The contribution of faint active galactic nuclei to the hard X-ray background

Hard X-ray selection is the most efficient way to discriminate between accr etion-powered sources, such as active galactic nuclei (AGN), and sources do minated by starlight. Hard X-rays are also less affected than other bands b y obscuration. We have therefore carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the largely unexplored 5-10 keV band, finding 180 sources in similar to 50 deg(2) of sky with flux greater than or similar to 5 x 10(-14) ergcm(-2)s(-1). After correction for the non-uniform sky cover age this corresponds to resolving about 30 per cent of the hard cosmic X-ra y background (XRB). Here we report on a first optical spectroscopic identif ication campaign, finding 12 AGN out of 14 X-ray error boxes studied. Seven AGN show evidence for obscuration in X-ray and optical bands, a fraction h igher than in previous ROSAT or ASCA-ROSAT surveys (at 95-99 and 90 per cen t confidence levels respectively), thus supporting the scenario in which a significant fraction of the XRB is created by obscured AGN.