Accretion disk temperatures and continuum colors in QSOs

Accretion disks around supermassive black holes are widely believed to be the dominant source of the optical-ultraviolet continuum in many classes of active galactic nuclei (AGN). We study here the relationship between the continuum colors of AGN and the characteristic accretion disk temperature (T-max). Based on non-LTE (NLTE) models of accretion disks in AGN computed as described by Hubeny and coworkers, we find that continuum intensity ratios for several pairs of wavelengths between 1350 and 5100 angstrom should show a trend of bluer colors for higher Tmax, notwithstanding random disk inclinations. We compare this theoretical expectation with observed colors of QSOs in the Sloan Digital Sky Survey (SDSS), deriving black holemass and thence Tmax from the width of the Mg II broad emission line. The observed colors generally do not show the expected trend, and in some cases show a reverse trend of redder colors with increasing Tmax. The cause of this discrepancy does not appear to be dust reddening or galaxy contamination, but may relate to the accretion rate, as the offset objects are accreting above similar to 30% of the Eddington limit. The derived disk temperature depends primarily on line width, with little or no dependence on luminosity.