The HELLAS2XMM survey

Abstract

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 (), the source breakdown includes Broad Line AGN, Narrow Emission–Line Galaxies, and optically “normal” galaxies. All the ten sources at are spectroscopically classified as Broad Line AGNs. A few of them show significant intrinsic X–ray absorption ( 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.