Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.8405c&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #84.05D; Bulletin of the American Astronomical Society, Vol. 24, p.1258
Astronomy and Astrophysics
Astronomy
Scientific paper
The release of gravitational energy by matter accreting onto a supermassive black hole is the most widely accepted energy production mechanism for active galactic nuclei (AGN). The Shakura and Sunyaev model of accretion through a thin alpha -disk has been, with slight modifications, the favored model of accretion for two decades. The disk model has proved a promising candidate for the source of the Blue Bump in AGN optical-ultraviolet spectra. As more detailed observations have become available, the disk model has come under closer scrutiny. We examine here two questions of importance to the viability of the disk model: (1) what is the nature of the Lyman edge in the disk emission spectrum, and (2) to what degree are soft X-rays present in the disk thermal emission? Supermassive disk spectra have heretofore been modeled chiefly in two ways: as radiation from relatively simple atmospheres involving free-free absorption, bound-free opacity in LTE, and electron-scattering; or as emission from superposed stellar atmospheres. We find that extremely low effective gravities (geff) characterize the disk atmospheres in the regions of interest, and that these low gravities, along with non-LTE effects, are important in shaping the emission spectrum. The aforementioned approaches have difficulty in properly taking into account the extremely low geffs expected. We have explored the implications of low geff and non-LTE effects by calculating density and temperature gradients for disk atmospheres using stellar atmosphere techniques and using non-LTE radiative transfer to compute the emergent spectrum. Locally emitted spectra are integrated over the disk surface, with relativistic effects included, to predict observed spectra for various viewing angles and disk parameters. Model spectra and implications for observable Lyman edge and soft X-ray emission will be discussed. This work is supported by NASA grants NAGW-1807 and NGT-50659.
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