The HELLAS2XMM survey - III. Multiwavelength observations of hard X-ray selected sources in the PKS 0312-77 field

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 (similar to15%) 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.