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This paper presents the analysis of a statistically complete sample of 28serendipitous X-ray sources selected in 82 pointed XMM-Newton fields down to acount-rate of 0.002 counts s^-1(4.5-7.5 keV energy band). This is the firstsample selected in this energy range to have complete spectroscopicidentifications and redshift determinations for all the objects. Apart from oneGalactic source (an interacting binary), all the objects are AGNs. Theiroptical and X-ray properties (derived from the spectral analysis of theXMM-EPIC data) are compared together. The good correlation between the opticalspectral type and the X-ray absorption properties supports the AGN unifiedmodel. Only one object that does not fit the relation between optical and X-rayabsorption is found, namely a Seyfert 1.9 with no evidence of obscuration inthe X-ray band (NH<1.3x10^20 cm^-2). In total, 7 sources out of 27 are heavilyobscured in the X-ray (NH>10^22 cm^-2), corresponding to a surface density of0.7 deg^-2 at the flux limit the sample (4-7x10^-14 erg s^-1 cm^-2 in the4.5-7.5 keV energy band). Among these obscured objects, two sources show alarge (intrinsic) luminosity (L_[2-10keV]>10^44 erg s^-1) and are thusclassified as type 2 QSO. Finally, we have compared the fraction of X-rayabsorbed AGNs (26%) with that predicted by the current XRB synthesis models atthe flux limit of the survey. We find that the models significantly (~90%confidence level) over predict the fraction of absorbed AGNs thus confirmingalso in this hard energy band (4.5-7.5 keV) similar results recently obtainedin the 2-10 keV band.Comment: Accepted for publication in A&

The XMM-NewtonHBS28 sample: Studying the obscuration in hard X-ray selected AGNs