The BeppoSAX High Energy Large Area Survey (HELLAS) - III. Testing synthesis models for the X-ray background

The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed several te ns of deg(2) of the sky in the 5-10 keV band down to a flux of about 5 x 10 (-14) erg cm(-2) s(-1). The source surface density of 16.9 +/- 6.4 deg(-2) at the survey limit corresponds to a resolved fraction of the 5-10 keV X-ra y background (XRB) of the order of 20-30 per cent. The extrapolation of the HELLAS log N-log S towards fainter fluxes with a Euclidean slope is consis tent with the first XMM-Newton measurements, in the same energy band, which are a factor of 20 times more sensitive. The source counts in the hardest band so far surveyed by X-ray satellites are used to constrain XRB models. It is shown that in order to reproduce the 5-10 keV counts over the range o f fluxes covered by BeppoSAX and XMM-Newton a large fraction of highly abso rbed (log N-H = 23-24 cm(-2)), luminous (L-x > 10(44) erg s(-1)) active gal actic nuclei is needed. A sizeable number of more heavily obscured, Compton -thick, objects cannot be ruled out but they are not required by the presen t data. The model predicts an absorption distribution consistent with that found from the hardness ratios analysis of the so far identified HELLAS sou rces. Interestingly enough, there is evidence of a decoupling between X-ray absorption and optical reddening indicators, especially at high redshifts/ luminosities where several broad-line quasars show hardness ratios typical of absorbed power-law models with log N-H = 22-24 cm(-2).