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We investigate a novel technique for the astrophysical detection of axions oraxion-like particles in the dark matter halo of the Milky Way based onstimulated decay of axions, which we call axion gegenschein emission. Photonsfrom the brightest known radio sources with a frequency equal to half the axionmass stimulate axion decay while propagating through the dark matter halo,causing radio emission in a direction precisely opposite to the incoming photonin the axion rest-frame and creating a countersource for every radio source,with an image smoothed by the dark matter velocity dispersion. We calculate theflux of the axion gegenschein countersource of Cygnus A, the brightestextragalactic radio source, and the limits that can be set with SKA to theaxion-photon coupling constant $g_{a\gamma}$. We find this method to be morepowerful than previous proposals based on searching for radio emission fromaxion decay in nearby dwarf galaxies or the Milky Way. The forecasted limitsremain considerably higher than predictions from QCD axion models, and limitsthat can be set with laboratory searches of radio waves generated in resonantcavities with strong magnetic fields similar to the ADMX experiment, althoughthis observation would directly measure a column density of dark matter throughthe Galactic halo and is therefore not affected by possible substructure in thedark matter distribution.

Axion Gegenschein: Probing Back-scattering of Astrophysical Radio Sources Induced by Dark Matter