The BeppoSAX High Energy Large Area Survey (HELLAS) has found so far 200 sources with a 5-10 flux higher than 5 x 10(-14) erg cm(-2) s(-1) in similar to 80 deg(2) of sky. A summary on the optical identifications of a subsample of these sources is presented here, Most of the identified sources are AGN at 0.04 < z < 3.5. The fraction of sources showing absorption/extinction in X-ray/optical to the total is 0.53 +/- 0.10, higher than in previous ROSAT, and ASCA-ROSAT surveys, but in line with scenarios in which the bulk of the hard Cosmic X-ray background (XRB) is made by highly obscured AGN (e.g. Setti & Woltjer 1989, Comastri et al. 1995).
Fiore, F., LA FRANCA, F. (1999). The BeppoSAX HELLAS survey: the nature of the hard X-ray selected sources. ASTRONOMISCHE NACHRICHTEN, 320(4-5), 253-254 [10.1002/1521-3994(199908)320:4/5<253::AID-ASNA253>3.3.CO;2-R].