| definition |
An X-ray source that is not in the nucleus of a galaxy, and is more luminous than
10^39 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 (10^3 - 10^5 solar
masses). NGC 1313X-1, NGC 5408X-1, and NGC 6946X-1 are three ULXs with X-ray luminosities
up to ~10^40 erg s^-1.
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