A signal from stimulated decays of axion dark matter in the Milky Way

In the presence of radio waves having a wavelength equivalent to half of an axion mass, an axion undergoes a stimulated decay and emits two photons. Those two photons travel in opposite directions along the background radiation fields, and thus, the radio image and its counterimage of background fields would be enhanced. Assuming all the dark matter consists of axions, the radio signals from decays of an axion in the Milky Way stimulated by galactic, extragalactic, and CMB photons are computed. We also consider what could affect the signal: the time evolution of the flux from supernova remnants, the free-free absorption with Galactic electrons, the formation of dilute axion stars, and the mass segregation between dilute axion stars and ordinary stars. The resulting signal turns out to be bright enough to be detected by the first phase of the Square Kilometer Array for the axion-photon coupling g_aγ ∼ 2-3 × 10^-11 GeV^-1 and an axion mass m_a ∼ 10^-6 eV.