Ultraviolet variability of quasars: dependence on the accretion rate
  We compiled a catalogue of about 4000 SDSS quasars including individual
estimators V for the variability strength, virial black hole masses M, and mass
accretion rates dM/dt from the Davis-Laor scaling relation. We confirm
significant anti-correlations between V and dM/dt, the Eddington ratio, and the
bolometric luminosity L, respectively. A weak, statistically not significant
positive trend is indicated for the dependence of V on M. As a side product, we
find a strong correlation of the radiative efficiency with M and show that this
trend is most likely produced by selection effects in combination with the mass
errors and the use of the scaling relation for dM/dt. The anti-correlations
found for V cannot be explained in such a way. The strongest anti-correlation
is found with dM/dt. However, it is difficult to decide which of the quantities
(L, Eddington ratio, dM/dt) is intrinsically correlated with V and which of the
observed correlations are produced by the relations between these quantities. A
V-dM/dt anti-correlation is qualitatively expected for the strongly
inhomogeneous accretion disks. We argue that several observed variability
properties are not adequately explained by the simple multi-temperature
black-body model of a standard disk and suggest to check whether the strongly
inhomogeneous disk model is capable of reproducing these observations better.
