Ensemble spectral variability study of Active Galactic Nuclei from the XMM-Newton serendipitous source catalogue

TL;DR

This study analyzes the spectral variability of a large sample of quasars using XMM-Newton data, finding that they tend to be softer when brighter, similar to local AGNs, thus extending understanding across luminosity ranges.

Contribution

It provides the first ensemble analysis of spectral variability in high-luminosity quasars using a large, multi-epoch X-ray dataset, revealing consistent softer-when-brighter behavior.

Findings

Quasars exhibit softer spectra when brighter.

Spectral variability behavior is similar across different AGN luminosities.

Large sample analysis confirms previous local AGN results.

Abstract

The variability of the X-ray spectra of active galactic nuclei (AGN) usually includes a change of the spectral slope. This has been investigated for a small sample of local AGNs by Sobolewska and Papadakis, who found that slope variations are well correlated with flux variations, and that spectra are typically steeper in the bright phase (softer when brighter behaviour). Not much information is available for the spectral variability of high-luminosity AGNs and quasars. In order to investigate this phenomenon, we use data from the XMM-Newton Serendipitous Source Catalogue, Data Release 5, which contains X-ray observations for a large number of active galactic nuclei in a wide luminosity and redshift range, for several different epochs. This allows to perform an ensemble analysis of the spectral variability for a large sample of quasars. We quantify the spectral variability through the spectral variability parameter $\beta$, defined as the ratio between the change in spectral slope and the corresponding logarithmic flux variation. We find that the spectral variability of quasars has a softer when brighter behaviour, similarly to local AGNs.