Constraints on light QCD and CP-violating axions from the death line of rotation-powered pulsars

TL;DR

This paper explores how axion hair around neutron stars affects pulsar lifetimes and emission, providing new constraints on light QCD and CP-violating axions through pulsar observations and simulations.

Contribution

It introduces the concept of axion hair influencing pulsar death lines, extending pulsar lifetimes, and offers novel limits on axion properties based on pulsar emission data.

Findings

Axion hair can extend pulsar lifetimes significantly.

Non-observation of old pulsars constrains axion-nucleon interactions.

Emission from both poles of pulsars can set limits on CP-violating axions.

Abstract

For axions that couple to nucleons, the presence of dense nuclear matter can displace the axion from its vacuum minimum, sourcing large field gradients around neutron stars (and, more generally, compact objects). These gradients, which we refer to as axion hair, couple to the local background magnetic field, inducing a large voltage drop near the surface of the star; here, we demonstrate that the presence of axion hair decouples local near-field particle acceleration in the open magnetic field line bundle from the rotational frequency of the pulsar itself. This is significant as the non-observation of old slowly-rotating pulsars is attributed to the fact the rotationally-induced electric fields are not strong enough to sustain $e^\pm$ pair production. In this work, we review the evidence for the existence for `pulsar death', i.e. the threshold at which $e^\pm$ pair production (and thus, by association, coherent radio emission) ceases, and demonstrate using both semi-analytics and particle-in-cell simulations that the existence of axion hair can dramatically extend pulsar lifetimes. We show that the non-observation of extremely old, slowly rotating, pulsars allows for a new probe of light QCD and CP-violating axions. We also demonstrate how the observation of emission from both poles of pulsars with nearly orthogonal rotational and magnetic axes, as seen e.g. in PSR J1906+0746, can be used to set competitive limits on CP-violating axion-nucleon interactions.