The initial spin matters: the impact of rapid rotation on magnetic-field amplification at merger

TL;DR

This study investigates how rapid rotation affects magnetic field amplification during neutron star mergers, revealing that spin orientation influences the growth rate and topology of magnetic fields through Kelvin-Helmholtz instability.

Contribution

First systematic high-resolution simulations showing the impact of different neutron star spin configurations on magnetic field amplification during merger.

Findings

Anti-aligned spins produce the largest vorticity and magnetic energy growth.

Different spins lead to distinct initial magnetic field topologies.

All configurations eventually converge to a similar magnetic field topology.

Abstract

A couple of milliseconds after the merger of a binary system of neutron stars can play a fundamental role in amplifying the comparatively low initial magnetic fields into magnetar strengths. The basic mechanism responsible for this amplification is the Kelvin-Helmholtz instability (KHI) and we here report the first systematic study of the impact of rapid rotation on the KHI-amplification process exploiting general-relativistic magnetohydrodynamic simulations at very high-resolutions of $35\,{\rm m}$. Concentrating on four different spinning configurations, we find that aligned, anti-aligned, and mixed (aligned/anti-aligned) spin configurations lead to markedly different growth rates of the electromagnetic (EM) energy, field topologies, and vortex properties when compared to the irrotational case. These differences arise from intrinsic variations in the system dynamics, such as tidal deformation, collision strength, and contact surface area, with the anti-aligned configuration producing the largest vorticity and growth in EM energy. Importantly, while different spin configurations lead to significantly different initial growth rates of the poloidal/toroidal components, all systems converge to a specific topological partition. Our simulations are confined to a short window in time, but the different EM energies produced as a result of spin will imprint the EM emission at merger and provide information on the spinning state at merger.