Radio Signals from Axion Dark Matter Conversion in Neutron Star Magnetospheres

TL;DR

This paper proposes that axion dark matter could be detected via narrow radio signals generated by resonant conversion in neutron star magnetospheres, offering a new method to probe axion properties.

Contribution

It provides a detailed calculation of axion-photon conversion probabilities in neutron star environments, including plasma effects, and discusses observational prospects for detecting QCD axions.

Findings

Resonant conversion occurs at specific radii where plasma frequency matches axion mass.

The predicted radio signals are narrow and depend on neutron star magnetic fields and plasma profiles.

Potential to probe QCD axion parameter space in the mass range 0.2-40 μeV.

Abstract

We show that axion dark matter (DM) may be detectable through narrow radio lines emitted from neutron stars. The neutron star magnetosphere hosts a strong magnetic field and a plasma frequency that increases towards the neutron star surface. As the axions pass through the magnetosphere, they can resonantly convert into radio photons in a narrow region around the radius at which the plasma frequency equals the axion mass. The bandwidth of the signal is set by the small DM velocity dispersion far away from the neutron star. We solve the axion-photon mixing equations, including a full treatment of the magnetized plasma and associated anisotropic dielectric tensor, to obtain the conversion probability. We discuss possible neutron-star targets and how they may probe the QCD axion parameter space in the mass range of ~0.2-40 $\mu$eV.