Averaging the AGN X-ray spectra from deep Chandra fields

¹ Instituto de Física de Cantabria (CSIC-UC), 39005 Santander, Spain
e-mail: falocco@ifca.unican.es
² Astropysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London, SW7 2AZ, UK
³ Max Planck Institute für Extraterrestriche Physik, 85748 Garching, Germany
⁴ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁵ Osservatorio Astronomico di Brera INAF, via Brera 28, CAP 20121, Milano, Italy

Received: 29 August 2011
Accepted: 10 November 2011

Abstract

Context. The emission line most often observed in the X-ray spectra of active galactic nuclei (AGN) is Fe K_α. It is known that it can be broadened and deformed by relativistic effects if emitted close enough to the central super massive black hole (SMBH). Recent statistical studies of the X-ray spectra of AGN samples have found that a narrow Fe line is ubiquitous, while whether the broad features are as common it is still uncertain.

Aims. We present here the results of an investigation on the characteristics of the Fe line in the average X-ray spectra of AGN in deep Chandra fields.

Methods. The average spectrum of the AGN was computed using Chandra spectra with more than 200 net counts from the AEGIS, Chandra deep field north (CDFN) and Chandra deep field south (CDFS) surveys. The sample spans a broader range of X-ray luminosities than other samples studied with stacking methods up to z ~ 3.5. We analyzed the average spectra of this sample using our own averaging method, checking the results against extensive simulations. Subsamples defined in terms of column density of the local absorber, redshift, and luminosity were also investigated.

Results. We found a very significant Fe line with a narrow profile in all our samples and in almost all the subsamples that we constructed. The equivalent width of the narrow line estimated in the average spectrum of the full sample is 74 eV. In the subsample of AGN with L < 1.43 × 10⁴⁴ erg s^-1 and z < 0.76, the equivalent width is 108 eV.