Jets, winds and bursts from coalescing neutron stars

Abstract

Recent high-resolution calculations of neutron star coalescences are used to investigate whether νν̄ annihilation can provide sufficient energy to power gamma-ray bursts, especially those belonging to the short duration category. Late time slices of the simulations, where the neutrino emission has reached a maximum, stationary level are analysed to address this question. We find that νν̄ annihilation can provide the necessary driving stresses that lead to relativistic jet expansion Maximum Lorentz factors of the order of 15 are found along the binary rotation axis, but larger values are expected to arise from higher numerical resolution. Yet the accompanying neutrino-driven wind must be absent from the axis when the burst occurs or prohibitive baryon loading may occur. We argue that even under the most favourable conditions, νν̄ annihilation is unlikely to power a burst in excess of ∼1048 erg. Unless the emission is beamed into less than one per cent of the solid angle, which we argue is improbable if it is collimated by gas pressure, this may fail to satisfy the apparent isotropic energies inferred at cosmological distances. This mechanism may none the less drive a precursor fireball, thereby evacuating a cavity into which a later magnetically driven jet could expand. A large range of time delays between the merger and the black hole formation are to be expected. If the magnetically driven jet occurs after the black hole has formed, a time span as long as weeks could pass between the neutrino powered precursor and the magnetically driven GRB.