Gamma-ray bursts as hypernovae

A standard fireball/afterglow model of a gamma-ray burst (GRB) relates the event to a merging neutron star binary, or a neutron star-black hole binary, which places the events far away from star-forming regions, and is thought to have an energy of ~10⁵¹ erg. A hypernova, the death of a massive and rapidly spinning star, may release ~10⁵⁴ erg of kinetic energy by tapping the rotational energy of a Kerr black hole formed in the core collapse. Only a small fraction of all energy is in the debris ejected with the largest Lorentz factors, those giving rise to the GRB itself, but all energy is available to power the afterglow for a long time. In this scenario GRBs should be found in star-forming regions, the optical afterglows may be obscured by dust, and the early thermal emission of the massive ejecta may give rise to X-ray precursors, as observed by Ginga. The optical and X-ray afterglows of GRB970228, GRB97508, and GRB97828 provide some evidence that these bursts were located in galaxies, most likely in dwarf galaxies, in or near star-forming regions.