  We report on the spectral properties of a sample of 90 hard X-ray selected
serendipitous sources detected in 12 XMM observations with 1<F(2-10)<80
10^(-14) erg/cm2/s. Approximately 40% of the sources are optically identified
with 0.1<z<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 ~65% of the source spectra. This fit yields an average photon index of
<Gamma>~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>~1.8-1.9)
between 0<z<2, with no hints of particular trends emerging along z. An
additional cold absorption component with 10^(21)<Nh<10^(23) cm^(-2) is
required in ~30% of the sources. Considering only subsamples that are complete
in flux, we find that the observed fraction of absorbed sources (i.e. with
Nh>~10^(22) cm^(-2)) is ~30%, with little evolution in the range 2<F(2-10)<80
10^(-14) erg/cm2/s. Interestingly, this value is a factor ~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/cm2/s). 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.
