Radio Signals from Axion Dark Matter Conversion in Neutron Star
  Magnetospheres
  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.
