The impact of superconductivity and the Hall effect in models of magnetized neutron stars

TL;DR

This paper introduces a numerical scheme to model magnetic fields in neutron stars, incorporating superconductivity and the Hall effect, revealing that the toroidal magnetic energy remains limited and confined to the crust in superconducting cores.

Contribution

The authors develop a flexible numerical method to solve the nonlinear Grad-Shafranov equation for neutron star magnetic fields, including superconductivity and Hall effects, extending previous parameter space exploration.

Findings

Toroidal magnetic energy does not exceed 5% of total energy.

Superconductivity confines the toroidal field to the crust.

Implications for pulsar glitch models with superconducting cores.

Abstract

Equilibrium configurations of the internal magnetic field of a pulsar play a key role in modeling astrophysical phenomena, from glitches to gravitational wave emission. In this paper we present a numerical scheme for solving the Grad-Shafranov equation and calculating equilibrium configurations of pulsars, accounting for superconductivity in the core of the neutron star, and for the Hall effect in the crust of the star. Our numerical code uses a finite-difference method in which the source term appearing in the Grad-Shafranov equation, used to model the magnetic equilibrium is nonlinear. We obtain solutions by linearizing the source and applying an under-relaxation scheme at each step of computation to improve the solver's convergence. We have developed our code in both C++ and Python, and our numerical algorithm can further be adapted to solve any nonlinear PDEs appearing in other areas of computational astrophysics. We produce mixed toroidal-poloidal field configurations, and extend the portion of parameter space that can be investigated with respect to previous studies. We find that even in the more extreme cases the magnetic energy in the toroidal component does not exceed approximately 5\% of the total. We also find that if the core of the star is superconducting, the toroidal component is entirely confined to the crust of the star, which has important implications for pulsar glitch models which rely on the presence of a strong toroidal field region in the core of the star, where superfluid vortices pin to superconducting fluxtubes.