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 (${\sigma }_{\epsilon }$). The asymptotic variability amplitude of a DHO process is quantified by ${\sigma }_{\mathrm{DHO}}$ -- a function of ${\omega }_0$, $\xi$, ${\tau }_{\mathrm{perturb}}$, and ${\sigma }_{\epsilon }$. We find that both ${\tau }_{\mathrm{perturb}}$ and ${\sigma }_{\epsilon }$ follow different dependencies with rest-frame wavelength (${\lambda }_{\mathrm{RF}}$) on either side of 2500 Å, 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 ${\sigma }_{\epsilon }$) 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.

26 pages, 14 figures, revised after referee report, accepted for publication in ApJ

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