Ultraluminous X-ray sources as magnetically powered sub-Eddington advective accretion flows around stellar mass black holes

Abstract

ABSTRACT In order to explain unusually high luminosity and spectral nature of ultraluminous X-ray sources (ULXs), some of the underlying black holes are argued to be of intermediate mass, between several tens to million solar masses. Indeed, there is a long standing question of missing mass of intermediate range of black holes. However, as some ULXs are argued to be neutron stars too, often their unusual high luminosity is argued by super-Eddington accretions. Nevertheless, all the models are based on non-magnetized or weakly magnetized accretion. There are, however, pieces of evidence that magnetic fields in accretion discs/flows around a stellar mass black hole could be million Gauss. Such a magnetically arrested accretion flow plausibly plays a key role to power many combined disc-jet/outflow systems. Here we show that flow energetics of a 2.5-dimensional advective magnetized accretion disc/outflow system around a stellar mass black hole are sufficient to explain power of ULXs in their hard states. Hence, they are neither expected to have intermediate mass black holes nor super-Eddington accretors. We suggest that at least some ULXs are magnetically powered sub-Eddington accretors around a stellar mass black hole.