  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 (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.
