  Recently, twin-peak 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 ~1000 solar mass)
radiating very near the Eddington limit and (2) a conventional black hole (M ~
10 solar mass) accreting at a highly super-Eddington rate with its emission
beamed along the rotation axis. We suggest that it may be possible to
distinguish 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 ~100 Hz value observed for the Galactic
microquasars.
