Models of Ultraluminous X-Ray Sources with Intermediate-Mass Black Holes

We have computed models for ultraluminous X-ray sources (ULXs) consisting of a black hole accretor of intermediate mass (IMBH; e.g., ~1000 M☉) and a captured donor star. For each of four different sets of initial donor masses and orbital separations we computed 30,000 binary evolution models using a full Henyey stellar evolution code. To our knowledge, this is the first time that a population of X-ray binaries this large has been carried out with other than approximation methods, and it serves to demonstrate the feasibility of this approach to large-scale population studies of mass transfer binaries. In the present study, we find that in order to have a plausible efficiency for producing active ULX systems with IMBHs having luminosities ≳1040 ergs s-1, there are two basic requirements for the capture of companion/donor stars. First, the donor stars should be massive, i.e., ≳8 M☉. Second, the initial orbital separations after circularization should be close, i.e., ≲6-30 times the radius of the donor star when on the main sequence. Even under these optimistic conditions, we show that the production rate of IMBH-ULX systems may fall short of the observed values by factors of 10-100.