Quasar spectral variability from the XMM-Newton serendipitous source catalogue
2017
Context. X-ray spectral variability analyses of Active Galactic Nuclei (AGN)
with moderate luminosities and redshifts typically show a softer when brighter
behaviour. Such trend has been rarely investigated for high-luminosity AGNs ($
L_bol\gtrsim 10^44$ erg/s), nor for a wider redshift range (e.g.,
$0\lesssim z\lesssim 5$). Aims. We present an analysis of the spectral
variability based on a large sample of 2,700 quasars, measured at several
different epochs, extracted from the fifth release of the XMM-Newton
Serendipitous Source Catalogue. Methods. We quantify the spectral variability
through the parameter $\beta$ defined as the ratio between the change in the
photon index $\Gamma$ and the corresponding logarithmic flux variation,
$\beta=-\Delta\Gamma/\Delta\log F_X$. Results. Our analysis confirms a softer
when brighter behaviour also for our sample, extending to high luminosity and
redshift the general trend previously found. We estimate an ensemble value of
the spectral variability parameter $\beta=-0.69\pm0.03$. We do not find
dependence of $\beta$ on redshift, X-ray luminosity, black hole mass, Eddington
ratio. A subsample of radio-loud sources shows a smaller spectral variability
parameter. There is also some change with the X-ray flux, with smaller $\beta$
(in absolute value) for brighter sources. We also find significant correlations
for a few individual sources, indicating more negative values for some sources.
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