Generalized Ray Tracing for Axions in Astrophysical Plasmas

TL;DR

This paper develops a covariant ray tracing framework to predict radio signals from axion-photon conversion near neutron stars, accounting for plasma anisotropy and gravity, and assesses observational prospects for detecting axions.

Contribution

It introduces a fully covariant ray tracing method for axion-induced radio signals in neutron star environments, incorporating realistic plasma and gravitational effects.

Findings

Quantifies the impact of gravity and plasma anisotropy on photon propagation.

Shows current and future telescopes could detect axions with masses around a few millielectronvolts.

Revisits sensitivity estimates using realistic magnetosphere models.

Abstract

Ray tracing plays a vital role in black hole imaging, modeling the emission mechanisms of pulsars, and deriving signatures from physics beyond the Standard Model. In this work we focus on one specific application of ray tracing, namely, predicting radio signals generated from the resonant conversion of axion dark matter in the strongly magnetized plasma surrounding neutron stars. The production and propagation of low-energy photons in these environments are sensitive to both the anisotropic response of the background plasma and curved spacetime; here, we employ a fully covariant framework capable of treating both effects. We implement this both via forward and backward ray tracing. In forward ray tracing, photons are sampled at the point of emission and propagated to infinity, whilst in the backward-tracing approach, photons are traced backwards from an image plane to the point of production. We explore various approximations adopted in prior work, quantifying the importance of gravity, plasma anisotropy, the neutron star mass and radius, and imposing the proper kinematic matching of the resonance. Finally, using a more realistic model for the charge distribution of magnetar magnetospheres, we revisit the sensitivity of current and future radio and sub-mm telescopes to spectral lines emanating from the Galactic Center Magnetar, showing such observations may extend sensitivity to axion masses $m_a \sim \mathcal{O}({\rm few}) \times 10^{-3}$ eV, potentially even probing parameter space of the QCD axion.