2008A&A...487..119D

We investigate here the X-ray luminosity function (XLF) of absorbed (N_H between 4x10²¹ and 10²⁴/cm2) and unabsorbed (N_H<4x10²¹/cm2) 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/cm2) AGN. 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>7x10^–14erg/cm2/s) 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<4x10²¹/cm2) and 22 are absorbed (N_H between 4x10²¹ and ∼10²⁴/cm2). 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 4x10²¹ and 10²⁴/cm2 divided the sources with N_H below 10²⁴/cm2, corrected for the bias due to the photoelectric absorption) with L_2–10keV>3x10⁴²erg/s 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/cm2. 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/cm2) and Compton thin AGN (N_H≤10²⁴/cm2) decreases from 1.08±0.44 at ∼10⁴³erg/s to 0.57±0.22 at ∼10⁴⁴erg/s to 0.23±0.15 at ∼10⁴⁵erg/s. The results presented here on the anti-correlation between F and -L_x 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.