Examining AGN UV/Optical Variability Beyond the Simple Damped Random Walk. II. Insights from 22 Years Observations of SDSS, PS1 and ZTF

TL;DR

This work demonstrates that a noise-driven damped harmonic oscillator (DHO) model provides a superior description of AGN UV/optical variability compared with the traditional DRW approach, using 22 years of multi-survey quasar light curves from SDSS Stripe 82, PS1, and ZTF. The DHO is characterized by four parameters ($\sigma_{DHO}$, $\tau_{decay}$, $\sigma_{\epsilon}$, $\tau_{perturb}$) that separate long- and short-term variability, and its parameters correlate with rest-frame wavelength, $L/L_{\rm Edd}$, and $M_{\rm BH}$, suggesting long-term disk-origin fluctuations and short-term corona-related reprocessing of X-ray variability. The extended baselines and calibrations enable robust DHO fits and highlight the need to move beyond DRW in analyzing AGN light curves from upcoming wide-field time-domain surveys. These results provide a framework for interpreting AGN variability in terms of accretion-disk physics and X-ray reprocessing, with implications for how future surveys model and extract physical information from quasar variability signals.

Abstract

A damped random walk (DRW) process is often used to describe the temporal UV/optical continuum variability of active galactic nuclei (AGN). However, recent investigations have shown that this model fails to capture the full spectrum of AGN variability. In this work, we model the 22-year-long light curves of $21,767$ quasars, spanning the redshift range $0.28 < z < 2.71$, as a noise-driven damped harmonic oscillator (DHO) process. The light curves, in the optical $g$ and $r$ bands, are collected and combined from the Sloan Digital Sky Survey, the Panoramic Survey Telescope and Rapid Response System, and the Zwicky Transient Facility. A DHO process can be defined using four parameters, two for describing its long-term behavior/variability, and the other two for describing its short-term behavior/variability. We find that the best-fit DHO model describes the observed variability of our quasar light curves better than the best-fit DRW model. Furthermore, the best-fit DHO parameters exhibit correlations with the rest-frame wavelength, the Eddington ratio, and the black hole mass of our quasars. Based on the power spectral density shape of the best-fit DHOs and these correlations, we suggest that the observed long-term variability of our quasars can be best explained by accretion rate or thermal fluctuations originating from the accretion disk, and the observed short-term variability can be best explained by reprocessing of X-ray variability originating from the corona. The additional information revealed by DHO modeling emphasizes the need to go beyond DRW when analyzing AGN light curves delivered by next-generation wide-field time-domain surveys.

Figures (2)

• Figure 1: $\chi^2$ per degree of freedom ($\chi^2_{dof}$) distribution for standard star light curves compiled from the SDSS-I Legacy Survey, the SDSS-II Supernova Survey, PS1, and ZTF (Zubercal). The dashed lines show the median of the $\chi^2$ distribution computed using the pipeline photometric errors, as a function of the star's mean magnitude. The solid lines and the shaded regions show the $\chi^2$ distribution computed using the re-calibrated photometric errors. Recalibration is not performed for SDSS-I photometry.
• Figure 2: Three example combined SDSS-PS1-ZTF light curves. The ID shown at the bottom left corner of each panel gives the identifier of each corresponding quasar's SDSS spectrum, in the format of PLATE-MJD-FIBERID.