Dynamical onset of superconductivity and retention of magnetic fields in cooling neutron stars

TL;DR

This paper demonstrates through neutron star cooling simulations that magnetic fields in neutron star cores are retained longer than previously thought due to rapid cooling, challenging the expectation of flux expulsion by the Meissner effect.

Contribution

It provides the first detailed simulation-based analysis showing magnetic flux retention in neutron star cores during early cooling phases.

Findings

Magnetic fields remain in the core for at least 10^6-10^7 years.

Flux free regions are estimated to be less than 100 meters at 10^7 years.

Magnetic flux may be expelled from a thin shell after 10^5 years in certain models.

Abstract

A superconductor of paired protons is thought to form in the core of neutron stars soon after their birth. Minimum energy conditions suggest magnetic flux is expelled from the superconducting region due to the Meissner effect, such that the neutron star core is largely devoid of magnetic fields for some nuclear equation of state and proton pairing models. We show via neutron star cooling simulations that the superconducting region expands faster than flux is expected to be expelled because cooling timescales are much shorter than timescales of magnetic field diffusion. Thus magnetic fields remain in the bulk of the neutron star core for at least 10^6-10^7 yr. We estimate the size of flux free regions at 10^7 yr to be <~ 100 m for a magnetic field of 10^11 G and possibly smaller for stronger field strengths. For proton pairing models that are narrow, magnetic flux may be completely expelled from a thin shell of approximately the above size after 10^5 yr. This shell may insulate lower conductivity outer layers, where magnetic fields can diffuse and decay faster, from fields maintained in the highly conducting deep core.