X-ray observations of ultraluminous X-ray sources

TL;DR

This paper reviews X-ray observations of ultraluminous X-ray sources (ULXs), highlighting how recent high-quality data from XMM-Newton helps distinguish between models of their extreme luminosities and underlying compact objects.

Contribution

It provides a comprehensive review of recent X-ray studies of ULXs, emphasizing the role of XMM-Newton data in understanding their nature and the physical models involved.

Findings

Deep XMM-Newton observations reveal diverse spectral states of ULXs.

Data quality enables discrimination between super-Eddington accretion and intermediate-mass black hole models.

Insights into the accretion physics and compact object types in ULXs.

Abstract

Ultraluminous X-ray sources (ULXs) are amongst the most intriguing of X-ray source classes. Their extreme luminosities - greater than 10^39 erg/s in the 0.3 - 10 keV band alone - suggest either the presence of black holes larger than those regularly encountered in our own Galaxy (the Galactic centre excepted), or sources apparently radiating well above the Eddington limit. We review the insights afforded us by studies of their X-ray emission, focussing on what this reveals about the underlying compact object. In particular, we discuss recent deep observations of ULXs by the XMM-Newton observatory, and how the unprecedented data quality provided by this mission is starting to discriminate between the different physical models for these extraordinary X-ray emitters.