Magnetic dynamo caused by axions in neutron stars

TL;DR

This paper explores how axion-photon coupling can induce a magnetic dynamo in neutron stars, significantly increasing their magnetic energy and thermal luminosity, which constrains axion properties based on astrophysical observations.

Contribution

It demonstrates that axion-induced dynamo effects in neutron stars can lead to observable consequences, providing new bounds on axion parameters.

Findings

Axion-photon coupling modifies Maxwell's equations in neutron stars.

Dynamo mechanism increases magnetic energy and internal heating.

Constraints on axion parameters are derived from astrophysical observations.

Abstract

The coupling between axions and photons modifies Maxwell's equations, introducing a dynamo term in the magnetic induction equation. In neutron stars, for critical values of the axion decay constant and axion mass, the magnetic dynamo mechanism increases the total magnetic energy of the star. We show that this generates substantial internal heating due to enhanced dissipation of crustal electric currents. These mechanisms would lead magnetized neutron stars to increase their magnetic energy and thermal luminosity by several orders of magnitude, in contrast to observations of thermally-emitting neutron stars. To prevent the activation of the dynamo, bounds on the allowed axion parameter space can be derived.