The deviation of optical variability of radio-quiet quasars from damped random walk

Abstract

In this work, we examine the consistency of quasar optical flux variability with the damped random walk (DRW) model. Our sample includes 144 radio quiet quasars selected from DR7 SDSS quasar catalog randomly, with the redshift $0.3 < z < 1.0 $ , $10^{8.0} M_{\odot} < M_{\mathrm {BH}} < 10^{9.9} M_{\odot}$ and the luminosity $\lambda L(\lambda, {5100~\mathring{\mathrm{A}}} )$ larger than $10^{45}~\mbox{erg}\,\mbox{s}^{-1}$ . We measure the first-order structure function of their light curves and model the results with four parameters described by the power exponential (PE) covariance matrix, and we further utilize the Markov Chain Monte Carlo (MCMC) method to obtain four reliable parameter value. The main results are listed as follows: (1) The ensemble structure function of 144 sources is consistent with the DRW model. (2) From the individual analytical result, we find 10 objects deviated from the DRW model ( $\beta=1$ ) at 5 $\sigma$ levels and the simulation result indicate the reliability of $70\%$ for our identified deviation from the DRW model. It suggest that AGNs variability is a complex phenomenon and the DRW model is not suitable for the optical variability of all AGNs.