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

We have computed models for ultraluminous X-ray sources ("ULXs") consistingof a black-hole accretor of intermediate mass ("IMBH"; e.g., ~1000 Msun) and acaptured donor star. For each of four different sets of initial donor massesand orbital separations, we computed 30,000 binary evolution models using afull Henyey stellar evolution code. To our knowledge this is the first timethat a population of X-ray binaries this large has been carried out with otherthan approximation methods, and it serves to demonstrate the feasibility ofthis approach to large-scale population studies of mass-transfer binaries. Inthe present study, we find that in order to have a plausible efficiency forproducing active ULX systems with IMBHs having luminosities > 10^{40} ergs/sec,there are two basic requirements for the capture of companion/donor stars.First, the donor stars should be massive, i.e., > 8 Msun. Second, the initialorbital separations, after circularization, should be close, i.e., < 6-30 timesthe radius of the donor star when on the main sequence. Even under theseoptimistic conditions, we show that the production rate of IMBH-ULX systems mayfall short of the observed values by factors of 10-100.Comment: 5 pages, 2 figures, submitted to Ap