Magnetically Inspired Explosive Outflows from Neutron-star Mergers

Binary neutron-star mergers have long been associated with short-duration gamma-ray bursts (GRBs). This connection was confirmed with the first coincident detection of gravitational waves together with electromagnetic radiation from GW170817. The basic paradigm for short-duration GRBs includes an ultra-relativistic jet, but the low-luminosity prompt emission together with follow-up radio and X-ray observations have hinted that this picture may be different in the case of GW170817. In particular, it has been proposed that large amounts of the magnetic energy that is amplified after the merger, can be released when the remnant collapses to a black hole, giving rise to a quasi-spherical explosion impacting on the merger ejecta. Through numerical simulations we investigate this scenario for a range of viewing angles, injected energies and matter densities at the time of the collapse. Depending on the magnitude of the energy injection and the remnant density, we find two types of outflows: one with a narrow relativistic core and one with a wide-angle, but mildly relativistic outflow. Furthermore, very wide outflows are possible, but require energy releases in excess of 10⁵² erg.