Exploring the Nature of the Brightest Hyper-luminous X-ray Source

TL;DR

This paper reviews the discovery and multi-wavelength observations of HLX-1, a hyper-luminous X-ray source providing strong evidence for an intermediate mass black hole exceeding 500 solar masses.

Contribution

It compiles recent observational data and analysis of HLX-1, reinforcing the case for an intermediate mass black hole based on spectral, flux variability, and optical counterpart studies.

Findings

HLX-1's luminosity exceeds 10^42 erg/s.

Spectral and flux variability similar to stellar-mass black holes.

Optical observations support a black hole mass over 500 solar masses.

Abstract

The small subset of hyper-luminous X-ray sources with luminosities in excess of ~1E41 erg/s are hard to explain without the presence of an intermediate mass black hole, as significantly super-Eddington accretion and/or very small beaming angles are required. The recent discovery of HLX-1, the most luminous object in this class with a record breaking luminosity of ~1E42 erg/s in the galaxy ESO 243-49, therefore currently provides some of the strongest evidence for the existence of intermediate mass black holes. HLX-1 is almost an order of magnitude brighter than the other hyper-luminous sources, and appears to exhibit X-ray spectral and flux variability similar to Galactic stellar mass black hole X-ray binaries. In this paper we review the current state of knowledge on this intriguing source and outline the results of multi-wavelength studies from radio to ultra-violet wavelengths, including imaging and spectroscopy of the recently identified optical counterpart obtained with the Very Large Telescope. These results continue to support an intermediate mass black hole in excess of 500 Msun