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 deg2 of sky down to a flux of F5–10keV∼5×10−14 erg cm−2 s−1. 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 1022.5–23.5 cm−2, 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 ef...