A new approach to the variability characterization of active galactic nuclei

TL;DR

This paper identifies a bias in the normalized excess variance used for AGN variability studies due to cosmological time dilation and proposes a correction method based on the structure function, validated with XMM-Newton and SDSS data.

Contribution

It introduces a new correction formula for the normalized excess variance to account for cosmological effects in AGN variability analysis.

Findings

The normalized excess variance is affected by cosmological time dilation.

The proposed correction improves the reliability of variability estimates.

Validation with XMM-Newton and SDSS data confirms the correction's effectiveness.

Abstract

The normalized excess variance is a popular method used by many authors to estimate the variability of active galactic nuclei (AGNs), especially in the X-ray band. We show that this estimator is affected by the cosmological time dilation, so that it should be appropriately corrected when applied to AGN samples distributed in wide redshift intervals. We propose a formula to modify this estimator, based on the use of the structure function. To verify the presence of the cosmological effect and the reliability of the proposed correction, we use data extracted from the XMM-Newton Serendipitous Source Catalogue, data release 5 (XMMSSC-DR5), and cross-matched with the Sloan Digital Sky Survey quasar catalogue, of data release 7 and 12.