Axion dark matter-induced echo of supernova remnants
  Axions are a theoretically promising dark matter (DM) candidate. In the
presence of radiation from bright astrophysical sources at radio frequencies,
nonrelativistic DM axions can undergo stimulated decay to two nearly
back-to-back photons, meaning that bright sources of radio waves will have a
counterimage ("gegenschein") in nearly the exact opposite spatial direction.
The counterimage will be spectrally distinct from backgrounds, taking the form
of a narrow radio line centered at $\nu = m_a/4\pi$ with a width determined by
Doppler broadening in the DM halo, $\Delta \nu/\nu \sim 10^{-3}$. In this work,
we show that the axion decay-induced echoes of supernova remnants may be bright
enough to be detectable. Their non-detection may be able to set the strongest
limits to date on axion DM in the $\sim 1-10 \, \mu$eV mass range where there
are gaps in coverage from existing experiments.
