  In the context of black hole spin in X-ray binaries, we propose that certain
type of quasi-period oscillations (QPOs) observed in the light curves of black
hole binaries (BHBs) are produced by X-ray modulation at the precession
frequency of accretion disks, due to relativistic dragging of inertial frames
around spinning black holes. These QPOs tend to be relatively stable in their
centroid frequencies. They have been observed in the frequency range of a few
to a few hundred Hz for several black holes with dynamically determined masses.
By comparing the computed disk precession frequency with that of the observed
QPO, we can derive the black hole angular momentum, given its mass. When
applying this model to GRO J1655-40, GRS 1915+105, Cyg X-1, and GS 1124-68, we
found that the black holes in GRO J1655-40 and GRS 1915+105, the only known
BHBs that occasionally produce superluminal radio jets, spin at a rate close to
the maximum limit, while Cyg X-1 and GS 1124-68, typical (persistent and
transient) BHBs, contain only moderately rotating ones. Extending the model to
the general population of black hole candidates, the fact that only
low-frequency QPOs have been detected is consistent with the presence of only
slowly spinning black holes in these systems. Our results are in good agreement
with those derived from spectral data, thus strongly support the classification
scheme that we proposed previously for BHBs.
