The HELLAS2XMM Survey. II. Multiwavelength Observations of P3: An X-Ray-bright, Optically Inactive Galaxy

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray-bright galaxies is emerging at 2-10 keV fluxes of the order of 10^-14 ergs cm^-2 s^-1. Although they might constitute an important fraction of the sources responsible for the hard X-ray background, their nature is still unknown. With the aim of better understanding the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOU J031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z=0.159, usually known as FIORE P3. Analysis of the broadband energy distribution suggests the presence of a heavily obscured active nucleus. This work is based on observations collected at the European Southern Observatory (proposal 66.B-0472A), La Silla and Paranal, Chile, and observations made with the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA).