2004ApJ...616L.119L

The majority of the ultraluminous X-ray sources (ULXs) in external galaxies are believed to be accreting black holes in binary systems; some of the black holes could be as massive as ∼100-1000 M_☉. We have performed evolution calculations for intermediate-mass black hole X-ray binaries, assuming they are formed in dense star clusters via tidal capture. The results are compared with those for stellar-mass black hole X-ray binaries. We find that these two types of black holes may have similar companion stars and binary orbits if observed as ULXs. However, intermediate-mass black holes seem to be favored in explaining the most luminous ULXs. We also discuss the possibilities of transient behavior and beamed emission in the evolution of these binary systems.