Quasar Accretion Disks Are Strongly Inhomogeneous
  Active galactic nuclei (AGN) have been observed to vary stochastically with
10-20 rms amplitudes over a range of optical wavelengths where the emission
arises in an accretion disk. Since the accretion disk is unlikely to vary
coherently, local fluctuations may be significantly larger than the global rms
variability. We investigate toy models of quasar accretion disks consisting of
a number of regions, n, whose temperatures vary independently with an amplitude
of \sigma_T in dex. Models with large fluctuations (\sigma_T=0.35-0.50) in
100-1000 independently fluctuating zones for every factor of two in radius can
explain the observed discrepancy between thin accretion disk sizes inferred
from microlensing events and optical luminosity while matching the observed
optical variability. For the same range of \sigma_T, inhomogeneous disk spectra
provide excellent fits to the HST quasar composite without invoking global
Compton scattering atmospheres to explain the high levels of observed UV
emission. Simulated microlensing light curves for the Einstein cross from our
time-varying toy models are well fit using a time-steady power-law temperature
disk, and produce magnification light curves that are consistent with current
microlensing observations. Deviations due to the inhomogeneous, time-dependent
disk structure should occur above the 1% level in the light curves, detectable
in future microlensing observations with millimag sensitivity.
