Ultra-luminous X-ray sources (ULX) are normally defined as sources with luminosities in the X-ray band above 1039 erg s^1. They were first detected with the Einstein Observatory and at that time named super-Eddington sources, because their luminosity was well above the Eddington limit for a neutron star (~2x1038 erg s^1), suggesting accreting objects with masses of 100 M0 or more (see, e.g., review ). Such masses exceed those of stellar mass black holes in XRBs and may indicate that ULXs represent a new class of astrophysical objects. Certainly several ULXs can be explained as bright background objects or even as X-ray luminous SN in the host galaxy (see, e.g., SN 1993J in M 81), but several of them are recurrent and/or connected to star forming regions in galaxies excluding such models. They could represent sources that fill the gap in the black hole mass distribution between stellar mass black holes detected in X-ray binaries and supermassive black holes found in the nuclei of many galaxies. These sources have been called intermediate mass black holes (IMBHs). The high spatial resolution of Chandra allows to resolve these sources also in more distant galaxies and with the collecting power of XMM-Newton detailed X-ray spectra were collected. Several of the X-ray spectra of the brightest ULX sources showed emission components that have been modeled by cool accretion disks. As the temperature is inversely related to black hole mass, this has been taken as an argument for them containing an IMBH (see e.g. ).
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