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%.Comment: 16 pages, 12 figures. Emulateapj version. To appear on ApJ, v570, May
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