Non-axisymmetric instabilities of neutron star with toroidal magnetic fields

The aim of this paper is to clarify the stabilities of neutron stars with strong toroidal magnetic fields against non-axisymmetric perturbation. The motivation comes from the fact that super magnetized neutron stars of $\sim 10^{15}$G, magnetars, and magnetized proto-neutron stars born after the magnetically-driven supernovae are likely to have such strong toroidal magnetic fields. Long-term, three-dimensional general relativistic magneto-hydrodynamic simulations are performed, preparing isentropic neutron stars with toroidal magnetic fields in equilibrium as initial conditions. To explore the effects of rotations on the stability, simulations are done for both non-rotating and rigidly rotating models. We find the emergence of the Parker and/or Tayler instabilities in both the non-rotating and rotating models. For both non-rotating and rotating models, the Parker instability is the primary instability as predicted by the local linear perturbation analysis. The interchange instability also appears in the rotating models. It is found that rapid rotation is not enough to suppress the Parker instability, and this finding does not agree with the perturbation analysis. The reason for this is that rigidly and rapidly rotating stars are marginally stable, and hence, in the presence of stellar pulsations by which the rotational profile is deformed, unstable regions with negative gradient of angular momentum profile is developed. After the onset of the instabilities, a turbulence is excited. Contrary to the axisymmetric case, the magnetic fields never reach an equilibrium state after the development of the turbulence. This conclusion suggests that three-dimensional simulation is indispensable for exploring the formation of magnetars or prominence activities of magnetars such as giant flares.

💡 Research Summary

The paper investigates the stability of neutron stars that possess strong toroidal magnetic fields against non‑axisymmetric perturbations, using long‑term three‑dimensional general‑relativistic magnetohydrodynamic (GRMHD) simulations. The motivation stems from observations of magnetars (∼10¹⁴–10¹⁵ G) and magnetized proto‑neutron stars formed after magnetically driven supernovae, which are expected to contain substantial toroidal magnetic components.

Model construction
The authors construct equilibrium neutron‑star models with a polytropic equation of state (Γ = 2) and a purely toroidal magnetic field confined inside the star. The magnetic field profile is prescribed as

Bφ = B₀