Ultraluminous X-ray Sources as Intermediate Mass Black Holes Fed by Tidally Captured Stars

The nature of ultraluminous X-ray sources (ULXs) is presently unknown. A possible explanation is that they are accreting intermediate mass black holes (IBHs) that are fed by Roche lobe overflow from a tidally captured stellar companion. We show that a star can circularize around an IBH without being destroyed by tidal heating (in contrast to the case of M•>10 M⊙ massive black holes in galactic centers, where survival is unlikely). We find that the capture and circularization rate is ∼ 5×10−8 yr, almost independently of the cluster’s relaxation time. We follow the luminosity evolution of the binary system during the main sequence Roche lobe overflow phase and show it can maintain ULXlike luminosities for > 10 yr. In particular, we show that the ULX in the young cluster MGG-11 in star-burst galaxy M82, which possibly harbors an IBH, is well explained by this mechanism, and we predict that &10% of similar clusters with IBHs have a tidally captured circularized star. The cluster can evaporate on a time-scale shorter than the lifetime of the binary. This raises the possibility of a ULX that outlives its host cluster, or even lights up only after the cluster has evaporated, in agreement with observations of host-less ULXs. Subject headings: black hole physics — stellar dynamics — galaxies: star clusters — X-ray binaries