The correlations between optical variability and physical parameters of
  quasars in SDSS Stripe 82
  We investigate the optical variability of 7658 quasars from SDSS Stripe 82.
Taking advantage of a larger sample and relatively more data points for each
quasar, we estimate variability amplitudes and divide the sample into small
bins of redshift, rest-frame wavelength, black hole mass, Eddington ratio and
bolometric luminosity respectively, to investigate the relationships between
variability and these parameters. An anti-correlation between variability and
rest-frame wavelength is found. The variability amplitude of radio-quiet
quasars shows almost no cosmological evolution, but that of radio-loud ones may
weakly anti-correlate with redshift. In addition, variability increases as
either luminosity or Eddington ratio decreases. However, the relationship
between variability and black hole mass is uncertain; it is negative when the
influence of Eddington ratio is excluded, but positive when the influence of
luminosity is excluded. The intrinsic distribution of variability amplitudes
for radio-loud and radio-quiet quasars are different. Both radio-loud and
radio-quiet quasars exhibit a bluer-when-brighter chromatism. Assuming that
quasar variability is caused by variations of accretion rate, the
Shakura-Sunyaev disk model can reproduce the tendencies of observed
correlations between variability and rest-frame wavelength, luminosity as well
as Eddington ratio, supporting that changes of accretion rate plays an
important role in producing the observed optical variability. However, the
predicted positive correlation between variability and black hole mass seems to
be inconsistent with the observed negative correlation between them in small
bins of Eddington ratio, which suggests that other physical mechanisms may
still need to be considered in modifying the simple accretion disk model.
