A toy model for the electromagnetic output of neutron star merger prompt collapse to a black hole: magnetised neutron star collisions

We present a systematic study of magnetised neutron star head on collisions. We investigate the resulting magnetic field geometries as the two neutron stars merge. Furthermore, we analyze the luminosity produced in these collisions and monitor the evolution of the magnetic fields from the time of merger until the subsequent production of a black hole. At the time of black hole formation the luminosity peaks and rings-down following the decay of the electromagnetic fields. A comparison is presented for three different cases, one where the initial magnetic field in both neutron stars is aligned, one where they are anti-aligned and also one case where they have unequal magnetic field strength initially. We identify regions and set limits that pair creation and magnetic reconnection would occur in this scenario, and further discuss limits and differences in the radiated energy. This study should be regarded as a toy model of the case where the remnant, of a binary neutron star merger, undergoes a prompt collapse to a black hole with a negligible surrounding disk. We note that the generated electromagnetic pulses resembles the fast radio bursts phenomenology. We consider implications on the high mass mergers leading to a fast prompt collapse to a black hole and the expected flux to be observed in a distance similar to the binary neutron star gravitational wave detection GW190425.