Ultraluminous X-ray sources as super-Eddington accretion disks

TL;DR

This paper reviews the nature of Ultraluminous X-ray sources, suggesting they are likely stellar-mass black holes with super-Eddington accretion, supported by spectral evidence and similarities to SS 433.

Contribution

It provides a comprehensive review of ULX properties, emphasizing optical spectroscopy evidence supporting supercritical accretion models.

Findings

ULXs show spectral features indicating hot winds from supercritical accretion disks.

Optical spectra of ULX counterparts resemble that of SS 433.

Super-Eddington accretion is a plausible explanation for ULX luminosities.

Abstract

The origin of Ultraluminous X-ray sources (ULXs) in external galaxies whose X-ray luminosities exceed those of the brightest black holes in our Galaxy by hundreds and thousands of times is mysterious. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. Here we review the ULX properties, their X-ray spectra indicate a presence of hot winds in their accretion disks supposing the supercritical accretion. However, the strongest evidences come from optical spectroscopy. The spectra of the ULX counterparts are very similar to that of SS 433, the only known supercritical accretor in our Galaxy.