High Resolution Calculations of Merging Neutron Stars III: Gamma-Ray Bursts

Abstract

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 $\nu\bar{\nu}$ and magnetohydrodynamic processes- are explored in order to investigate their viability in launching GRBs. We find that $\nu\bar{\nu}$ annihilation can provide the necessary stresses to drive a highly relativistic expansion. But unless the outflow is beamed into less than one percent 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 $\nu\bar{\nu}$-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 $10^{17}$ 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 $10^{52}$ ergs/s even without beaming.