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 v = m(a)/4 pi with a width determined by Doppler broadening in the DM halo, Av/v similar to 10(-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 similar to 1-10 mu eV mass range where there are gaps in coverage from existing experiments.