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

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 ($ω_{0}$), a damping ratio ($ξ$), a characteristic perturbation timescale ($τ_{\mathrm{perturb}}$), and an amplitude for the perturbing white noise ($σ_{\mathrmε}$). The asymptotic variability amplitude of a DHO process is quantified by $σ_{\mathrm{DHO}}$ -- a function of $ω_{0}$, $ξ$, $τ_{\mathrm{perturb}}$, and $σ_{\mathrmε}$. We find that both $τ_{\mathrm{perturb}}$ and $σ_{\mathrmε}$ follow different dependencies with rest-frame wavelength ($λ_{\mathrm{RF}}$) on either side of 2500 Å, whereas $σ_{\mathrm{DHO}}$ follows a single power-law relation with $λ_{\mathrm{RF}}$. After correcting for wavelength dependence, $σ_{\mathrm{DHO}}$ exhibits anti-correlations with both the Eddington ratio and the black hole mass, while $τ_{\mathrm{perturb}}$ -- 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 $τ_{\mathrm{perturb}}$ and $σ_{\mathrmε}$) 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.

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