How far is quasar UV/optical variability from damped random walk at low frequency?

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.