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

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posted on 2016-08-03, 12:30 authored by R. Della Ceca, A. Caccianiga, P. Severgnini, T. Maccacaro, H. Brunner, F. J. Carrera, F. Cocchia, S. Mateos, M. J. Page, Jonathan A. Tedds
Aims. We investigate here the X-ray luminosity function (XLF) of absorbed ($N_{\rm H}$ between 4 $\times$ 1021 and 1024 cm-2) and unabsorbed ($N_{\rm H}$ < 4 $\times$ 1021 cm-2) AGN, the fraction of absorbed AGN as a function of $L_{\rm X}$ (and z), the intrinsic $N_{\rm H}$ distribution of the AGN population, and the XLF of Compton thick ($N_{\rm H}$ > 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 ($f_{\rm x}$ $\ga$ 7 $\times$ 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; $N_{\rm H}$ < 4 $\times$ 1021 cm-2) and 22 are absorbed ($N_{\rm H}$ between 4 $\times$ 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 $N_{\rm H}$ between 4 $\times$ 1021 and 1024 cm-2 divided the sources with $N_{\rm H}$ below 1024 cm-2, corrected for the bias due to the photoelectric absorption) with $L_{2{-}10~{\rm keV}}$ $\ga$ 3 $\times$ 1042 erg s-1 is 0.57 $\pm$ 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 $N_{\rm H}$ distribution for $N_{\rm H}$ 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 ($N_{\rm H}$ $\geq$ 1024 cm-2) and Compton thin AGN ($N_{\rm H}$ $\leq$ 1024 cm-2) decreases from 1.08 $\pm$ 0.44 at ~1043 erg s-1 to 0.57 $\pm$ 0.22 at ~1044 erg s-1 to 0.23 $\pm$ 0.15 at ~1045 erg s-1. Conclusions. The results presented here on the anti-correlation between F and $-L_{\rm x}$ 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.



ASTRONOMY & ASTROPHYSICS, 2008, 487 (1), pp. 119-130

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/Organisation/COLLEGE OF MEDICINE, BIOLOGICAL SCIENCES AND PSYCHOLOGY/School of Medicine/Department of Health Sciences


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