Super-Eddington Accretion through a Multiwavelength Lens: Searching for Counterparts of Ultraluminous X-ray Sources

TL;DR

This paper reviews the current understanding of ultraluminous X-ray sources (ULXs), emphasizing their role as laboratories for super-Eddington accretion, and discusses recent discoveries, open questions, and future observational prospects.

Contribution

It provides a comprehensive overview of ULX properties, recent findings, and highlights key open questions and future directions in the study of super-Eddington accretion.

Findings

Detection of coherent pulsations in some ULXs confirms neutron star accretors.

ULXs can host either neutron stars or intermediate-mass black holes.

Understanding of ULX accretion mechanisms and donor star types remains incomplete.

Abstract

Ultraluminous X-ray sources (ULXs) represent the closest and most accessible laboratories to study sustained super-Eddington accretion onto compact objects. Over the past decade, the discoveries of coherent pulsations in a few ULXs has proved that these systems can be powered by accreting neutron stars, while the most luminous and distant ones remain strong candidates for hosting intermediate-mass black holes. Despite the increasing number of available X-ray data and the significant progress in theoretical modeling and simulations, our understanding of ULXs remains incomplete. Key open questions include the nature and mass distribution of the compact objects, the type of the donor stars, the geometry of the accretion disc and its contribution to the observed broadband emission, the mechanisms responsible for the wide spectral and temporal phenomenology, the duration of the super-Eddington accretion phase and its feedback on the host-galaxy environment. Future ground-based facilities will play a crucial role in addressing these issues.