Portal:Radiation astronomy/X-ray astronomy article/12

Ultraluminous X-ray sources (ULXs) are pointlike, nonnuclear X-ray sources with luminosities above the Eddington limit of 3 × 1039 ergs s−1 for a 20 M☉ black hole.

Many ULXs show strong variability and may be black hole binaries. To fall into the class of intermediate-mass black holes (IMBHs), their luminosities, thermal disk emissions, variation timescales, and surrounding emission-line nebulae must suggest this. However, when the emission is beamed or exceeds the Eddington limit, the ULX may be a stellar-mass black hole. The nearby spiral galaxy NGC 1313 has two compact ULXs, X-1 and X-2. For X-1 the X-ray luminosity increases to a maximum of 3 × 1040 ergs s−1, exceeding the Eddington limit, and enters a steep power-law state at high luminosities more indicative of a stellar-mass black hole, whereas X-2 has the opposite behavior and appears to be in the hard X-ray state of an IMBH.