Optical Variability of Quasars with 20-Year Photometric Light Curves

Abstract

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 ∼ 1998 − 2020 with SDSS, PanSTARRS-1, the Dark Energy Survey, and dedicated follow-up monitoring with Blanco 4m/DECam. With on average ∼ 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 100 epochs. We find that the average damping timescale 𝜏 DRW continues to rise with increased baseline, reaching a median value of ∼ 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 𝜏 DRW (less than 20% of the baseline), we confirm a weak wavelength dependence of 𝜏 DRW ∝ 𝜆 0.51±0.20 . 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 ∼ a month for these luminous quasars, and this second break point correlates with the longer DRW damping timescale.

Wavelength-resolved reverberation mapping of quasar CTS C30.10: Dissecting Mg II and Fe II emission regions

“…However, the overall source variability is still not much higher than before, which supports the view that the frequency break in the power spectra of quasars is typically one to two years (e.g. Kozłowski 2016;Stone et al 2022), and the variability amplitude rises much more slowly with a longer observing time. This saturation of the amplitude was already well seen in CTS C30.10 in the previous data (see Fig.…”

Wavelength-resolved Reverberation Mapping of quasar CTSC30.10: Dissecting MgII and FeII emission regions

“…However, the overall source variability is still not much higher then before, which supports the view that frequency break in the power spectra of quasars is typically at 1-2 years (e.g. Kozłowski 2016;Stone et al 2022), and the variability amplitude rises much more slowly with extension of observing time. This saturation of the amplitude was already well seen in CTS C30.10 in the previous data (see Fig.…”

Observational window effects on multi-object Reverberation Mapping

“…The light curve model we used does not include these deviations, as they are still poorly understood. Recently Stone et al (2022) have found that the DRW damping timescale, 𝜏 𝐷 , constrained with 20-year light curves, continues to increase as the baseline of the light curves increases. They also observe smooth, long-term variations in some sources, leading them to suggest that the current DRW model may be insufficient for these sources.…”