Ultraluminous X-ray sources (ULXs) are among the most luminous objects in the Universe. First identified in external galaxies, ULXs have also been found in our own Milky Way. These powerful X-ray emitters are thought to be powered by either stellar-mass or intermediate-mass black holes accreting at super-Eddington rates.
Although ULXs were first detected in other galaxies, several dozen ULX candidates have since been discovered in our own Milky Way. The majority of these Galactic ULXs are associated with star-forming regions, and may represent a young population of accreting black holes. In contrast, a small number of Galactic ULXs are found in elliptical galaxies, which contain little ongoing star formation. These “elliptical ULXs” may instead be powered by older stellar-mass black holes that have been kicked out of their birthplaces and onto wide orbits around their host galaxies.
Most ULX studies to date have focused on individual objects or small samples. However, recent surveys with Chandra and XMM-Newton have begun to reveal the demographics of the larger population of extragalactic ULXs. These surveys suggest that the majority of bright extragalactic ULXs are powered by stellar-mass black holes accreting at supercritical rates. A smaller fraction (~10%) may instead be intermediate-mass black holes; however, this fraction could be higher if some stellar-mass black holes are hidden behind large columns of gas and dust.
The high luminosities achieved by accreting black holes make them important tools for studying a variety of astrophysical phenomena. For example, recent observations of tidal disruption events – where a star is torn apart as it falls into a supermassive black hole – suggest that these events can power “mini” quasars that outshine all other known sources in the ultraviolet/optical bands. Similarly, extreme examples of binary systems containing an accreting neutron star or white dwarf can be used to probe the strong gravity regime near compact objects and test predictions of Einstein’s General Theory of Relativity