Ultra-luminous X ray source
An ultra-luminous X-ray source (ULX) is an astronomical source of X-rays that is less luminous than an active galactic nucleus but is more consistently luminous than any known stellar process (> 1039erg/s, or 1032watts), assuming that it radiates isotropically (the same in all directions). Typically there is about one ULX per galaxy in galaxies which host them, but some galaxies contain many. The Milky Way has not been shown to contain a ULX. The main interest in ULXs stems from the fact that their luminosity exceeds the Eddington luminosity of neutron stars and even stellar black holes. It is not known what powers ULXs; models include beamed emission of stellar mass objects, accreting intermediate-mass black holes, and super-Eddington emission.
ULXs were first discovered in the 1980s by the Einstein Observatory. Later observations were made by ROSAT. Great progress has been made by the X-ray observatories XMM-Newton and Chandra, which have a much greater spectral and angular resolution. A survey of ULXs by Chandra observations shows that there is approximately one ULX per galaxy in galaxies which host ULXs (most do not). ULXs are found in all types of galaxies, including elliptical galaxies but are more ubiquitous in star-forming galaxies and in gravitationally interacting galaxies. Tens of percents of ULXs are in fact background quasars; the probability for a ULX to be a background source is larger in elliptical galaxies than in spiral galaxies.
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