Long-term evolution of a merger-remnant neutron star in general relativistic magnetohydrodynamics I: Effect of magnetic winding

TL;DR

This study uses general relativistic magnetohydrodynamics simulations to explore how magnetic winding affects the evolution of neutron star remnants after binary mergers, highlighting magnetic amplification, rotational dynamics, and mass ejection processes.

Contribution

It provides the first detailed analysis of long-term magnetic winding effects on merger remnants in full general relativity, including resistive effects and neutrino interactions.

Findings

Magnetic fields can be amplified to 10% of rotational energy within ~1 second.

Magnetic winding enforces rigid rotation after amplification.

Magnetic effects have minor influence on post-merger mass ejection.

Abstract

Long-term ideal and resistive magnetohydrodynamics (MHD) simulations in full general relativity are performed for a massive neutron star formed as a remnant of binary neutron star mergers. Neutrino radiation transport effects are taken into account as in our previous papers. The simulation is performed in axial symmetry and without considering dynamo effects as a first step. In the ideal MHD, the differential rotation of the remnant neutron star amplifies the magnetic-field strength by the winding in the presence of a seed poloidal field until the electromagnetic energy reaches $\sim 10\%$ of the rotational kinetic energy, $E_{\rm kin}$, of the neutron star. The timescale until the maximum electromagnetic energy is reached depends on the initial magnetic-field strength and it is $\sim 1$ s for the case that the initial maximum magnetic-field strength is $\sim 10^{15}$ G. After a significant amplification of the magnetic-field strength by the winding, the magnetic braking enforces the initially differentially rotating state approximately to a rigidly rotating state. In the presence of the resistivity, the amplification is continued only for the resistive timescale, and if the maximum electromagnetic energy reached is smaller than $\sim 3\%$ of $E_{\rm kin}$, the initial differential rotation state is approximately preserved. In the present context, the post-merger mass ejection is induced primarily by the neutrino irradiation/heating and the magnetic winding effect plays only a minor role for the mass ejection.