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 Å should show a trend of bluer colors for higher T max , notwithstanding random disk inclinations. We compare this theoretical expectation with observed colors of QSOs in the Sloan Digital Sky Survey (SDSS), deriving black hole mass and thence T max 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 T max . 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 ~30% of the Eddington limit. The derived disk temperature depends primarily on line width, with little or no dependence on luminosity.