3.1. The Optical/UV Big Blue Bump
The Big Blue Bump can be explained as thermal emission from gas in an accretion disk (AD) with a wide range of temperatures. ADs have a large number of parameters and can produce the variety of observed SED shapes (Siemiginowska et al. 1995). Individual fits which include the soft X-ray excess require scattering of the AD photons in a lower density, hot corona (Zycki et al. 1995, Laor et al. 1997). As noted by Blaes (this volume), to further constrain accretion disk models, an observational relation between optical/UV colors and the black hole mass are required. Mass measurements are available for a small number of low-redshift sources, although new methods have been developed to estimate the mass at higher redshift (Vestergaard, this volume).
As popular as accretion disks are, they are not unique in being able to explain the optical-soft X-ray continuum of AGN. Models invoking mildly optically thick cloud distributions (Czerny & Dumont 1998, Collin-Souffrin et al. 1996) provide an alternative.