Magnetic field evolution in neutron stars

TL;DR

This paper reviews the origin, configuration, and long-term evolution of magnetic fields in neutron stars, highlighting the roles of flux conservation, internal processes, and their impact on different neutron star types.

Contribution

It provides a comprehensive overview of magnetic field evolution mechanisms in neutron stars, emphasizing the influence of internal physics and initial conditions.

Findings

Magnetic fields likely originate from flux conservation during collapse.

Stable stratification helps contain magnetic fields within the star.

Field evolution is driven by processes overcoming stratification, affecting different neutron star classes.

Abstract

Neutron stars contain persistent, ordered magnetic fields that are the strongest known in the Universe. However, their magnetic fluxes are similar to those in magnetic A and B stars and white dwarfs, suggesting that flux conservation during gravitational collapse may play an important role in establishing the field, although it might also be modified substantially by early convection, differential rotation, and magnetic instabilities. The equilibrium field configuration, established within hours (at most) of the formation of the star, is likely to be roughly axisymmetric, involving both poloidal and toroidal components. The stable stratification of the neutron star matter (due to its radial composition gradient) probably plays a crucial role in holding this magnetic structure inside the star. The field can evolve on long time scales by processes that overcome the stable stratification, such as weak interactions changing the relative abundances and ambipolar diffusion of charged particles with respect to neutrons. These processes become more effective for stronger magnetic fields, thus naturally explaining the magnetic energy dissipation expected in magnetars, at the same time as the longer-lived, weaker fields in classical and millisecond pulsars.