A Method to Measure the Unbiased Decorrelation Timescale of the AGN Variable Signal from Structure Functions

Abstract

A simple, model-independent method to quantify the stochastic variability of active galactic nuclei (AGNs) is the structure function (SF) analysis. If the SF for the timescales shorter than the decorrelation timescale $\tau$ is a single power-law and for the longer ones becomes flat (i.e., the white noise), the auto-correlation function (ACF) of the signal can have the form of the power exponential (PE). We show that the signal decorrelation timescale can be measured directly from the SF as the timescale matching the amplitude 0.795 of the flat SF part (at long timescales), and only then the measurement is independent of the ACF PE power. Typically, the timescale has been measured at an arbitrarily fixed SF amplitude, but as we prove, this approach provides biased results because the AGN SF/PSD slopes, so the ACF shape, are not constant and depend on either the AGN luminosity and/or the black hole mass. In particular, we show that using such a method for the simulated SFs that include a combination of empirically known dependencies between the AGN luminosity $L$ and both the SF amplitude and the PE power, and having no intrinsic $\tau -L$ dependence, produces a fake $\tau \propto L^{\kappa }$ relation with $0.3\lesssim \kappa \lesssim 0.6$, that otherwise is expected from theoretical works ($\kappa \equiv 0.5$). Our method provides an alternative means for analyzing AGN variability to the standard SF fitting. The caveats, for both methods, are that the light curves must be sufficiently long (several years rest-frame) and the ensemble SF assumes AGNs to have the same underlying variability process.

7 pages, 3 figures, accepted for publication in ApJ

References in corpus (12)

• A Catalog of Quasar Properties from SDSS DR7
• Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk
• The Sloan Digital Sky Survey Quasar Catalog: twelfth data release
• Is Quasar Optical Variability a Damped Random Walk?
• Are the Variability Properties of the Kepler AGN Light Curves Consistent with a Damped Random Walk?
• Optical variability of AGN in the PTF/iPTF survey
• Limitations on the recovery of the true AGN variability parameters using Damped Random Walk modeling
• Quasar Selection Based on Photometric Variability
• Discovery of a ~5 day characteristic timescale in the Kepler power spectrum of Zw 229-15
• Virial Black Hole Mass Estimates for 280,000 AGNs from the SDSS Broad-Band Photometry and Single Epoch Spectra
• Ensemble X-ray variability of Active Galactic Nuclei. II. Excess Variance and updated Structure Function
• Measuring Quasar Variability with Pan-STARRS1 and SDSS

Cited by in corpus (10)

• How far is quasar UV/optical variability from damped random walk at low frequency?
• Discovery of a Candidate Binary Supermassive Black Hole in a Periodic Quasar from Circumbinary Accretion Variability
• Reliable Identification of Binary Supermassive Black Holes from Rubin Observatory Time-Domain Monitoring
• Estimation of the size and structure of the broad line region using Bayesian approach
• Conditional Neural Process for non-parametric modeling of AGN light curve
• Seeking Neutrino Emission from AGN through Temporal and Spatial Cross Correlation
• Modeling quasar variability through self-organizing map-based process
• AGN variability in the age of VRO
• Optical variability of eight FRII-type quasars with 13-yr photometric light curves
• Variability Selected Active Galactic Nuclei from ASAS-SN Survey: Constraining the Low Luminosity AGN Population