Relativistic reflection in the average X-ray spectrum of active galactic nuclei in the Véron-Cetty and Véron catalogue

¹ University Federico II, Via Cinthia, Building 6, 80126 Naples, Italy
e-mail: falocco@fisica.unina.it
² Instituto de Física de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain
³ Centro de Astrobiología (CSIC-INTA), Dep. de Astrofísica; ESAC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁴ Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing (IAASARS), National Observatory of Athens (NOA), Palaia Penteli, 15236 Athens, Greece

Received: 7 October 2013
Accepted: 14 June 2014

Abstract

Context. The X-ray spectra of active galactic nuclei (AGN) unveil properties of matter around the super-massive black hole (SMBH).

Aims. We investigate the X-ray spectra of AGN focusing on Compton reflection and fluorescence. These are two of the most important processes of interaction between primary radiation and circumnuclear material that is located far away from the SMBH, as indicated by the unresolved spectral emission lines (most notably the Fe line) in the X-ray spectra of AGN. Contributions from the inner accretion disk, affected by relativistic effects as expected, have also been detected in several cases.

Methods. We studied the average X-ray spectrum of a sample of 263 X-ray unabsorbed AGN that yield 419 023 counts in the 2−12 keV rest-frame band distributed among 388 XMM-Newton spectra.

Results. We fitted the average spectrum using a (basically) unabsorbed power law (representing the primary radiation). From a second model that represents the interaction (through Compton reflection and fluorescence) of this primary radiation with matter located far away from the central engine (e.g. the putative torus), we found that it was very significantly detected. Finally, we added a contribution from interaction with neutral material in the accretion disk close to the central SMBH, which is therefore smeared by relativistic effects, which improved the fit at a 6 sigma. The reflection factors are 0.65 for the accretion disk and 0.25 for the torus. Replacing the neutral disk-reflection with low-ionisation disk reflection, also relativistically smeared, fits the data equally well, suggesting that we do not find evidence for significant ionisation of the accretion disk.

Conclusions. We detect distant neutral reflection associated with a narrow Fe line in the average spectrum of unabsorbed AGN with ⟨ z ⟩ = 0.8. Adding the disk-reflection component associated with a relativistic Fe line improves the data description at a 6 sigma confidence level, suggesting that both reflection components are present. The disk-reflection component accounts for about 70% of the total reflected flux.