2007ApJ...671L..41B

Knowing the Kerr parameters, we can make quantitative calculations of the rotational energy of black holes. We show that Nova Sco, Il Lupi, XTE J1550-564, and GS 2023+338 are relics of gamma-ray bursts (GRBs) and hypernova explosions. They had more than enough rotational energy to power themselves. In fact, they had so much energy that they would have disrupted the accretion disk of the black hole that powered them by the communicated rotational energy, so that the energy delivery would have been self-limiting. The most important feature in producing high rotational energy in the binary is low donor (secondary star) mass. We suggest that V4641 Sgr and GRS 1915+105 underwent less energetic explosions, because of their large donor masses. Cyg X-1 had an even less energetic explosion, because of an even larger donor mass. We find that in the evolution of the soft X-ray transient sources the donor is tidally locked with the helium star, which evolved from the giant, as the hydrogen envelope is stripped off in common-envelope evolution. Depending on the mass of the donor, the black hole can be spun up to the angular momentum necessary to power the GRB and hypernova explosion. The donor decouples, acting as a passive witness to the explosion which, for the given angular momentum, then proceeds as in the Woosley collapsar model. High-mass donors which tend to follow from low metallicity give long GRBs because their lower energy can be accepted by the central engine.