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 \buildrel < \over { \sim } F _ { 2 - 10 } \buildrel < \over { \sim } 80 \times 10 ^ { -14 } erg cm ^ { -2 } s ^ { -1 } .
Approximately 40 % of the sources are optically identified with 0.1 \buildrel < \over { \sim } z \buildrel < \over { \sim } 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 \sim 
65 % of the source spectra .
This fit yields an average photon index of \langle \Gamma \rangle \approx 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 ( \langle \Gamma \rangle \approx 1.8–1.9 ) between 0 \buildrel < \over { \sim } z \buildrel < \over { \sim } 2 , with no hints of particular trends emerging along z .
An additional cold absorption component with 10 ^ { 21 } \buildrel < \over { \sim } N _ { H } \buildrel < \over { \sim } 10 ^ { 23 } cm ^ { -2 } is required in \sim 30 % 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 } \buildrel > \over { \sim } 10 ^ { 22 } cm ^ { -2 } ) is \sim 30 % , with little evolution in the range 2 \buildrel < \over { \sim } F _ { 2 - 10 } \buildrel < \over { \sim } 80 \times 10 ^ { -14 } erg cm ^ { -2 } s ^ { -1 } .
Interestingly , this value is a factor \sim 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 ( F _ { 2 - 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 .