Characteristic QSO accretion disk temperatures from spectroscopic continuum variability

Using Sloan Digital Sky Survey (SDSS) QSO spectra taken at multiple epochs, we find that the composite flux differences in the rest-frame wavelength range 1300-6000 Å can be fit by a standard thermal accretion disk model in which 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 T̄*. In turn, in a standard disk the characteristic temperature is dependent on the ratio of the mass accretion rate to the square of the black hole mass. Therefore, provided enough time has elapsed for a composite disk spectrum to adjust to a new average characteristic temperature, reasonably consistent with the standard model, we conclude that most of the UV-optical variability observed in QSOs may be due to processes involving changes in disk accretion rates. This is consistent with the conclusion that a significant fraction of a QSO's UV-optical spectrum comes directly from the disk.