Ultra-Luminous X-ray sources (ULX) in which many researchers believe to harbor IMBHs, we have chosen 6 ULXs from the XMM-Newton archive whose spectra have high signal-to-noise and can be fitted solely with a disk model without requiring any power-law component. To estimate systematic errors in the inferred parameters, we fit every spectrum to two different disk models, one based on local blackbody emission (KERRBB) and one based on the detailed atmosphere calculation (BHSPEC). Both incorporate full general relativistic treatment of the disk surface brightness profile, photon Doppler shifts, and photon trajectories. We found in every case that they give almost identical fits and similar acceptable parameters and this indistinguishableness could be attributed to both low-level populations of unstripped heavy elements which generate the atomic features and strong Doppler shift which smears those atomic features. The best-fit value of the most interesting parameter, the mass of the central object, is between 23 and 73 [Special characters omitted.] in 5 of the 6 examples. In every case, the best-fit inclination angle and mass are correlated, in the sense that large mass corresponds to high inclination. Even after allowing for this degeneracy, we find that, with >= 99.9% formal statistical confidence, 3 of the 6 objects have mass >= 25 [Special characters omitted.] ; for the other 3, these data are consistent with a wide range of masses. A mass greater than several hundred [Special characters omitted.] is unlikely for the 3 best-constrained objects. These fits also suggest comparatively rapid black hole spin in the 3 objects whose masses are relatively well-determined, but our estimate of the spin is subject to significant systematic error having to do with uncertainty in the underlying surface brightness profile.
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