Gamma-Ray Bursts as Hypernovae

The energetics of optical and radio afterglows following BeppoSAX and BATSE gamma-ray bursts (GRBs) suggests that gamma-ray emission is not narrowly collimated, but a moderate beaming is possible, so the total energy of a GRB may be in the range 10^{50} - 10^{51} erg. All attempts to generate a fireball powered by neutrino-antineutrino annihilation have failed so far, and a rapid rotation combined with a magnetic field of $ \sim 10^{15} $ gauss gains popularity. In this paper a hypernova scenario is described: a collapse of a massive member in a close binary system, similar to the `failed' Type Ib supernova model proposed by Woosley (1993). The collapse may lead to explosion, with energy transmitted from the rapidly spinning hot neutron core or black hole to the envelope by a strong magnetic field, as in a supernova model proposed by Ostriker and Gunn (1971). However, because of a large mass and rapid rotation of the core the explosion of a hypernova may release up to 10^{54} erg of kinetic energy, creating a `dirty' fireball. In this scenario a moderately non-spherical explosion may accelerate a very small fraction of matter to a very large Lorentz factor, and this may give rise to a gamma-ray burst and its afterglow, just like in a conventional fireball model. However, the highest velocity ejecta from a hypernova are followed with matter which expands less rapidly but carries the bulk of kinetic energy, providing a long term power source for the afterglow. If the afterglows remain luminous for a very long time then the proposed hypernova scenario may provide an explanation. Pre-hypernovae, being massive stars, are likely to be located in the young, star forming regions.