An Etymological Dictionary of Astronomy and Astrophysics

An X-ray source that is not in the nucleus of a galaxy, and is more luminous than 10³⁹ ergs s^-1, brighter than the → Eddington luminosity of a 10 → solar mass → black hole. In general, there is about one ULX per galaxy in galaxies which host ULXs. The Milky Way contains no such objects. ULXs are thought to be powered by → accretion onto a → compact object. Possible explanations include accretion onto → neutron stars
with strong → magnetic fields, onto → stellar black holes (of up to 20 → solar masses) at or in excess of the classical Eddington limit, or onto → intermediate-mass black holes (10³-10⁵ solar masses). NGC 1313X-1, NGC 5408X-1, and NGC 6946X-1 are three ULXs with X-ray luminosities up to ~ 10⁴⁰ erg s^-1 (Ciro Pinto et al., 2016, Nature 533, N) 7601).

See also: → ultraluminous; → X-ray source.