High-resolution calculations of merging neutron stars - III. Gamma-ray bursts

Recent three-dimensional, high-resolution simulations of neutron star coalescences are analysed to assess whether short gamma-ray bursts (GRBs) could originate from such encounters. The two most popular modes of energy extraction - namely the annihilation of and magnetohydrodynamic processes - are explored in order to investigate their viability in launching GRBs. We find that annihilation can provide the necessary stresses to drive a highly relativistic expansion. However, unless the outflow is beamed into less than 1 per cent of the solid angle, this mechanism may fail to explain the apparent isotropized energies implied for short GRBs at cosmological distances. We argue that the energetic, neutrino-driven wind that accompanies the merger event will have enough pressure to provide adequate collimation to the -annihilation-driven jet, thereby comfortably satisfying constraints on event rate and apparent luminosity. We also assess magnetic mechanisms to transform the available energy into a GRB. If the central object does not collapse immediately into a black hole, it will be convective and it is expected to act as an effective large scale dynamo, amplifying the seed magnetic fields to a few times 10¹⁷ G within a small fraction of a second. The associated spindown time-scale is 0.2 s, coinciding with the typical duration of a short GRB. The efficiencies of the various assessed magnetic processes are high enough to produce isotropized luminosities in excess of 10⁵² erg s^-1 even without beaming.