Quasar spectral variability from the XMM-Newton serendipitous source catalogue
1 Dipartimento di Fisica, Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
2 Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
3 Dipartimento di Matematica e Fisica, Università Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
Received: 12 December 2016
Accepted: 20 December 2016
Context. X-ray spectral variability analyses of active galactic nuclei (AGN) with moderate luminosities and redshifts typically show a “softer when brighter” behaviour. Such a trend has rarely been investigated for high-luminosity AGNs (Lbol ≳ 1044 erg/s), nor for a wider redshift range (e.g. 0 ≲ z ≲ 5).
Aims. We present an analysis of spectral variability based on a large sample of 2700 quasars, measured at several different epochs, extracted from the fifth release of the XMM-Newton Serendipitous Source Catalogue.
Methods. We quantified the spectral variability through the parameter β defined as the ratio between the change in the photon index Γ and the corresponding logarithmic flux variation, β = −ΔΓ/Δlog FX.
Results. Our analysis confirms a softer when brighter behaviour for our sample, extending the previously found general trend to high luminosity and redshift. We estimate an ensemble value of the spectral variability parameter β = −0.69 ± 0.03. We do not find dependence of β on redshift, X-ray luminosity, black hole mass or 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 β (in absolute value) for brighter sources. We also find significant correlations for a small number of individual sources, indicating more negative values for some sources.
Key words: surveys / galaxies: active / quasars: general / X-rays: galaxies
© ESO, 2017