Ultraluminous X-ray sources.

We review observations of ultraluminous X-ray sources (ULXs). X-ray spectroscopic and timing studies of ULXs suggest a new accretion state distinct from those seen in Galactic stellar-mass black hole binaries. Optical and X-ray line profiles and the properties of radio and optical nebulae suggest that ULXs produce powerful outflows, while dynamic constraints on the mass of the compact object in one ULX and the detection of coherent pulsations from another show that they exhibit super-Eddington luminosities. We discuss models of super-Eddington accretion and their relation to the observed behaviors of ULXs. We review the evidence for intermediate mass black holes in ULXs. We consider the implications of ULXs for super-Eddinton accretion in active galactic nuclei, heating of the early universe, and the origin of the black hole binary recently detected via gravitational waves.