An XMM-Newton study of the hard X-ray sky

We report on the spectral properties of a sample of 90 hard X- ray selected serendipitous sources detected in 12 XMM Newton observations with 1 less than or similar to F2- 10 less than or similar to 80 x 10(-14) erg cm(-2) s(-1). Approximately 40% of the sources are optically identified with 0.1 less than or similar to z less than or similar to 2 and most of them are classified as broad line AGNs. A simple model consisting of power law modified by Galactic absorption offers an acceptable fit to similar to 65% of the source spectra. This fit yields an average photon index of (Gamma) approximate to 1.55 over the whole sample. We also find that the mean slope of the QSOs in our sample turns out to remain nearly constant ((Gamma) approximate to 1.8- 1.9) between 0 less than or similar to z less than or similar to 2, with no hints of particular trends emerging along z. An additional cold absorption component with 10(21) less than or similar to N-H less than or similar to 10(23) cm(-2) is required in similar to30% of the sources. Considering only subsamples that are complete in flux, we find that the observed fraction of absorbed sources ( i. e. with N-H greater than or similar to 10(22) cm(-2)) is similar to 30%, with little evolution in the range 2 less than or similar to F2- 10 less than or similar to 80 x 10(-14) erg cm(-2) s(-1). Interestingly, this value is a factor similar to 2 lower than predicted by the synthesis models of the CXB. This finding, detected for the first time in this survey, therefore suggests that most of the heavily obscured objects which make up the bulk of the CXB will be found at lower fluxes ( F2-10 < 10(-14) erg cm(-2) s(-1)). This mismatch together with other recent observational evidences which contrast with CXB model predictions suggest that one ( or more) of the assumptions usually included in these models need to be revised.