Optical Variability of Quasars with 20-Year 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 $\sim 1998-2020$ with SDSS, PanSTARRS-1, the Dark Energy Survey, and
dedicated follow-up monitoring with Blanco 4m/DECam. With on average $\sim 200$
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 $\lesssim 100$ epochs. We find that the
average damping timescale $\tau_{\rm DRW}$ continues to rise with increased
baseline, reaching a median value of $\sim 750$ days ($g$ 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 $\tau_{\rm DRW}$ (less than 20\% of
the baseline), we confirm a weak wavelength dependence of $\tau_{\rm
DRW}\propto \lambda^{0.51\pm0.20}$. We further quantify optical variability of
these quasars over days to decades timescales using structure function (SF) and
power spectrum density (PSD) analyses. The SF and PSD measurements
qualitatively confirm the measured (hundreds of days) damping timescales from
the DRW fits. However, the ensemble PSD is steeper than that of a DRW on
timescales less than $\sim$ a month for these luminous quasars, and this second
break point correlates with the longer DRW damping timescale.
