Quasar Spectral Slope Variability in the Optical Band

We performed a new analysis of B and R light curves of a sample of PGquasars. We confirm the variability-redshift correlation and its explanation interms of spectral variability, coupled with the increase of rest-frameobserving frequency for quasars at high redshift. The analysis of theinstantaneous spectral slope for the whole quasar samples indicates both aninter-QSO and intra-QSO variability-luminosity correlation. Numericalsimulations show that the latter correlation cannot be entirely due to theaddition of the host galaxy emission to a nuclear spectrum of variableluminosity but constant shape, implying a spectral variability of the nuclearcomponent. Changes of accretion rate are also insufficient to explain theamount of spectral variation, while hot spots possibly caused by local diskinstabilities can explain the observations.Comment: 20 pages, 6 figures, to appear in Ap.J., January 200