| Abstract
| - The spectroscopic analysis of 117 serendipitous sources in the HELLAS2XMM 1df (1 degree field) survey is described. Of these, 106 sources, of which 86% have a spectroscopic redshift, are used to evaluate the fraction of X-ray absorbed (log $N_{\rm H}>22$) Active Galactic Nuclei (AGN) in the 2-10 keV flux range $0.8{-}20\times10^{-14}$ erg cm -2 s -1. This fraction turns out lower than what is predicted by two well known Cosmic X-Ray Background synthesis models, and the discrepancy is significant at the 99.999% level. This result consolidates the findings recently obtained by other authors. In the flux interval explored, the data are consistent with an intrinsic distribution of the absorbing columns (flat per decade above log $N_{\rm H}>21$) independent of luminosity and redshift, together with an AGN luminosity function evolving purely in luminosity. It is shown that, on the other hand, extrapolation to lower fluxes fails to reproduce the results inferred from the Chandra Deep Field North survey. It is found that about 40% of the high luminosity sources in our sample have best fit log $N_{\rm H}>22$, and the surface density of these X-ray obscured QSOs can then be estimated at about 48 per square degree, at the flux limit of ${\sim}10^{-14}$ erg cm -2 s -1 of the HELLAS2XMM 1df survey. As a side issue, 5 or 6 out of 60 sources, that is about 10%, identified with broad line AGN, turn out to be affected by log $N_{\rm H}>22$ absorption.
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