The BeppoSAX view of the hard X-ray background

Abstract First results on a medium-deep X-ray survey in the “new” 5–10 keV band carried out with the MECS detectors onboard BeppoSAX are presented. The High Energy Llarge Area Survey (HELLAS) is aimed to directly explore a band where the energy density of the X-ray background is more than twice than that in the soft (0.5-2.0 keV) band. The optical identification follow-up of the first ten HELLAS hard X-ray sources indicate that Active Galactic Nuclei are the dominant population at 5–10 keV fluxes of the order of 10 −13 erg cm −2 s −1 . We discuss the implications of these findings for the AGN synthesis models for the XRB.