Nonlinear Alfv\'en-wave Dynamics and Premerger Emission from Crustal Oscillations in Neutron Star Mergers

TL;DR

This paper models how crustal oscillations in neutron stars during mergers excite nonlinear Alfvén waves, leading to observable precursor emissions in radio and X-ray wavelengths.

Contribution

It provides the first numerical simulation of Alfvén wave dynamics in neutron star mergers using general-relativistic force-free electrodynamics.

Findings

Alfvén waves steepen nonlinearly before reaching the light cylinder.

Formation of flares and energy dissipation in transient current sheets.

Prediction of radio and X-ray precursor emissions.

Abstract

Neutron stars have solid crusts threaded by strong magnetic fields. Perturbations in the crust can excite non-radial oscillations, which can in turn launch Alfven waves into the magnetosphere. In the case of a compact binary close to merger involving at least one neutron star, this can happen through tidal interactions causing resonant excitations that shatter the neutron star crust. We present the first numerical study that elucidates the dynamics of Alfven waves launched in a compact binary magnetosphere. We seed a magnetic field perturbation on the neutron star crust, which we then evolve in fully general-relativistic force-free electrodynamics using a GPU-based implementation. We show that Alfven waves steepen nonlinearly before reaching the orbital light cylinder, form flares, and dissipate energy in a transient current sheet. Our results predict radio and X-ray precursor emission from this process.