Axion Gegenschein: Probing Back-scattering of Astrophysical Radio
  Sources Induced by Dark Matter
  We investigate a novel technique for the astrophysical detection of axions or
axion-like particles in the dark matter halo of the Milky Way based on
stimulated decay of axions, which we call axion gegenschein emission. Photons
from the brightest known radio sources with a frequency equal to half the axion
mass stimulate axion decay while propagating through the dark matter halo,
causing radio emission in a direction precisely opposite to the incoming photon
in the axion rest-frame and creating a countersource for every radio source,
with an image smoothed by the dark matter velocity dispersion. We calculate the
flux of the axion gegenschein countersource of Cygnus A, the brightest
extragalactic radio source, and the limits that can be set with SKA to the
axion-photon coupling constant $g_{a\gamma}$. We find this method to be more
powerful than previous proposals based on searching for radio emission from
axion decay in nearby dwarf galaxies or the Milky Way. The forecasted limits
remain considerably higher than predictions from QCD axion models, and limits
that can be set with laboratory searches of radio waves generated in resonant
cavities with strong magnetic fields similar to the ADMX experiment, although
this observation would directly measure a column density of dark matter through
the Galactic halo and is therefore not affected by possible substructure in the
dark matter distribution.
