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

TL;DR

This paper proposes that some ultra-luminous X-ray sources are powered by magnetically arrested accretion flows around stellar mass black holes, explaining their high luminosity without requiring intermediate mass black holes or super-Eddington accretion.

Contribution

It introduces a magnetically powered accretion model that accounts for ULX luminosities around stellar mass black holes, challenging previous intermediate mass or super-Eddington explanations.

Findings

Magnetized accretion flows can produce ULX luminosities in stellar mass black holes.

ULXs can be explained as sub-Eddington, magnetically powered systems.

The model negates the need for intermediate mass black holes.

Abstract

In order to explain unusually high luminosity and spectral nature of ultra-luminous 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, evidences 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.