# Ultraluminous X-ray sources

## TL;DR

Ultraluminous X-ray sources are extremely bright binary systems, now believed to predominantly involve neutron stars with super-Eddington accretion, challenging previous black hole models and revealing strong outflows and pulsations.

## Contribution

This paper reviews recent advances showing that many ULXs are neutron stars with super-Eddington accretion, shifting the understanding of their nature.

## Key findings

- Super-Eddington accretion models are now favored for ULXs.
- Pulsations found in four ULXs indicate neutron star hosts.
- Strong outflows are observed from supercritical accretion disks.

## Abstract

Ultraluminous X-ray sources (ULXs) represent a class of binary systems that are more luminous than any black hole in our Galaxy. The nature of these objects remained unclear for a long time. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. In the last few years our understating of these objects was significantly improved, which made the model of super-Eddington accretion much preferable. Both the X-ray and optical spectra provide evidence for the strong outflows coming from supercritical accretion disk. Another surprising result was discovery of pulsations in four ULXs, which claims that these systems must host neutron stars. Besides the presence of pulsations, there is no sharp difference between ultraluminous pulsars and normal ULXs. This fact implies that significant number of known ULXs might eventually be neutron stars.