The mass and radii of strongly magnetized neutron stars

TL;DR

This study investigates how strong magnetic fields inside neutron stars influence their mass and radius, revealing that magnetic field geometry significantly affects these properties and challenging previous assumptions.

Contribution

It provides the first detailed analysis of magnetic field effects on neutron star structure considering different geometries and complex nuclear physics.

Findings

Magnetic fields can alter neutron star maximum mass by up to 30%.

Field geometry determines whether mass and radius increase or decrease.

Equipartition magnetic fields have a significant but geometry-dependent impact.

Abstract

It has been clear for some time now that super-critical surface magnetic fields, exceeding 4 x 10^13 G, exist on a subset of neutron stars. These magnetars may harbor interior fields many orders of magnitude larger, potentially reaching equipartition values. However, the impact of these strong fields on stellar structure has been largely ignored, potentially complicating attempts to infer the high density nuclear equation of state. Here we assess the effect of these strong magnetic fields on the mass-radius relationship of neutron stars. We employ an effective field theory model for the nuclear equation of state that includes the impact of hyperons, anomalous magnetic moments, and the physics of the crust. We consider two magnetic field geometries, bounding the likely magnitude of the impact of magnetic fields: a statistically isotropic, tangled field and a force-free configuration. In both cases even equipartition fields have at most a 30% impact on the maximum mass. However, the direction of the effect of the magnetic field depends on the geometry employed - force-free fields leading to reductions in the maximum neutron star mass and radius while tangled fields increase both - challenging the common intuition in the literature on the impact of magnetic fields.