Spectral Transition for ULXs - Super-Eddington Luminosity from Rapidly Spinning Moderate Mass BHs?

Abstract.

The X-ray luminosity of ultraluminous compact X-ray sources (ULXs) reaches 10^39-40 erg/s. To avoid super-Eddington luminosity high black hole masses of up to $100 M_{\rm solar}$ are required. But severe problems are encountered with this interpretation. We suggest that the high luminosities are truly super-Eddington, but that the source of energy is not accretion, but magnetic extraction of energy from a rapidly rotating spinning black hole. The dissipation of this energy fills the corona above the disk with moderately relativistic particles, part of which impinge on the accretion disk underneath and provide its super-Eddington luminosity. At the same time the super-Eddington radiation pressure from below is confined by the particle impact pressure from above. We discuss requirements and derive order of magnitude estimates for magnetic field strength, particle density, relativistic $\gamma$-factor and surface density of the disk.