Improved Accretion Disk Models of Continuum Emission from Active Galactic Nuclei

Abstract

We present calculations of the spectra of black hole accretion disks which may power Active Galactic Nuclei. We use a simple model which is more realistic than previous ones in two important ways. Firstly, we have accounted for the dominance of electron scattering opacity in the disk, by approximating the locally emerging flux with low-gravity non-LTE stellar atmospheres. This disk spectrum constructed with stellar atmospheres is harder than the disk spectrum from a sum of blackbodies. It also shows a moderately strong Lyman limit absorption edge, up to 0.25 dex in depth. Such an edge appears to have been observed in several intermediate redshift quasars. The Balmer absorption edge is somewhat weaker.

We have also included the strong general relativistic effects of gravitational focusing and disk inclination on the observed spectrum. The observed spectrum becomes significantly harder when the disk is viewed at higher inclinations. We find that the spectrum of an edge-on disk is approximately 5 times harder than that of the same disk viewed face-on. The inferred black hole masses and accretion rates change by comparable factors.