Axion-photon conversion in strongly magnetised plasmas

TL;DR

This paper presents the first 3D calculation of axion-photon conversion in neutron star magnetospheres, revealing significant differences from previous simplified models and impacting radio signal predictions for dark matter detection.

Contribution

It introduces a fully three-dimensional model for axion-photon conversion in anisotropic, magnetised plasmas, improving accuracy over prior one-dimensional approaches.

Findings

Orders-of-magnitude differences in conversion rates compared to 1D models

Altered directionality of the emitted photons

Implications for radio signal detection strategies

Abstract

Axion dark matter can resonantly convert to photons in the magnetosphere of neutron stars, possibly giving rise to radio signals observable on Earth. This method for the indirect detection of axion dark matter has recently received significant attention in the literature. The calculation of the radio signal is complicated by a number of effects; most importantly, the gravitational infall of the axions onto the neutron star accelerates them to semi-relativistic speed, and the neutron star magnetosphere is highly anisotropic. Both of these factors complicate the calculation of the conversion of axions to photons. In this work, we present the first fully three-dimensional calculation of the axion-photon conversion in highly magnetised anisotropic media. Depending on the axion trajectory, this calculation leads to orders-of-magnitude differences in the conversion compared to the simplified one-dimensional calculation used so far in the literature, altering the directionality of the produced photons. Our results will have important implications for the radio signal one would observe in a telescope.