Magnetic amplification in premerger neutron stars through resonance-induced magnetorotational instabilities

Abstract

Tidal resonances in the final seconds of a binary neutron-star inspiral can excite oscillation modes in one or both of the constituents to large amplitudes. Under favorable circumstances, resonant pulsations can overstrain the stellar crust and unleash a torrent of magnetoelastic energy that manifests as a gamma-ray ``precursor flare.'' We show that for realistic, stratified stars rotating with a spin frequency of $\gtrsim30\,$Hz, the fundamental $g$ or its first overtone can also execute a differential rotation in the crust such that a magnetic field of strength $\gtrsim10^{13}\,$G is generated via magnetorotational instabilities. This may help to explain observed precursor rates and their luminosities. Premerger magnetic growth would also provide seed magnetic energy for the postmerger remnant.