The cosmological properties of AGN in the XMM-Newton Hard Bright Survey ^*,**

¹ Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Brera, via Brera 21, 20121 Milano, Italy e-mail: roberto.dellaceca@brera.inaf.it
² Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany
³ Instituto de Física de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain
⁴ INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ X-ray & Observational Astronomy Group, Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK
⁶ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK

Received: 22 December 2007
Accepted: 28 April 2008

Abstract

Aims. We investigate here the X-ray luminosity function (XLF) of absorbed (N_H between 4 10²¹ and 10²⁴ cm^-2) and unabsorbed (N_H < 4 10²¹ cm^-2) AGN, the fraction of absorbed AGN as a function of L_X (and z), the intrinsic N_H distribution of the AGN population, and the XLF of Compton thick (N_H > 10²⁴ cm^-2) AGN.

Methods. To carry out this investigation, we have used the XMM-Newton Hard Bright Serendipitous Sample (HBSS), a complete sample of bright X-ray sources (f_x 7 10^-14 erg cm^-2 s^-1) at high galactic latitude (|b| > 20°) selected in the 4.5-7.5 keV energy band. The HBSS sample is now almost completely identified (97% spectroscopic identifications) and it can be safely used for a statistical investigation. The HBSS contains 62 AGN out of which 40 are unabsorbed (or marginally absorbed; N_H < 4 10²¹ cm^-2) and 22 are absorbed (N_H between 4 10²¹ and ~10²⁴ cm^-2).

Results. Absorbed and unabsorbed AGN are characterised by two different XLF with the absorbed AGN population being described by a steeper XLF, if compared with the unabsorbed ones, at all luminosities. The intrinsic fraction F of absorbed AGN (i.e., the fraction of sources with N_H between 4 10²¹ and 10²⁴ cm^-2 divided the sources with N_H below 10²⁴ cm^-2, corrected for the bias due to the photoelectric absorption) with 3 10⁴² erg s^-1  is 0.57 ± 0.11; we find that F decreases with the intrinsic luminosity, and probably, increases with the redshift. Our data are consistent with a flat Log N_H distribution for N_H between 10²⁰ and 10²⁴ cm^-2. Finally, by comparing the results obtained here with those obtained using an optically-selected sample of AGN we derive, in an indirect way, the XLF of Compton thick AGN; the latter is well described by a XLF similar, in shape, to that of absorbed AGN, but having a normalization of about a factor of 2 above. The density ratio between Compton thick AGN (N_H ≥ 10²⁴ cm^-2) and Compton thin AGN (N_H ≤ 10²⁴ cm^-2) decreases from 1.08 ± 0.44 at ~10⁴³ erg s^-1  to 0.57 ± 0.22 at ~10⁴⁴ erg s^-1  to 0.23 ± 0.15 at ~10⁴⁵ erg s^-1.

Conclusions. The results presented here on the anti-correlation between F and are fully consistent with the hypothesis of a reduction of the covering factor of the gas as a function of the luminosity and are clearly inconsistent with the simplest unified scheme of AGN. These results strongly support the recently proposed radiation-limited clumpy dust torus model although alternative physical models are also consistent with the observations.