Thick accretion disks around black holes and the UV/soft X-ray excess in quasars

The optical/UV bump seen in quasars is often modeled as the integrated thermal emission from an optically thick, geometrically thin accretion disk around a supermassive black hole. Soft X-ray excesses recently detected in optically selected (PG) quasars have consequently been attributed to the hot tail of the accretion disk spectrum. However, the high temperatures and luminosities inferred favor a picture in which supercritical, radiation-supported tori around 10⁷^-10⁸^ Mₛun_ black holes are responsible for the UV/soft X-ray thermal component. We investigate models of thick accretion disks and, in particular, the effects of the toroidal shape on the observed thermal emission. The occultation of the innermost disk region due to self-shadowing and the reflection effect of photons off the funnel walls are taken into account. The dependence of the observed spectrum on the viewing angle is discussed. It is suggested that high- and low-inclination systems are responsible for the optical/UV and UV/soft X-ray excesses, respectively.