The Milky Way as a Kiloparsec-Scale Axionscope

TL;DR

This paper investigates how photon-axion-like-particle oscillations in the Milky Way's magnetic field could explain the unexpected transparency of the universe to very high energy gamma-rays from distant sources.

Contribution

It demonstrates that photon-ALP conversion in the Milky Way's magnetic field can account for observed gamma-ray transparency, offering a novel explanation involving axion-like particles.

Findings

Photon-ALP conversion can be efficient in the Milky Way's magnetic field.

The mechanism explains the transparency of the universe to high-energy gamma-rays.

Features can distinguish this scenario from other models.

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 axion-like-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: 1ES1101-232 at redshift z=0.186 and H 2356-309 at z=0.165. We also discuss features of this scenario which could be used to distinguish it from standard or other exotic models.