Examining AGN UV/optical Variability Beyond the Simple Damped Random Walk

Abstract

We present damped harmonic oscillator (DHO) light-curve modeling for a sample of 12,714 spectroscopically confirmed quasars in the Sloan Digital Sky Survey Stripe 82 region. DHO is a second-order continuous-time autoregressive moving-average (CARMA) process, which can be fully described using four independent parameters: a natural oscillation frequency ($\omega_{0}$), a damping ratio ($\xi$), a characteristic perturbation timescale ($\tau_{\mathrm{perturb}}$), and an amplitude for the perturbing white noise ($\sigma_{\mathrm{\epsilon}}$). The asymptotic variability amplitude of a DHO process is quantified by $\sigma_{\mathrm{DHO}}$ -- a function of $\omega_{0}$, $\xi$, $\tau_{\mathrm{perturb}}$, and $\sigma_{\mathrm{\epsilon}}$. We find that both $\tau_{\mathrm{perturb}}$ and $\sigma_{\mathrm{\epsilon}}$ follow different dependencies with rest-frame wavelength ($\lambda_{\mathrm{RF}}$) on either side of 2500 \AA, whereas $\sigma_{\mathrm{DHO}}$ follows a single power-law relation with $\lambda_{\mathrm{RF}}$. After correcting for wavelength dependence, $\sigma_{\mathrm{DHO}}$ exhibits anti-correlations with both the Eddington ratio and the black hole mass, while $\tau_{\mathrm{perturb}}$ -- with a typical value of days in the rest-frame -- shows an anti-correlation with the bolometric luminosity. Modeling AGN variability as a DHO offers more insight into the workings of accretion disks close to the supermassive black holes (SMBHs) at the center of AGN. The newly discovered short-term variability (characterized by $\tau_{\mathrm{perturb}}$ and $\sigma_{\mathrm{\epsilon}}$) and its correlation with bolometric luminosity pave the way for new algorithms that will derive fundamental properties (e.g., Eddington ratio) of AGN using photometric data alone.