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 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 (perturb)), and an amplitude for the perturbing white noise (sigma ( epsilon )). The asymptotic variability amplitude of a DHO process is quantified by sigma (DHO)-a function of omega (0), xi, tau (perturb), and sigma ( epsilon ). We find that both tau (perturb) and sigma ( epsilon ) follow different dependencies with rest-frame wavelength (lambda (RF)) on either side of 2500 angstrom, whereas sigma (DHO) follows a single power-law relation with lambda (RF). After correcting for wavelength dependence, sigma (DHO) exhibits anticorrelations with both the Eddington ratio and the black hole mass, while tau (perturb)-with a typical value of days in the rest frame-shows an anticorrelation with the bolometric luminosity. Modeling active galactic nuclei (AGN) variability as a DHO offers more insight into the workings of accretion disks close to the supermassive black holes at the center of AGN. The newly discovered short-term variability (characterized by tau (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.