A long-term study of AGN X-ray variability

Structure function analysis on a ROSAT-XMM quasar sample

¹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
e-mail: riccardo.middei@uniroma3.it
² Dipartimento di Fisica, Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
³ Dip. di Fisica Ettore Pancini, Università di Napoli Federico II, C.U. Monte Sant’Angelo, via Cinthia, 80126 Napoli, Italy
⁴ INFN Sezione di Napoli, via Cinthia, 80126 Napoli, Italy
⁵ Agenzia Spaziale Italiana – Science Data Center, via del Politecnico snc, 00133 Roma, Italy
⁶ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France

Received: 21 October 2016
Accepted: 23 December 2016

Abstract

Context. Variability in the X-rays is a key ingredient in understanding and unveiling active galactic nuclei (AGN) properties. In this band, flux variations occur on short timescales (hours) as well as on larger timescales. While short timescale variability is often investigated in single source studies, only a few works are able to explore flux variation on very long timescales.

Aims. This work aims to provide a statistical analysis of the AGN long term X-ray variability. We study variability on the largest time interval ever investigated for the 0.2−2 keV band, up to approximately 20 yr rest-frame for a sample of 220 sources. Moreover, we study variability for 2700 quasars up to approximatley eight years rest-frame in the same (soft) band.

Methods. We built our source sample using the 3XMM serendipitous source catalogue data release 5, and data from ROSAT All Sky Survey Bright and Faint source catalogues. To ensure that we selected AGN only, we used the Sloan Digital Sky Survey quasar catalogues data releases 7 and 12. Combining ROSAT and XMM-Newton observations, we investigated variability using the structure function analysis which describes the amount of variability as a function of the lag between the observations.

Results. Our work shows an increase of the structure function up to 20 yr. We find no evidence of a plateau in the structure function on these long timescales.

Conclusions. The increase of the structure function at long time lags suggests that variability in the soft X-rays can be influenced by flux variations originated in the accretion disk or that they take place in a region large enough to justify variation on such long timescales.