Detecting Axion Dark Matter with Radio Lines from Neutron Star
  Populations
  It has been suggested that radio telescopes may be sensitive to axion dark
matter that resonantly converts to radio photons in the magnetospheres
surrounding neutron stars (NSs). In this work, we closely examine this
possibility by calculating the radiated power from and projected sensitivity to
axion dark matter conversion in ensembles of NSs within astrophysical systems
like galaxies and globular clusters. We use population synthesis and evolution
models to describe the spatial distributions of NSs within these systems and
the distributions of NS properties. Focusing on three specific targets for
illustration, the Galactic Center of the Milky Way, the globular cluster M54 in
the Sagittarius dwarf galaxy, and the Andromeda galaxy, we show that
narrow-band radio observations with telescopes such as the Green Bank Telescope
and the future Square Kilometer Array may be able to probe the quantum
chromodynamics axion over roughly two orders of magnitude in mass, starting at
a fraction of a $\mu$eV.
