  We show that the 5-GHz radio flux of transient ballistic jets in black hole
binaries correlates with the dimensionless black hole spin parameter a*
estimated via the continuum-fitting method. The data suggest that jet power
scales either as the square of a* or the square of the angular velocity of the
horizon. This is the first direct evidence that jets may be powered by black
hole spin energy. The observed correlation validates the continuum-fitting
method of measuring spin. In addition, for those black holes that have
well-sampled radio observations of ballistic jets, the correlation may be used
to obtain rough estimates of their spins.
