The cosmological properties of AGN in the XMM-Newton Hard Bright Survey
Page, M. J.; Carrera, F. J.; Caccianiga, A.; Severgnini, P.; Della Ceca, R.; Maccacaro, T.; Brunner, H.; Cocchia, F.; Mateos, S.; Tedds, J. A.
Italy, Germany, Spain, United Kingdom
Abstract
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 L2{-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 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 -Lx 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.