  In our second paper on long-term quasar variability, we employ a much larger
database of quasars than in de Vries, Becker & White. This expanded sample,
containing 35165 quasars from the Sloan Digital Sky Survey Data Release 2, and
6413 additional quasars in the same area of the sky taken from the 2dF QSO
Redshift Survey, allows us to significantly improve on our earlier conclusions.
As before, all the historic quasar photometry has been calibrated onto the SDSS
scale by using large numbers of calibration stars around each quasar position.
We find the following: (1) the outbursts have an asymmetric light-curve
profile, with a fast-rise, slow-decline shape; this argues against a scenario
in which micro-lensing events along the line-of-sight to the quasars are
dominating the long-term variations in quasars; (2) there is no turnover in the
Structure Function of the quasars up to time-scales of ~40 years, and the
increase in variability with increasing time-lags is monotonic and constant;
and consequently, (3) there is not a single preferred characteristic outburst
time-scale for the quasars, but most likely a continuum of outburst
time-scales, (4) the magnitude of the quasar variability is a function of
wavelength: variability increases toward the blue part of the spectrum, (5)
high-luminosity quasars vary less than low-luminosity quasars, consistent with
a scenario in which variations have limited absolute magnitude. Based on this,
we conclude that quasar variability is intrinsic to the Active Galactic
Nucleus, is caused by chromatic outbursts / flares with a limited luminosity
range and varying time-scales, and which have an overall asymmetric light-curve
shape. Currently the model that has the most promise of fitting the
observations is based on accretion disk instabilities.
