The importance of discovering a 3:2 twin-peak quasi-periodic oscillation in an ultraluminous x-ray source, or how to solve the puzzle of intermediate-mass black holes

Recently, twin-peak quasi-periodic oscillations (QPOs) have been observed in a 3 : 2 ratio for three Galactic black hole microquasars with frequencies that have been shown to scale as 1/M, as expected for general relativisitic motion near a black hole. It may be possible to extend this result to distinguish between the following two disparate models that have been proposed for the puzzling ultraluminous X-ray sources (ULXs): ( 1) an intermediate-mass black hole (M similar to 10(3) M-.) radiating very near the Eddington limit and (2) a conventional black hole (M similar to 10 M-.), accreting at a highly super-Eddington rate with its emission beamed along the rotation axis. We suggest that one could discriminate between these models by detecting the counterpart of a Galactic twin-peak QPO in a ULX: the expected frequency for the intermediate-mass black hole model is only about 1 Hz, whereas for the conventional black hole model the expected frequency would be the similar to 100 Hz value observed for the Galactic microquasars.