Observations of Ultraluminous X-ray Sources

Ultraluminous X-ray sources (ULXs) have luminosities exceeding that of the Eddington limit for spherical accretion onto stellar-mass black holes (~3-20 M(solar)). Such high luminosities have provoked debate over the true nature of these sources. Some argue that such objects are intermediate-mass black holes (IMBHs) residing in standard accretion states, while others propose that these are stellar-mass compact objects residing in a (new) extreme, super-Eddington accretion state. I will review some of the observational evidence supporting each side of the debate and present current ideas on the nature of these systems.