Cosmological gamma-ray bursts are potentially associated with systems harboring a rotating black hole .
Calorimetry on yet “ unseen ” emissions in gravitational waves from a surrounding torus may provide a measure for its rotational energy .
We introduce a compactness parameter \alpha = 2 \pi \int fdE for the fluence E and frequency f in graviational waves .
For black hole-torus systems , \alpha \simeq 2 \pi Ef \simeq 0.005 - 0.05 may exceed the upper bound \alpha ^ { * } \simeq 0.005 for rapidly rotating neutron stars .
A duration in gravitational radiation consistent with the redshift corrected durations of the BATSE catalogue defines an association with long GRBs .
This promises a new test for Kerr black holes as objects in Nature .