Ultraluminous X-Ray Sources

TL;DR

Ultraluminous X-ray sources are a distinct class of objects with luminosities exceeding typical black hole limits, now understood to involve supercritical accretion disks and neutron stars, based on extensive multi-wavelength observations.

Contribution

This review consolidates observational data and theoretical understanding of ULXs and SS 433, emphasizing the role of supercritical accretion and recent neutron star discoveries.

Findings

ULXs exhibit supercritical accretion disk characteristics.

Neutron stars have been identified in some ULXs.

Observations support supercritical accretion as the main mechanism.

Abstract

Ultraluminous X-ray sources (ULXs) were identified as a separate class of objects in 2000 based on data from the Chandra X-Ray Observatory. These are unique objects: their X-ray luminosities exceed the Eddington limit for a typical stellar-mass black hole. For a long time, the nature of ULXs remained unclear. However, the gradual accumulation of data, new results of X-ray and optical spectroscopy, and the study of the structure and energy of nebulae surrounding ULXs led to the understanding that most of the ultraluminous X-ray sources must be supercritical accretion disks like SS 433. The discovery of neutron stars in a number of objects only increased the confidence of the scientific community in the conclusions obtained, since the presence of neutron stars in such systems clearly indicates a supercritical accretion regime. In this review, we systematize the main facts about the observational manifestations of ULXs and SS 433 in the X-ray and optical ranges and discuss their explanation from the point of view of the supercritical accretion theory.