Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Brera, via Brera 21, 20121 Milano, Italy e-mail: email@example.com
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
Accepted: 28 April 2008
Aims. We investigate here the X-ray luminosity function (XLF) of absorbed (NH between 4 1021 and 1024 cm-2) and unabsorbed (NH < 4 1021 cm-2) AGN, the fraction of absorbed AGN as a function of LX (and z), the intrinsic NH distribution of the AGN population, and the XLF of Compton thick (NH > 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 (fx 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; NH < 4 1021 cm-2) and 22 are absorbed (NH 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 NH between 4 1021 and 1024 cm-2 divided the sources with NH 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 NH distribution for NH 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 (NH ≥ 1024 cm-2) and Compton thin AGN (NH ≤ 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.
Key words: surveys / galaxies: active / galaxies: evolution / X-rays: diffuse background / X-ray: galaxies / quasars: general
© ESO, 2008