Optical variability of quasars with 20-yr photometric light curves

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