The HELLAS2XMMSurvey. 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.