# Ultraluminous X-Ray Sources

## TL;DR

This review discusses the nature of ultraluminous X-ray sources (ULXs), highlighting evidence for super-Eddington accretion, neutron-star accretors, and potential intermediate-mass black holes, with implications for broader astrophysical phenomena.

## Contribution

It synthesizes observational evidence and models to advance understanding of ULXs, emphasizing their role in super-Eddington accretion and black hole evolution.

## Key findings

- Detection of neutron-star accretors in ULXs
- Evidence for super-Eddington outflows and luminosities
- Discussion of intermediate-mass black hole candidates

## Abstract

We review observations of ultraluminous X-ray sources (ULXs). X-ray spectroscopic and timing studies of ULXs suggest a new accretion state distinct from those seen in Galactic stellar-mass black hole binaries. The detection of coherent pulsations indicates the presence of neutron-star accretors in three ULXs and therefore apparently super-Eddington luminosities. Optical and X-ray line profiles of ULXs and the properties of associated radio and optical nebulae suggest that ULXs produce powerful outflows, also indicative of super-Eddington accretion. We discuss models of super-Eddington accretion and their relation to the observed behaviors of ULXs. We review the evidence for intermediate mass black holes in ULXs. We consider the implications of ULXs for super-Eddington accretion in active galactic nuclei, heating of the early universe, and the origin of the black hole binary recently detected via gravitational waves.