Quasar Microlensing Variability Studies Favor Shallow Accretion Disk Temperature Profiles

Abstract

We compare the microlensing-based continuum emission region size measurements in a sample of 15 gravitationally lensed quasars with estimates of luminosity-based thin disk sizes to constrain the temperature profile of the quasar continuum accretion region. If we adopt the standard thin disk model, we find a significant discrepancy between sizes estimated using the luminosity and those measured by microlensing of log( r L / r μ ) = −0.57 ± 0.08 dex. If quasar continuum sources are simple, optically thick accretion disks with a generalized temperature profile T ( r ) ∝ r − β , the discrepancy between the microlensing measurements and the luminosity-based size estimates can be resolved by a temperature profile slope 0.37 <  β  < 0.56 at 1 σ confidence. This is shallower than the standard thin disk model ( β  = 0.75) at 3 σ significance. We consider alternate accretion disk models that could produce such a temperature profile and reproduce the empirical continuum size scaling with black hole mass, including disk winds or disks with nonblackbody atmospheres.