Ultraluminous X‐Ray Sources Powered by Radiatively Efficient Two‐Phase 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 andsubsequent conversion into coronal photon power, the randomized gravitationalbinding energy responsible for powering ULX phenomena must separate from themass of its origin by a means other than, and quicker than, electronscattering-mediated radiative diffusion. Therefore, the release of accretionpower in ULXs is not necessarily subject to Eddington-limited photon trapping,as long as it occurs in a corona. Motivated by these basic considerations, wepresent a model of ULXs powered by geometrically thin accretion onto stellarmass black holes. We argue that the radiative efficiency of the flow remainshigh if the corona is magnetized or optically thin and the majority of theaccretion power escapes in the form of radiation rather than an outflow. Withinthe context of the current black hole X-ray binary paradigm, our ULX model maybe viewed as an extension of the very high state observed in Galactic sources.(abridged)Comment: 11 page