Ultraluminous X-ray Sources: Accretion Physics at the Extremes

Ultraluminous X-ray sources (ULXs) are the most accessible laboratories for studying the physics of accretion onto compact objects in the super-Eddington regime. While it is now established that ULXs can host stellar-mass compact objects, the nature of the compact object has been firmly determined only for a few cases, thanks to the discovery of coherent pulsations, which imply the presence of an accreting neutron star. In the majority of cases the nature of the compact object, whether a black hole or a neutron star, remains uncertain, as well as the population ratio between the two classes. However, indirect evidence can be used to identify and classify black hole and neutron star ULX candidates through alternative approaches. In this talk, I will introduce the phenomenology of ULXs and present recent results based on the application of artificial intelligence to archival X-ray data.