Ultraluminous X-ray Sources Powered by Radiatively Efficient Two-phased Super-eddington Accretion onto Stellar Mass Black Holes

The radiation spectra of many of the brightest ultraluminous X-ray sources (ULXs) are dominated by a hard power law component, likely powered by a hot, optically thin corona that Comptonizes soft seed photons emitted from a cool, optically thick black hole accretion disk. Before its dissipation and subsequent conversion into coronal photon power, the randomized gravitational binding energy responsible for powering ULX phenomena must separate from the mass of its origin by a means other than, and quicker than, electron scattering-mediated radiative diffusion. Therefore, the release of accretion power in ULXs is not necessarily subject to Eddington-limited photon trapping, as long as it occurs in a corona. Motivated by these basic considerations, we present a model of ULXs powered by geometrically thin accretion onto stellar mass black holes. In the region closest to the hole (region I), where the majority of the binding energy is released, cool thermal disk radiation is Comptonized by an adjacent corona covering the entire surface of the disk. The amount of reprocessed thermal emission in region I is quite small compared to the hard coronal output since the disk behaves as a near perfect reflector of X-rays, a result of the intense ionizing flux which leads to an extreme state of photo-ionization. If energy injection takes place within an optically thin corona, the conversion of binding energy into a wind is hampered by Compton drag and the wind’s low optical depth. Furthermore, if the magnetic field geometry of the corona is primarily closed, then magnetic fields of modest strength can in principle, prevent the launching of a wind. In the outer regions (region II), where the albedo is somewhat lower, thermal emission resulting from a combination of viscous dissipation within the body of the disk and reprocessed coronal power is emitted at relatively low temperatures, due to the large surface area. Within the context of the current black hole X-ray binary paradigm, our ULX model may be viewed as an extension of the very high state observed in Galactic sources. Subject headings: accretion, accretion disks – black hole physics – X-rays: binaries – X-rays: galaxies 1. OVERVIEW OF OBSERVATIONS, CURRENT INTERPRETATIONS, AND PLAN OF THIS WORK Per detected photon, ultraluminous X-ray sources (ULXs) garner a disproportionate amount of attention. The reason for such relative popularity results from the two most common interpretations of their observed behavior: 1) super-Eddington luminosities resulting from accretion onto stellar mass black holes 2) sub-Eddington accretion onto intermediate mass black holes. Both explanations are problematic from a theoretical point of view. With respect to super-Eddington luminosities, it is not clear how a radiatively efficient flow can release binding energy at a rate above the Eddington limit in a steady-state manner. As for intermediate mass black holes, a well-established evolutionary path that accounts for their birth does not exist. Below, we summarize some important observational features of ULXs and previous theoretical models of their behavior in hopes to motivate our own work.