Optical variability of quasars with 20-yr photometric light curves

We study the optical gri photometric variability of a sample of 190 quasars within the SDSS Stripe 82 region that have long-term photometric coverage during similar to 1998-2020 with SDSS, PanSTARRS-1, the Dark Energy Survey, and dedicated follow-up monitoring with Blanco 4m/DECam. With on average similar to 200 nightly epochs per quasar per filter band, we improve the parameter constraints from a Damped Random Walk (DRW) model fit to the light curves over previous studies with 10-15 yr baselines and less than or similar to 100 epochs. We find that the average damping time-scale tau(DRW) continues to rise with increased baseline, reaching a median value of similar to 750 d (g band) in the rest frame of these quasars using the 20-yr light curves. Some quasars may have gradual, long-term trends in their light curves, suggesting that either the DRW fit requires very long baselines to converge, or that the underlying variability is more complex than a single DRW process for these quasars. Using a subset of quasars with better-constrained tau(DRW) (less than 20 per cent of the baseline), we confirm a weak wavelength dependence of tau(DRW)proportional to lambda(0.51 +/- 0.20). We further quantify optical variability of these quasars over days to decades time-scales using structure function (SF) and power spectrum density (PSD) analyses. The SF and PSD measurements qualitatively confirm the measured (hundreds of days) damping time-scales from the DRW fits. However, the ensemble PSD is steeper than that of a DRW on time-scales less than similar to a month for these luminous quasars, and this second break point correlates with the longer DRW damping time-scale.