The freedom to choose neutron star magnetic field equilibria

TL;DR

This paper investigates whether magnetic field equilibria in neutron stars can be arbitrarily prescribed, considering various realistic models and symmetries, and finds that axisymmetry allows complete freedom while non-axisymmetry does not.

Contribution

It provides a comprehensive analysis of magnetic equilibrium freedom in neutron stars across different models and symmetries, including relativistic effects.

Findings

Axisymmetric equilibria allow complete freedom in magnetic field configuration.

Non-axisymmetric equilibria are constrained and not completely arbitrary.

Realistic models with superfluidity and superconductivity still permit free axisymmetric equilibria.

Abstract

Our ability to interpret and glean useful information from the large body of observations of strongly magnetised neutron stars rests largely on our theoretical understanding of magnetic field equilibria. We answer the following question: is one free to arbitrarily prescribe magnetic equilibria such that fluid degrees of freedom can balance the equilibrium equations? We examine this question for various models for neutron star matter; from the simplest single-fluid barotrope to more realistic non-barotropic multifluid models with superfluid/superconducting components, muons and entropy. We do this for both axi- and non-axisymmetric equilibria, and in Newtonian gravity and general relativity. We show that, in axisymmetry, the most realistic model allows complete freedom in choosing a magnetic field equilibrium whereas non-axisymmetric equilibria are never completely arbitrary.