The ensemble optical variability of type-1 AGN in the Sloan Digital Sky Survey Data Release 7

TL;DR

This study analyzes optical variability in over 5000 low-redshift AGN from SDSS, revealing a correlation between variability, time lag, and luminosity, extending previous quasar variability findings to fainter AGN.

Contribution

It provides the first evidence that the variability-luminosity anticorrelation extends to very faint AGN down to M_{AGN_i}~-18.5, with detailed analysis of structure function slopes.

Findings

Variability amplitude correlates with rest-frame time lag.

Lower luminosity AGN exhibit greater variability.

Anticorrelation between luminosity and variability extends to faint AGN.

Abstract

We use a sample of over 5000 active galactic nuclei (AGN) with extended morphologies at z<0.8 from the Sloan Digital Sky Survey (SDSS) to study the ensemble optical variability as a function of rest-frame time lag and AGN luminosity with the aim of investigating these parameter relationships at lower luminosities than previously studied. We compare photometry from imaging data with spectrophotometry obtained weeks to years later in the Sloan g, r, and i bands. We employ quasar and galaxy eigenspectra fitting to separate the AGN and host galaxy components. A strong correlation between the variability amplitude and rest-frame time lag is observed, in agreement with quasar structure functions but extending to AGN several magnitudes fainter than previously studied. The structure function slopes for our fainter AGN sample are slightly shallower than those found in quasars studies. An anticorrelation with luminosity is clearly detected, with lower luminosity AGN displaying greater variability amplitudes. We demonstrate for the first time that this anticorrelation extends to AGN as faint as $M_{AGN_i}\sim-18.5$, with a slight trend towards shallower slopes at luminosities fainter than $M_{AGN_i}\sim-20.2$.