  We investigate the soft X-ray transients with black hole primaries which may
have been the sources of gamma-ray bursts and hypernovae earlier in their
evolution. We find that the black-hole mass increases with the orbital period
of the binary, both for systems with main-sequence donors and for those with
evolved donors. This correlation can be understood in terms of angular-momentum
support in the helium star progenitor of the black hole, if the systems with
shorter periods had more rapidly rotating primaries prior to their explosion:
centrifugal support will then prevent more of its mass from collapsing into the
black hole. This trend of more rapidly rotating stars in closer binaries is
very generally seen, and in the present case can be understood in terms of
spin-up during spiral in and subsequent tidal coupling. We investigate the
relation quantitatively and obtain reasonable agreement with the observed
mass-period correlation. A natural consequence of this observation is that
black holes formed in soft X-ray transients acquire significant Kerr
parameters. This makes them good sources of power for gamma-ray bursts and
hypernovae, via the Blandford-Znajek mechanism, and thus supports our model for
the origin of gamma-ray bursts in soft X-ray transients.

Astrophysics