Towards discrimination between galactic and intergalactic axion-photon mixing

Understand

There exists a growing evidence for the anomalous transparency of the Universe for energetic gamma rays.

• Popular explanations include conversion of photons into hypothetical axion-like particles (ALPs) and back in astrophysical magnetic fields.
• Two distinctive scenarios of this conversion have been put forward: either it happens in the (host galaxy of the) gamma-ray source and in the Milky Way, or the photon-ALP oscillations take place in the intergalactic magnetic fields all along the way between the source and the observer.
• Here we point out that, given recent astrophysical constraints on ALPs and on intergalactic magnetic fields, these two mechanisms imply very different ALP masses and couplings.