Anatomy of astrophysical echoes from axion dark matter

Understand

If the dark matter in the Universe is made of $\mu$eV axion-like particles (ALPs), then a rich phenomenology can emerge in connection to their stimulated decay into two photons.

• We discuss the ALP stimulated decay induced by electromagnetic radiation from Galactic radio sources.
• Three signatures, made by two echoes and one collinear emission, are associated with the decay, and can be simultaneously detected, offering a unique opportunity for a clear ALP identification.
• We derive the formalism associated with such signatures starting from first principles, and providing the relevant equations to be applied to study the ALP phenomenology.