Observational evidence for a correlation between jet power and black hole spin

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 as the square of the angular velocity of the horizon Ω_H. 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.