Ultraluminous X-ray sources powered by radiatively efficient two-phase super-Eddington accretion onto stellar-mass black holes

A hard power-law component dominates the radiation spectra of many of the brightest ultraluminous X-ray sources (ULXs). Therefore, a hot, optically thin corona likely powers the emission by Comptonizing seed photons emitted by a cooler, optically thick accretion disk. Before its dissipation and conversion into coronal radiation, the randomized gravitational binding energy responsible for powering ULXs must separate from the mass of its origin by a means quicker than electron-scattering-mediated radiative diffusion. Therefore, the accretion power released in ULXs is not necessarily subject to photon trapping, as long as it occurs in a corona. Motivated by these considerations, we present a model of ULXs powered by geometrically thin accretion onto stellar-mass black holes. In the innermost region of the disk, where the majority of the binding energy is released, an adjacent corona covering the entire disk surface Comptonizes the cool thermal radiation. The intense photoionizing flux and subsequent high ionization level produces an albedo near unity for X-ray photons. Therefore, the amount of reprocessed emission in this region is small relative to the non-thermal output. If dissipation takes place within an optically thin corona, Compton drag and the wind's low optical depth hamper the driving of a wind. If the magnetic field geometry of the corona is primarily closed, then fields of modest strength can, in principle, prevent the launching of a wind. In the outer region, the albedo is lower, and the surface area is larger. Therefore, the disk emits lower temperature thermal radiation resulting from both viscous dissipation within the body of the disk and reprocessed coronal power. Thus, this model qualitatively reproduces the main features of most highly luminous ULX spectra.