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

TL;DR

ULXs are accessible laboratories for sustained super-Eddington accretion, with luminosities surpassing $L_ ext{Edd}$ for a $10\,M_\odot$ black hole ($L_ ext{Edd}\approx 10^{39}$ erg s$^{-1}$). The paper synthesizes evidence for accreting neutron stars from X-ray pulsations and discusses how optical/IR counterparts, donor stars, disc winds, and reprocessing shape the broadband emission. It advocates a coordinated, multiwavelength, time-domain strategy to identify counterparts, measure orbital parameters, and constrain the compact-object mass distribution, including IMBH possibilities. Finally, it outlines technology and data-handling requirements for the 2040s, emphasizing fast optical/UV/IR timing facilities in concert with NewAthena and future GW detectors to quantify the duration and impact of the super-Eddington phase.

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.