Magnetic effects on the low-T/|W| instability in differentially rotating neutron stars

TL;DR

This study investigates how magnetic fields influence the low-T/|W| instability in differentially rotating neutron stars through 3D magnetohydrodynamics simulations, revealing that magnetic effects are limited to specific field strengths and depend on numerical methods.

Contribution

The paper introduces a new 3D magnetohydrodynamics simulation approach to analyze magnetic effects on the low-T/|W| instability, highlighting the limited impact of magnetic fields across most strengths.

Findings

Strong toroidal fields only suppress the instability in a narrow range.

Below 4e13 G, poloidal fields have minimal effect before saturation.

Magnetic instabilities can amplify quadrupole modes above 5e14 G.

Abstract

Dynamical instabilities in protoneutron stars may produce gravitational waves whose observation could shed light on the physics of core-collapse supernovae. When born with sufficient differential rotation, these stars are susceptible to a shear instability (the "low-T/|W| instability"), but such rotation can also amplify magnetic fields to strengths where they have a considerable impact on the dynamics of the stellar matter. Using a new magnetohydrodynamics module for the Spectral Einstein Code, we have simulated a differentially-rotating neutron star in full 3D to study the effects of magnetic fields on this instability. Though strong toroidal fields were predicted to suppress the low-T/|W| instability, we find that they do so only in a small range of field strengths. Below 4e13 G, poloidal seed fields do not wind up fast enough to have an effect before the instability saturates, while above 5e14 G, magnetic instabilities can actually amplify a global quadrupole mode (this threshold may be even lower in reality, as small-scale magnetic instabilities remain difficult to resolve numerically). Thus, the prospects for observing gravitational waves from such systems are not in fact diminished over most of the magnetic parameter space. Additionally, we report that the detailed development of the low-T/|W| instability, including its growth rate, depends strongly on the particular numerical methods used. The high-order methods we employ suggest that growth might be considerably slower than found in some previous simulations.