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 $\ensuremath{\nu}={m}_{a}/4\ensuremath{\pi}$ with a width determined by Doppler broadening in the DM halo, $\mathrm{\ensuremath{\Delta}}\ensuremath{\nu}/\ensuremath{\nu}\ensuremath{\sim}{10}^{\ensuremath{-}3}$. In this work, we show that the axion decay-induced echoes of supernova remnants may be bright enough to be detectable. Their nondetection may be able to set the strongest limits to date on axion DM in the $\ensuremath{\sim}1--10\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$ mass range where there are gaps in coverage from existing experiments.