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 _ { H } > 22 ) Active Galactic Nuclei ( AGN ) in the 2–10 keV flux range 0.8–20 \times 10 ^ { -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 _ { 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 logN _ { 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 logN _ { H } > 22 absorption .