The freedom to choose neutron star magnetic field 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.

11 pages, 1 table; some typos fixed, this version matches the published paper

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• Precession of magnetars: dynamical evolutions and modulations on polarized electromagnetic waves
• Magnetic equilibria of relativistic axisymmetric stars: The impact of flow constants
• The radio shut-off, glitch, and X-ray burst in 1E 1547.0-5408 interpreted through magnetic reconfiguration
• Magnetar structure in non-linear electrodynamics with mixed poloidal-toroidal fields
• Interlinking internal and external magnetic fields of relativistically rotating neutron stars