Gamma-ray bursters are serendipitously discovered transients of nonthermal emissions of cosmological origin. They come in two varieties: (a) short bursts with durations of a few tenths of a second, and (b) long bursts with durations of a few tens of seconds. The latter are now observed in association with supernovae, while no such association is observed for the former. The parent population of Type Ib/c supernovae may well represent the outcome of binary evolution of massive stars, such as SN1993J. In light of these observations, a complete theory is to explain GRBs as a rare kind of supernovae. Long-duration GRB-supernovae require a baryon-poor inner engine operating for similar durations, for which the most promising candidate is a rapidly rotating Kerr black hole. Formed in core collapse of a massive star, the black hole is parametrized by its mass, angular momentum, and kick velocity (M, JH, K).

At low kick velocity K, core-collapse produces a high-mass and rapidly rotating black hole. The Kerr solution predicts a large energy reservoir in angular momentum. Per unit of mass, this far surpasses the energy stored in any baryonic object, including a rapidly rotating neutron star.