Optical variability of quasars with 20-yr photometric light curves

Zachary Stone , Yue Shen , Colin J. Burke , Yu-Ching Chen , Qian Yang , Xin Liu , R. A. Gruendl , M. Adamów , F. Andrade-Oliveira , J. Annis , D. Bacon , E. Bertin (1, 2) , S. Bocquet , D. Brooks , D. L. Burke , A. Carnero Rosell , M. Carrasco Kind , J. Carretero , L. N. da Costa , M. E. S. Pereira , J. de Vicente , S. Desai , H. T. Diehl , P. Doel , I. Ferrero , D. N. Friedel , J. Frieman , J. García-Bellido , E. Gaztanaga , D. Gruen , G. Gutierrez , S. R. Hinton , D. L. Hollowood , K. Honscheid , D. J. James , K. Kuehn , N. Kuropatkin , C. Lidman , M. A. G. Maia , F. Menanteau , R. Miquel , R. Morgan , F. Paz-Chinchón , A. Pieres , A. A. Plazas Malagón , M. Rodriguez-Monroy , E. Sanchez , V. Scarpine , S. Serrano , I. Sevilla-Noarbe , M. Smith , E. Suchyta , M. E. C. Swanson , G. Tarlé , C. To

1 CFHT - Canada-France-Hawaii Telescope Corporation
2 IAP - Institut d'Astrophysique de Paris

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 time-scale τ_DRW continues to rise with increased baseline, reaching a median value of ~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 τ_DRW (less than 20 per cent 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 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 ~ a month for these luminous quasars, and this second break point correlates with the longer DRW damping time-scale.

Mots clés: surveys; quasars: general; quasars: supermassive black holes; Astrophysics - Astrophysics of Galaxies

Domaines: Planète et Univers [physics]

Zachary Stone, Yue Shen, Colin J. Burke, Yu-Ching Chen, Qian Yang, et al.. Optical variability of quasars with 20-yr photometric light curves. Monthly Notices of the Royal Astronomical Society, 2022, 514, pp.164-184. ⟨10.1093/mnras/stac1259⟩. ⟨insu-03748242⟩