EDP Sciences
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Volume 462, Number 1, January IV 2007
Page(s) 57 - 66
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20066277

A&A 462, 57-66 (2007)
DOI: 10.1051/0004-6361:20066277

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

S. Sazonov1, 2, M. Revnivtsev1, 2, R. Krivonos2, 1, E. Churazov1, 2, and R. Sunyaev1, 2

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)

Aims.We study the hard X-ray luminosity function and absorption distribution of local ( $z\lesssim 0.1$) 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 (>$5\sigma$) 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 $\vert b\vert>5^\circ$, where the survey's identification completeness is $\sim $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 $f_{\rm a}$ of obscured (log $N_{\rm H}>22$) objects is much higher (~70%) among the low-luminosity AGN ( $L_{\rm hx}<10^{43.6}$ erg s-1) than among the high-luminosity ones ( $L_{\rm hx}>10^{43.6}$ erg s-1), $f_{\rm a}\sim 25$%, where $L_{\rm hx}$ 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 40<log $L_{\rm hx}<45.5$ with a characteristic luminosity of log $L_\ast=43.40\pm 0.28$. The estimated local luminosity density due to AGN with log $L_{\rm hx}>40$ is $(1.4\pm 0.3)\times
10^{39}$ 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 $N_{\rm H}$ distribution of AGN (for a given $L_{\rm hx}/L_\ast(z)$ ratio) has not changed significantly since $z\sim 1.5$, while the AGN luminosity function has experienced pure luminosity evolution.

Key words: surveys -- galaxies: active -- galaxies: evolution -- galaxies: Seyfert -- X-rays: diffuse background

© ESO 2007