An ultraluminous X-ray source is an apparently off-nuclear compact X-ray source with inferred isotropic luminosity above roughly W. Luminosity alone does not establish an intermediate-mass black hole: supercritical accretion, beaming and magnetized neutron-star accretion can also produce large apparent luminosities. Spatial association, spectra, timing and dynamical constraints help distinguish models.
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An Ultraluminous X-ray Source (ULX) is a type of astronomical object that emits an extraordinary amount of X-ray radiation, significantly more than typical X-ray binaries. ULXs are defined as having luminosities greater than 10^39 erg/s (or about 10^36 watts), which exceeds the Eddington limit for a stellar black hole (a theoretical maximum luminosity based on the balance between the outward radiation pressure and the inward gravitational force).