The contrasting magnetic fields of superconducting pulsars and magnetars

TL;DR

This paper investigates how superconducting protons in neutron star cores influence magnetic field configurations, revealing differences between pulsars and magnetars and implications for magnetic field decay and activity.

Contribution

It introduces a model for equilibrium magnetic fields in neutron stars with superconducting cores, highlighting the role of the lower critical field in determining field geometry.

Findings

Pulsar and magnetar fields have distinct configurations due to superconductivity.

Field decay can cause internal rearrangements, affecting magnetar activity.

Calculated ellipticities suggest observable gravitational wave signals.

Abstract

We study equilibrium magnetic field configurations in a neutron star (NS) whose core has type-II superconducting protons. Unlike the equations for normal matter, which feature no special field strength, those for superconductors contain the lower critical field, of order $10^{15}$ G. We find that the ratio of this critical field to the smooth-averaged stellar field at the crust-core boundary is the key feature dictating the field geometry. Our results suggest that pulsar and magnetar-strength fields have notably different configurations. Field decay for NSs with $B_\textrm{pole}\sim 10^{14}$ G could thus result in substantial internal rearrangements, pushing the toroidal field component out of the core; this may be related to observed magnetar activity. In addition, we calculate the magnetically-induced ellipticities of our models.