Radio Signal of Axion-Photon Conversion in Neutron Stars: A Ray Tracing Analysis

TL;DR

This paper presents a comprehensive 3D analysis of axion-photon conversion in neutron star magnetospheres, demonstrating the robustness of the predicted radio signals and their detectability with SKA observations.

Contribution

It introduces the first three-dimensional computation of photon flux from axion-photon conversion, incorporating realistic axion distributions and magnetosphere structures.

Findings

Signal strength remains robust with realistic modeling.

Signal variance due to neutron star rotation is reduced.

SKA can detect axion-photon coupling around 3×10⁻¹³ GeV⁻¹ for specific axion masses.

Abstract

Axion dark matter can resonantly convert into photons in the magnetospheres of neutron stars (NSs). It has recently been shown that radio observations of nearby NSs can therefore provide a highly sensitive probe of the axion parameter space. Here we extend existing calculations by performing the first three-dimensional computation of the photon flux, taking into account the isotropic phase-space distribution of axions and the structure of the NS magnetosphere. In particular, we study the overall magnitude of the flux and its possible time variation. We find that overall signal strength is robust to our more realistic analysis. In addition, we find that the variance of the signal with respect to the NS rotation is washed out by the additional trajectories in our treatment. Nevertheless, we show that SKA observations towards J0806.4-4123 are sensitive to $g_{a\gamma\gamma}\sim 3\times10^{-13}\mathrm{\,GeV^{-1}}$ at $m_a\sim 7\times10^{-6}\mathrm{\,eV}$, even when accounting for Doppler broadening. Finally, we provide the necessary code to calculate the photon flux for any given NS system.