Discovery of a Black Hole Mass‐Period Correlation in Soft X‐Ray Transients and Its Implication for Gamma‐Ray Burst and Hypernova Mechanisms

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. For systems with evolved donors we are able to reconstruct the pre-explosion periods and find that the black-hole mass increases with the orbital period of the binary. 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 on a dynamical time. This trend of more rapidly rotating stars in closer binaries is usual in close binaries, 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. An important ingredient is the fact that the rapidly rotating new black hole powers both a GRB and the