Powering Short GRBs by Mergers of Moderately Magnetized Neutron Stars

We explore the process of amplification of the magnetic field initially contained in merging neutron stars on the light shed by the recent results of global and local numerical simulations. We show that the field growth proceeds in two regimes. First, the initial fields are amplified in an unstable KH-shear layer, which results when the two neutron stars touch each other. This amplification lasts, at most, until the central black hole if formed. Subsequently, an MRI unstable toroidal remnant amplifies further the magnetic field. No ultrarelativistic outflow is formed in by the action of the magnetic field, at least in the first similar to 30 ms after the neutron stars merge, since neither the magnetization, nor the magnetic flux across the event horizon are large enough. However, we conclude that thermally generated ultrarelativistic outflows, which ultimately give rise to short gamma-ray bursts, could be generated soon after the formation of the central BH by our models, if they would include the appropriate neutrino physics.