Observations of ultraluminous X-ray sources

TL;DR

This paper reviews observational studies of ultraluminous X-ray sources (ULXs) to clarify whether they are extreme stellar black holes or intermediate mass black holes, summarizing insights from multi-wavelength observations.

Contribution

It provides a comprehensive overview of current observational evidence and interpretations regarding the nature of ULXs, highlighting progress and remaining uncertainties.

Findings

ULXs may be stellar mass black holes or intermediate mass black holes.

Multi-wavelength observations have advanced understanding of ULX properties.

Current evidence is inconclusive about ULX true nature.

Abstract

Ultraluminous X-ray sources (ULXs), first observed ~30 years ago, have been argued as extreme stellar mass black hole binaries or a new class intermediate mass black hole. In order to settle this debate, scientists have utilised a wide range of telescopes, exploiting large sections of the electro-magnetic spectrum. Here we review some of the insight gained from these observational studies, collating an overview of our current position in ULX research.