Radio Signals from Axion Dark Matter Conversion in Neutron Star Magnetospheres

We show that axion dark matter may be detectable through narrow radio lines emitted from neutron stars. Neutron star magnetospheres host both 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 when the plasma frequency matches the axion mass. We solve the axion-photon mixing equations, including a full treatment of the magnetized plasma, 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 $\sim 0.2–40\mu \mathrm{eV}$.

Figure 1

Projected sensitivity to $g_{a\gamma \gamma }$ as a function of axion mass $m_a$ for $\mathrm{\Delta }{t}_{\mathrm{obs}}=100\mathrm{h}$ and $\mathrm{SEFD}=2\mathrm{Jy}$, alongside current and projected limits from ADMX and CAST. The QCD axion is predicted to lie within the orange band. We have taken ${\theta }_m=10°$ and the solid (dashed) curves assume $\theta =90°$ ($\theta =120.5°$). The lower mass cutoff is set by the lowest available frequency of current radio telescopes, while the high-mass cutoff comes from requiring the conversion radius to be outside the NS radius.