The Initial Spin Matters: The Impact of Rapid Rotation on Magnetic-field Amplification at Merger

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 m. Concentrating on four different spinning configurations, we find that aligned, antialigned, and mixed (aligned/antialigned) 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 antialigned 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 where the ratio of the poloidal to toroidal magnetic energy is approximately two.