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 tens of deg² of the sky in the 5-10keV band down to a flux of about 5×10^-14ergcm^-2s^-1. The source surface density of 16.9+/-6.4deg^-2 at the survey limit corresponds to a resolved fraction of the 5-10keV X-ray background (XRB) of the order of 20-30 per cent. The extrapolation of the HELLAS logN-logS towards fainter fluxes with a Euclidean slope is consistent 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 5-10keV counts over the range of fluxes covered by BeppoSAX and XMM-Newton a large fraction of highly absorbed (logN_H=23-24cm^-2), luminous (L_X>10⁴⁴ergs^-1) active galactic nuclei is needed. A sizeable number of more heavily obscured, Compton-thick, objects cannot be ruled out but they are not required by the present data. The model predicts an absorption distribution consistent with that found from the hardness ratios analysis of the so far identified HELLAS sources. 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 logN_H=22-24cm^-2.