Ultraluminous X-ray sources: three exciting years

TL;DR

This paper reviews recent advances in understanding Ultraluminous X-ray Sources (ULXs), highlighting the shift towards super-Eddington accretion interpretations and discussing notable exceptions like neutron star and intermediate-mass black hole candidates.

Contribution

It provides a comprehensive review of recent progress in ULX research, emphasizing the consensus on super-Eddington accretion and discussing notable outliers.

Findings

Super-Eddington accretion is now the prevailing explanation for most ULXs.

Some ULXs are confirmed to be neutron stars.

Evidence suggests the presence of intermediate-mass black hole candidates.

Abstract

The extreme extragalactic sources known as Ultraluminous X-ray Sources (ULX) represent a unique testing environment for compact objects population studies and the accretion process. Their nature has long been disputed. Their luminosity, well above the Eddington luminosity for a stellar-mass black hole, can imply the presence of an intermediate-mass black hole or a stellar black hole accreting above the Eddington limit. Both these interpretations are important to understand better the accretion process and the evolution of massive black holes. The last few years have seen a dramatic improvement of our knowledge of these sources. In particular, the super-Eddington interpretation for the bulk of the ULX population has gained a strong consensus. Nonetheless, exceptions to this general trend do exist, and in particular one ULX was shown to be a neutron star, and another was shown to be a very likely IMBH candidate. In this paper, I will review the progress done in the last few years.