A long term study of AGN X-ray variability. Structure function analysis
  on a ROSAT-XMM quasar sample
  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 time scales (hours) as well as on larger times scales. While short time
scale variability is often investigated in single source studies, only few
works are able to explore flux variation on very long time scales.This work
provides 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 $\sim 20$ years rest-frame for a sample of 220 sources.
Moreover, we study variability for 2,700 quasars up to $\sim 8$ years
rest-frame in the same (soft) band.We build our source sample using the 3XMM
serendipitous source catalogue data release 5, and data from ROSAT All Sky
Survey Bright and Faint source catalogues. In order to select only AGN we use
the Sloan Digital Sky Survey quasar catalogues data releases 7 and 12.
Combining ROSAT and XMM-Newton observations, we investigate variability using
the structure function analysis which describes the amount of variability as a
function of the lag between the observations.Our work shows an increase of the
structure function up to 20 years. We do not find evidence of a plateau in the
structure function on these long time scales.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 time
scales.
