While many observed ultra-luminous X-ray sources ( ULXs , L _ { X } \geq 10 ^ { 39 } erg s ^ { -1 } ) could be extragalactic X-ray binaries ( XRBs ) emitting close to the Eddington limit , the highest-luminosity ULXs ( L _ { X } > 3 \times 10 ^ { 39 } erg s ^ { -1 } ) exceed the isotropic Eddington luminosity for even high-stellar-mass accreting black hole XRBs .
It has been suggested that these highest-luminosity ULXs may contain accreting intermediate-mass black hole ( IMBH ) binaries .
We consider this hypothesis for dense , young ( \sim 10 ^ { 8 } yr ) stellar clusters where we assume that a 50-500 M _ { \odot } central IMBH has formed through runaway growth of a massive star .
Using numerical simulations of the dynamics and evolution of the central black hole ’ s captured companions , we obtain estimates of the incidence of mass transfer phases and possible ULX activity throughout the IMBH ’ s evolutionary history .
We find that , although it is common for the central black hole to acquire binary companions , there is a very low probability that these interacting binaries will become observable ULX sources .