Gamma-ray bursts as collimated jets from neutron star/black hole mergers

Abstract

THE distribution of more than 150 γ-ray bursts detected by the BATSE experiment is isotropic on the sky but radially non-uniform^1,2. This raises the possibility that bursts are cosmological (at z≲l) and therefore very energetic events, releasing ∼10⁵⁰ erg sr⁻¹ on a timescale of seconds. The coalescence of two neutron stars^3–7 or the accretion-induced collapse of a white dwarf⁸ can release up to 10⁵³ erg in the form of neutrino-antineutrino pairs, so that the conversion of <1% into γ-rays by annihilation⁹ could generate γ-ray bursts, but in all such models an optically thick wind tends to form^10,11, preventing the γ-rays from escaping and converting their energy into kinetic energy of the ejected material. We present here a possible solution to this difficulty. When a stellar-mass neutron star is disrupted by a black hole, it forms a thick disk which emits νν̄ pairs. These neutrinos expel a wind from the disk, but angular momentum conservation means that a clear funnel forms along the rotation axis. Neutrino annihilation within the matter-free funnel can then create γ-rays which escape to the distant observer.