  Microlensing perturbations to the flux ratios of gravitationally lensed
quasar images can vary with wavelength because of the chromatic dependence of
the accretion disk's apparent size. Multiwavelength observations of microlensed
quasars can thus constrain the temperature profiles of their accretion disks, a
fundamental test of an important astrophysical process which is not currently
possible using any other method. We present single-epoch broadband flux ratios
for 12 quadruply lensed quasars in eight bands ranging from 0.36 to 2.2
microns, as well as Chandra 0.5--8 keV flux ratios for five of them. We combine
the optical/IR and X-ray ratios, together with X-ray ratios from the
literature, using a Bayesian approach to constrain the half-light radii of the
quasars in each filter. Comparing the overall disk sizes and wavelength slopes
to those predicted by the standard thin accretion disk model, we find that on
average 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 than
the slope of 4/3 predicted by the standard thin disk theory. Though the error
bars on the slope are large for individual quasars, the large sample size lends
weight to the overall result. Our results present severe difficulties for a
standard thin accretion disk as the main source of UV/optical radiation from
quasars.
