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Microlensing perturbations to the flux ratios of gravitationally lensedquasar images can vary with wavelength because of the chromatic dependence ofthe accretion disk's apparent size. Multiwavelength observations of microlensedquasars can thus constrain the temperature profiles of their accretion disks, afundamental test of an important astrophysical process which is not currentlypossible using any other method. We present single-epoch broadband flux ratiosfor 12 quadruply lensed quasars in eight bands ranging from 0.36 to 2.2microns, as well as Chandra 0.5--8 keV flux ratios for five of them. We combinethe optical/IR and X-ray ratios, together with X-ray ratios from theliterature, using a Bayesian approach to constrain the half-light radii of thequasars in each filter. Comparing the overall disk sizes and wavelength slopesto those predicted by the standard thin accretion disk model, we find that onaverage the disks are larger than predicted by nearly an order of magnitude,with sizes that grow with wavelength with an average slope of ~0.2 rather thanthe slope of 4/3 predicted by the standard thin disk theory. Though the errorbars on the slope are large for individual quasars, the large sample size lendsweight to the overall result. Our results present severe difficulties for astandard thin accretion disk as the main source of UV/optical radiation fromquasars.

Sizes and Temperature Profiles of Quasar Accretion Disks from Chromatic Microlensing