  Studies have shown that UV/optical light curves of quasars can be described
with the prevalent damped random walk (DRW, also known as Ornstein-Uhlenbeck
process) model. A white noise power spectral density (PSD) is expected at low
frequency in this model, however, direct observational constraint to the low
frequency PSD slope is hard due to limited lengths of the light curves
available. Meanwhile, quasars show too large scatter in their DRW parameters to
be attributed to the uncertainties in the measurements and the dependence of
variation to known physical factors. In this work we present simulations
showing that, if the low frequency PSD deviates from DRW, the red noise leakage
can naturally produce large scatter in variation parameters measured from
simulated light curves. The steeper the low frequency PSD slope is, the larger
scatter we expect. Based on the observations of SDSS Stripe 82 quasars, we find
the low frequency PSD slope should be no steeper than -1.3. The actual slope
could be flatter, which consequently requires that quasar variabilities should
be influenced by other unknown factors. We speculate that magnetic field and/or
metallicity could be such additional factors.
