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

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 L_2{-10 keV} ⪆ 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 -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.

Appendix A is only available in electronic form at http://www.aanda.org^, Table 1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/487/119