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

Abstract

Aims. We study the hard X-ray luminosity function and absorption distribution of local (z ≲ 0.1) 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 (>5σ) 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 |b| > 5°, 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 fa of obscured (log NH > 22) objects is much higher (∼70%) among the low-luminosity AGN (Lhx < 1043.6 erg s-1) than among the high-luminosity ones (Lhx > 1043.6 erg s-1), fa ∼ 25%, where Lhx 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 40 < log L hx < 45.5 with a characteristic luminosity of log L* = 43.40 ±0.28. The estimated local luminosity density due to AGN with log L hx, > 40 is (1.4 ± 0.3) × 1039 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 NH distribution of AGN (for a given Lhx/L*(z) ratio) has not changed significantly since z ∼ 1.5, while the AGN luminosity function has experienced pure luminosity evolution. © ESO 2007.