We present extensive optical , radio and infrared follow-up observations of a sample of 35 hard X–ray ( 2–10 keV ) selected sources discovered serendipitously in the PV XMM– Newton observation of the radio–loud quasar PKS 0312–77 field , for which also an archival Chandra observation is available .
The observations have been carried out as part of the HELLAS2XMM survey , a program aimed to understand the nature of the sources responsible for the bulk of the hard X–ray Background ( XRB ) .
The identification of the optical counterparts greatly benefits from the positional accuracy obtained from Chandra and radio observations .
As a consequence , the spectroscopic completeness of the present sample ( 80 % ) is limited only by the faintness of the optical counterparts .
The multiwavelength coverage of our survey allows us to unveil a large spread in the overall properties of hard X–ray selected sources .
At low redshift ( z < 1 ) , the source breakdown includes Broad Line AGN , Narrow Emission–Line Galaxies , and optically “ normal ” galaxies .
All the ten sources at z > 1 are spectroscopically classified as Broad Line AGNs .
A few of them show significant intrinsic X–ray absorption ( N _ { H } > 10 ^ { 22 } cm ^ { -2 } ) , further supporting previous evidence of a decoupling between optical and X–ray properties at high luminosities and redshifts .
Finally , a non negligible fraction ( \sim 15 % ) of the hard X–ray sources are not detected down to the limiting magnitude of the optical images .
The corresponding high X–ray to optical flux ratio , X–ray and optical–infrared colors strongly suggest that they are high redshift , obscured AGN .