Probing the Optical/UV Continuum Variability of Active Galactic Nuclei with the Damped Random Walk Model

Optical/UV continuum variability over baselines of up to 20 yr is investigated using the damped random walk (DRW) model for 1214 active galactic nuclei (AGNs) compiled from Y. Shen et al. and C. J. Burke et al. Among these, 115 AGNs with reliable DRW fitting and unbiased DRW parameters (timescale τ 2500 and amplitude σ 2500 at the rest-frame 2500 Å) are used to examine the relations between the variability and the accretion properties. We found a significant correlation of τ 2500 with black hole mass (M BH) and dimensionless accretion rate of the form τ 2500 proportional to M BH 0.50 plus or minus0.07 0.27 plus or minus0.04, while the dependence on the continuum luminosity at 5100 Å and redshift becomes insignificant after controlling for M BH. τ 2500 also correlate with the mass accretion rate as τ 2500 proportional to 0.41 plus or minus0.04, which is consistent with the thermal timescale expected from the standard thin-disk model within 3σ uncertainties; while the average value is consistent with the expectations of the model, the scatter in the relation can be due to differences in the viscosity parameter (α) or in disk temperature profiles. Assuming that corresponds to the accretion disk thermal timescale, the disk-emitting radius at rest-frame 2500 Å (R 2500) is found to correlate with M BH as R 2500 proportional to M BH 0.53 plus or minus0.05. Using the microlensing-based R 2500-M BH relation of C. W. Morgan et al., α is statistically estimated to be 0.04 plus or minus0.01.