Hints of the existence of Axion-Like-Particles from the gamma-ray spectra of cosmological sources

TL;DR

This study investigates how Axion-Like Particles could influence gamma-ray spectra from distant sources through photon/axion mixing in magnetic fields, providing testable predictions for current gamma-ray observatories.

Contribution

It presents a comprehensive framework considering both source and intergalactic magnetic fields affecting photon/axion mixing, which was not simultaneously analyzed before.

Findings

30% attenuation in distant gamma-ray source spectra.

Attenuation depends on ALP properties and intergalactic magnetic field strength.

Predicted effects are observable with current gamma-ray telescopes.

Abstract

Axion Like Particles (ALPs) are predicted to couple with photons in the presence of magnetic fields. This effect may lead to a significant change in the observed spectra of gamma-ray sources such as AGNs. Here we carry out a detailed study that for the first time simultaneously considers in the same framework both the photon/axion mixing that takes place in the gamma-ray source and that one expected to occur in the intergalactic magnetic fields. An efficient photon/axion mixing in the source always means an attenuation in the photon flux, whereas the mixing in the intergalactic medium may result in a decrement and/or enhancement of the photon flux, depending on the distance of the source and the energy considered. Interestingly, we find that decreasing the value of the intergalactic magnetic field strength, which decreases the probability for photon/axion mixing, could result in an increase of the expected photon flux at Earth if the source is far enough. We also find a 30% attenuation in the intensity spectrum of distant sources, which occurs at an energy that only depends on the properties of the ALPs and the intensity of the intergalactic magnetic field, and thus independent of the AGN source being observed. Moreover, we show that this mechanism can easily explain recent puzzles in the spectra of distant gamma-ray sources... [ABRIDGED] The consequences that come from this work are testable with the current generation of gamma-ray instruments, namely Fermi (formerly known as GLAST) and imaging atmospheric Cherenkov telescopes like CANGAROO, HESS, MAGIC and VERITAS.