Close Binary Interactions of Intermediate‐Mass Black Holes: Possible Ultraluminous X‐Ray Sources?

Abstract

While many observed ultraluminous X-ray sources (ULXs;L_{X}{\gt}=10^{39} ergs s^{-1}) could beextragalactic X-ray binaries (XRBs) emitting close to the Eddingtonlimit, the highest luminosity ULXs(L_{X}{\gt}3{\times}10^{39} ergs s^{-1}) exceedthe isotropic Eddington luminosity for even high-stellar-mass-accretingblack hole XRBs. It has been suggested that these highest luminosityULXs may contain accreting intermediate-mass black hole (IMBH) binaries.We consider this hypothesis for dense, young (~100 Myr) stellar clusterswhere we assume that a 50-500 M_{solar} central IMBH has formedthrough runaway growth of a massive star. We develop numericalsimulations of such clusters' cores by combining single and binary starevolutionary syntheses with a simple treatment of dynamicalinteractions. We model interactions of the IMBH with single and binarystars, as well as single-binary and binary-binary interactions, but notthe formation of a cusp around the IMBH. The core density and velocitydispersion are assumed to be constant over 100 Myr. We investigate thesuccession of IMBH binary companions and the evolution of their orbitalparameters to obtain estimates of the incidence of mass transfer phasesand possible ULX activity involving the IMBH. We find that although itis common for the central black hole to acquire binary companions, thereis a very low probability that these interacting binaries will becomeobservable ULX sources.