On the Nature of X-ray Bright Optically Normal Galaxies

Recent X-ray surveys by Chandra and XMM-Newton have revealed a population of X-ray bright, optically normal galaxies (XBONGs) at moderate redshifts. By analogy with nearby low-luminosity active galactic nuclei, we propose that many XBONGs are powered by an inner radiatively inefficient accretion flow (RIAF) plus an outer standard thin accretion disk. The absence of optical/UV activity in XBONGs is explained by the truncation of the thin disk near the black hole, and the relatively strong X-ray emission is explained as inverse Compton emission from the hot RIAF. We show that the spectrum of the source P3, a prototypical XBONG, can be fit fairly well with such a model. By comparing P3 to other accreting black holes, we argue that XBONGs are intermediate in their characteristics between distant luminous active galactic nuclei and nearby low-luminosity nuclei. Subject headings: accretion, accretion disks – black hole physics – galaxies: active – galaxies: nuclei – hydrodynamics