The BeppoSAX hellas survey On the nature of faint hard X-ray selected sources

The BeppoSAX 4.5-10 keV High Energy Large Area Survey has covered about 80 deg<SUP>2</SUP> of sky down to a flux of F<SUB>5-10keV</SUB>~510<SUP>-14</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>. Optical spectroscopic identification of <~ half of the sources in the sample (62) shows that many (~50%) are highly obscured AGN, in line with the predictions of AGN synthesis models for the hard X-ray background (XRB, see e.g. Comastri et al. 1995). The X-ray data, complemented by optical, near-IR and radio follow-up, indicate that the majority of these AGN are ``intermediate'' objects, i.e. type 1.8-1.9 AGN, `red' quasars, and even a few broad line, blue continuum quasars, obscured in X-rays by columns of the order of 10<SUP>22.5-23.5</SUP> cm<SUP>-2</SUP>, but showing a wide dispersion in optical extinction. The optical and near-IR photometry of the obscured objects are dominated by galaxy starlight, indicating that a sizeable fraction of the accretion power in the Universe may actually have been missed in optical color surveys. This also implies that multicolor photometry techniques may be efficiently used to assess the redshift of the hard X-ray selected sources. .