Characteristic QSO Accretion Disk Temperatures from Spectroscopic   Continuum Variability
  Using Sloan Digital Sky Survey (SDSS) quasar spectra taken at multiple epochs, we find that the composite flux density differences in the rest frame wavelength range 1300-6000 AA can be fit by a standard thermal accretion disk model where the accretion rate has changed from one epoch to the next (without considering additional continuum emission components). The fit to the composite residual has two free parameters: a normalizing constant and the average characteristic temperature $\bar{T}^*$. In turn the characteristic temperature is dependent on the ratio of the mass accretion rate to the square of the black hole mass. We therefore conclude that most of the UV/optical variability may be due to processes involving the disk, and thus that a significant fraction of the UV/optical spectrum may come directly from the disk. 
Nicolas A. Pereyra, Daniel E. Vanden Berk, David A. Turnshek, D. John   Hillier, Brian C. Wilhite, Richard G. Kron, Donald P...