The BeppoSAX High‐Energy Large‐Area Survey. V. The Nature of the Hard X‐Ray Source Population and Its Evolution
La Franca, F. ; Fiore, F. ; Vignali, C. ; ...Giommi, P. ; et al.
May - 2002
ISSN : 0004-637X ;
journal : The Astrophysical Journal

Issue : 1
type: Article Journal

Abstract
We present optical spectroscopic identifications of hard X-ray (5-10 keV) selected sources belonging to the HELLAS sample obtained with BeppoSAX down to a 5-10 keV flux limit of f\_\5-10 keV\3x10\^-14 erg cm\^-2 s\^-1. The sample consists of 118 sources. 25 sources have been identified trough correlations with catalogues of known sources. 49 have been searched for spectroscopic identification at the telescope. 13 fields resulted empty down to R=21. 37 sources have been identified with type 1 AGN and 9 with type 2 AGN. The remaining are: 5 narrow emission line galaxies, 6 Clusters, 2 BL Lac, 1 Radio Galaxy and 1 Star. Combining these objects with other hard X-ray selected AGNs from ASCA and HEAO1, we find that the local luminosity function of type 1 AGN in the 2-10 keV band is fairly well represented by a double-power-law-function. There is evidence for significant cosmological evolution according to a pure luminosity evolution (PLE) model L\_X(z)(1+z)\^k, with k=2.12 and k=2.22 in a (Omega\_m,Omega\_lambda)=(1.0,0.0) and in a (Omega\_m,Omega\_lambda)=(0.3,0.7) cosmology, respectively. The data show an excess of faint high redshift type 1 AGN which is well modeled by a luminosity dependent density evolution (LDDE), similarly to what observed in the soft X-rays. However, in both cosmologies, the statistic is not significant enough to distinguish between the PLE and LDDE models. The fitted models imply a contribution of AGN1 to the 2-10 keV X-ray background from 35\% up to 60\%.

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