Ambipolar diffusion in superfluid neutron stars

TL;DR

This paper investigates how superfluidity in neutron star cores influences magnetic field diffusion, finding that superfluidity significantly prolongs ambipolar diffusion timescales and challenges previous assumptions about magnetar core activity.

Contribution

It introduces the effect of superfluidity on ambipolar diffusion in neutron stars, providing revised estimates and implications for magnetar magnetic field evolution.

Findings

Superfluidity drastically increases ambipolar diffusion timescales.

Magnetar core magnetic fields are unlikely to decay via ambipolar diffusion within their lifetimes.

Magnetic field evolution in magnetars may occur outside the core, such as in the crust.

Abstract

In this paper we reconsider the problem of magnetic field diffusion in neutron star cores. We model the star as consisting of a mixture of neutrons, protons and electrons, and allow for particle reactions and binary collisions between species. Our analysis is in much the same spirit as that of Goldreich & Reisenegger (1992), and we content ourselves with rough estimates of magnetic diffusion timescales, rather than solving accurately for some particular field geometry. However, our work improves upon previous treatments in one crucial respect: we allow for superfluidity in the neutron star matter. We find that the consequent mutual friction force, coupling the neutrons and charged particles, together with the suppression of particles collisions and reactions, drastically affect the ambipolar magnetic field diffusion timescale. In particular, the addition of superfluidity means that it is unlikely that there is ambipolar diffusion in magnetar cores on the timescale of the lifetimes of these objects, contradicting an assumption often made in the modelling of the flaring activity commonly observed in magnetars. Our work suggests that if a decaying magnetic field is indeed the cause of magnetar activity, the field evolution is likely to take place outside of the core, and might represent Hall/Ohmic diffusion in the stellar crust, or else that a mechanism other than standard ambipolar diffusion is active, e.g. flux expulsion due to the interaction between neutron vortices and magnetic fluxtubes.