Ferromagnetic neutron stars: axial anomaly, dense neutron matter, and pionic wall

TL;DR

This paper proposes that the axial anomaly in a chiral nonlinear sigma model coupled to dense neutrons can lead to a ferromagnetic phase with extremely strong magnetic fields, potentially explaining the origin of magnetars.

Contribution

It introduces a novel mechanism where the axial anomaly induces ferromagnetism in high-density neutron matter through neutral pion domain walls.

Findings

High-density neutron matter can become ferromagnetic due to axial anomaly effects.

The resulting magnetic fields could reach the QCD scale (~10^19 G).

This mechanism provides a microscopic explanation for magnetar magnetic fields.

Abstract

We show that a chiral nonlinear sigma model coupled to degenerate neutrons exhibits a ferromagnetic phase at high density. The magnetization is due to the axial anomaly acting on the parallel layers of neutral pion domain walls spontaneously formed at high density. The emergent magnetic field would reach the QCD scale ~ 10^19 [G], which suggests that the quantum anomaly can be a microscopic origin of the magnetars (highly magnetized neutron stars).