Ultraluminous X-ray Sources in Nearby Galaxies.

The observation of nearby galaxies with Einstein, ROSAT, ASCA, XMMNewton and Chandra has revealed a novel class of point-like, off-nuclear X-ray sources with isotropic luminosity largely in excess of the Eddington limit for one solar mass. From the available X-ray and multiwavelength observations, these ultraluminous X-ray sources turn out to be a composite population. The majority of them (probably all those found in star-forming galaxies) are accreting binaries, likely high mass X-ray binaries. The current debate is centered on understanding whether the compact objects is an intermediate mass (M > 100M ) black hole emitting isotropically or a stellar mass black hole with beamed emission. A short review of the topic is presented.