Stellar-mass black hole binaries as ULXs

Ultraluminous X-ray sources (ULXs) with L-x > 10(39) ergs s(-1) have been discovered in great numbers in external galaxies with ROSAT, Chandra, and XMM-Newton. The central question regarding this important class of sources is whether they represent an extension in the luminosity function of binary X-ray sources containing neutron stars and stellar-mass black holes (BHs), or a new class of objects, e.g., systems containing intermediate-mass black holes (100-1000 M-circle dot), We have carried out a theoretical study to test whether a large fraction of the ULXs, especially those in galaxies with recent star formation activity, can be explained with binary systems containing stellar-mass black holes. To this end, we have applied a unique set of binary evolution models for black-hole X-ray binaries, coupled to a binary population synthesis code, to model the ULXs observed in external galaxies. We find that for donor stars with initial masses > 10 Me the mass transfer driven by the normal nuclear evolution of the donor star is sufficient to potentially power most ULXs. This is the case during core hydrogen burning and, to an even more pronounced degree, while the donor star ascends the giant branch, though the latter phases last only 5% of the main sequence phase. We show that with only a modest violation of the Eddington limit, e,g., a factor of similar to 10, both the numbers and properties of the majority of the ULXs can be reproduced