Hard X-ray luminosity function and absorption distribution of nearby AGN: INTEGRAL all-sky survey

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany e-mail: sazonov@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

Received: 20 August 2006
Accepted: 24 September 2006

Abstract

Aims.We study the hard X-ray luminosity function and absorption distribution of local () active galactic nuclei (AGN) and discuss the implications for AGN cosmological evolution and for the cosmic X-ray background (CXB).

Methods.We use the INTEGRAL all-sky hard X-ray survey to perform a statistical study of a representative sample of nearby AGN. Our entire all-sky sample consists of 127 AGN, of which 91 are confidently detected (>) on the time-averaged map obtained with the IBIS/ISGRI instrument and 36 are detected only during single observations. Among the former there are 66 non-blazar AGN located at , where the survey's identification completeness is ∼93%, which we use for calculating the AGN luminosity function and X-ray absorption distribution.

Results.In broad agreement with previous studies, we find that the fraction f_a of obscured (log) objects is much higher (~%) among the low-luminosity AGN ( erg s^-1) than among the high-luminosity ones ( erg s^-1), %, where L_hx is the luminosity in the 17–60 keV energy band. We also find that locally the fraction of Compton-thick AGN is less than 20% unless there is a significant population of AGN that are so strongly obscured that their observed hard X-ray luminosities fall below ~ 10⁴⁰–10⁴¹ erg s^-1, the effective limit of our survey. The constructed hard X-ray luminosity function has a canonical, smoothly broken power-law shape in the range log with a characteristic luminosity of log. The estimated local luminosity density due to AGN with log is  erg s^-1 Mpc^-3 (17–60 keV). We demonstrate that the spectral shape and amplitude of the CXB are consistent with the simple scenario in which the N_H distribution of AGN (for a given ratio) has not changed significantly since , while the AGN luminosity function has experienced pure luminosity evolution.