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 .