Revisiting Stochastic Variability of AGNs with Structure Functions
Abstract
Discrepancies between reported structure function (SF) slopes and their overall flatness as compared to the expectations from the damped random walk (DRW) model, which generally well describes the variability of active galactic nuclei (AGNs), have triggered us to study this problem in detail. We review common AGN variability observables and identify their most common problems. Equipped with this knowledge, we study ∼9000 rband AGN light curves from Stripe 82 of the Sloan Digital Sky Survey, using SFs described by stochastic processes with the power exponential covariance matrix of the signal. We model the “subensemble” SFs in the redshiftabsolute magnitude bins with the full SF equation (including the turnover and the noise part) and a single power law (SPL; in the “red noise regime” after subtracting the noise term). The distribution of fullequation SF (SPL) slopes peaks at γ =0.55+/ 0.08 (0.52 ± 0.06) and is consistent with the DRW model. There is a hint of a weak correlation of γ with the luminosity and a lack of correlation with the black hole mass. The typical decorrelation timescale in the optical is τ =0.97+/ 0.46 year. The SF amplitude at one year obtained from the SPL fitting is {{SF}}_{0}=0.22+/ 0.06 mag and is overestimated because the SF is already at the turnover part, so the true value is {{SF}}_{0}=0.20+/ 0.06 mag. The asymptotic variability is {{SF}}_{∞ }=0.25+/ 0.06 mag. It is strongly anticorrelated with both the luminosity and the Eddington ratio and is correlated with the black hole mass. The reliability of these results is fortified with Monte Carlo simulations.
 Publication:

The Astrophysical Journal
 Pub Date:
 August 2016
 DOI:
 10.3847/0004637X/826/2/118
 arXiv:
 arXiv:1604.05858
 Bibcode:
 2016ApJ...826..118K
 Keywords:

 accretion;
 accretion disks;
 galaxies: active;
 methods: data analysis;
 quasars: general;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 16 pages, 12 figures, 1 table