  In order to investigate the dependence of quasar variability on fundamental
physical parameters like black hole mass, we have matched quasars from the
QUEST1 Variability Survey with broad-lined objects from the Sloan Digital Sky
Survey. The matched sample contains approximately 100 quasars, and the Sloan
spectra are used to estimate black hole masses and bolometric luminosities.
Variability amplitudes are measured from the QUEST1 light curves. We find that
black hole mass correlates with several measures of the variability amplitude
at the 99% significance level or better. The correlation does not appear to be
caused by obvious selection effects inherent to flux-limited quasar samples,
host galaxy contamination or other well-known correlations between quasar
variability and luminosity/redshift. We evaluate variability as a function of
rest-frame time lag using structure functions, and find further support for the
variability--black hole mass correlation. The correlation is strongest for time
lags of the order a few months up to the QUEST1 maximum temporal resolution of
approximately 2 years, and may provide important clues for understanding the
long-standing problem of the origin of quasar optical variability. We discuss
whether our result is a manifestation of a relation between characteristic
variability timescale and black hole mass, where the variability timescale is
typical for accretion disk thermal timescales, but find little support for
this. Our favoured explanation is that more massive black holes have larger
variability amplitudes, and we highlight the need for larger samples with more
complete temporal sampling to test the robustness of this result.
