Kerr parameters for stellar mass black holes and their consequences for gamma-ray bursts and hypernovae

Recent measurements of the Kerr parameters a_* for two black hole binaries in our Galaxy, GRO J1655-40 and 4U 1543-47, of a _* = 0.65-0.75 and a_* = 0.75-0.85, respectively, fit the predictions of Lee et al. of a_*= 0.8. They predicted a_* > 0.5 for 80% of the soft X-ray transient (SXT) sources. The maximum available energy in the Blandford-Znajek formalism for a_* > 0.5 gives E > 3 × 10⁵³ erg, orders of magnitude larger than the energy needed for the gamma-ray burst (GRB) and hypernova explosion. We interpret the SXTs to be relics of GRBs and hypernovae. We find that most galactic SXTs were subluminous given that they could use only a small part of the available rotational energy.