Ultraluminous X-ray Sources in the Chandra and XMM-Newton Era

TL;DR

This review synthesizes ten years of X-ray and multiwavelength observations of ULXs, discussing their properties, possible black hole types, and implications for accretion physics and black hole formation.

Contribution

It provides a comprehensive overview of ULX observational data, spectral models, and the evidence for different black hole mass classes, highlighting recent advances and remaining uncertainties.

Findings

ULXs may be powered by stellar, massive stellar, or intermediate mass black holes.

X-ray timing constrains black hole masses in ULXs.

Multiwavelength studies reveal diverse properties and environments of ULXs.

Abstract

Ultraluminous X-ray sources (ULXs) are accreting black holes that may contain the missing population of intermediate mass black holes or reflect super-Eddington accretion physics. Ten years of Chandra and XMM-Newton observations of ULXs, integrated by multiband studies of their counterparts, have produced a wealth of observational data and phenomenological classifications. We review the properties of their host galaxies, list popular spectral models and implications for standard and supercritical accretion physics, demonstrate how X-ray timing of these objects places constraints on their masses. We also review multiwavelength studies of ULXs, including the optical emission of the binary system and nebulosity around them. We summarize that three classes of black holes could power ULXs: normal stellar mass black holes (~10 solar masses), massive stellar black holes (~< 100 solar masses), and intermediate mass black holes (10^2 - 10^4 solar masses). We collect evidence for the presence of these three types of compact objects, including caveat of each interpretation, and briefly review their formation processes.