Accretion Disk Temperatures and Continuum Colors in QSOs

Accretion disks around supermassive black holes are widely believed to be thedominant source of the optical-ultraviolet continuum in many classes of activegalactic nuclei (AGN). We study here the relationship between the continuumcolors of AGN and the characteristic accretion disk temperature (T_max). Basedon NLTE models of accrection disks in AGN computed as described by Hubeny etal. (2000), we find that continuum intensity ratios for several pairs ofwavelengths between 1350 and 5100 A should show a trend of bluer colors forhigher T_max, notwithstanding random disk inclinations. We compare thistheoretical expectation with observed colors of QSOs in the Sloan Digital SkySurvey,deriving black hole mass and thence T_max from the width of the Mg IIbroad emission line. The observed colors generally do not show the expectedtrend and in some cases show a reverse trend of redder colors with increasingT_max. The cause of this discrepancy does not appear to be dust reddening orgalaxy contamination but may relate to the accretion rate, as the offsetobjects are accreting above ~30 % of the Eddington limit. The derived disktemperature depends primarily on line width, with little or no dependence onluminosity.Comment: 7 pages, 7 figures, accepted for publication in ApJ, uses emulateapj.cl