Milky Way as a kiloparsec-scale axionscope

Abstract

Very high energy gamma rays are expected to be absorbed by the extragalactic background light over cosmological distances via the process of electron-positron pair production. Recent observations of cosmologically distant gamma-ray emitters by ground based gamma-ray telescopes have, however, revealed a surprising degree of transparency of the universe to very high energy photons. One possible mechanism to explain this observation is the oscillation between photons and axionlike particles (ALPs). Here we explore this possibility further, focusing on photon-ALP conversion in the magnetic fields in and around gamma-ray sources and in the magnetic field of the Milky Way, where some fraction of the ALP flux is converted back into photons. We show that this mechanism can be efficient in allowed regions of the ALP parameter space, as well as in typical configurations of the galactic magnetic field. As case examples, we consider the spectrum observed from two HESS sources: $1\mathrm{ES}1101\mathrm{\text{−}}232$ at redshift $z=0.186$ and $\mathrm{H}2356\mathrm{\text{−}}309$ at $z=0.165$. We also discuss features of this scenario which could be used to distinguish it from standard or other exotic models.