No Indications of Axion-Like Particles From Fermi

TL;DR

This study uses Fermi data to analyze gamma ray spectra from distant sources, finding no evidence for axion-like particles as an explanation for the universe's transparency to very high energy gamma rays.

Contribution

It provides the first detailed spectral analysis combining Fermi and ground-based data to test the ALP hypothesis, finding no need for exotic physics.

Findings

Spectra fit well with intrinsic power-law sources without ALPs.

No evidence of ALPs or exotic physics in gamma ray attenuation.

Supports standard attenuation models for distant gamma ray sources.

Abstract

As very high energy (~100 GeV) gamma rays travel over cosmological distances, their flux is attenuated through interactions with the extragalactic background light. Observations of distant gamma ray sources at energies between ~200 GeV and a few TeV by ground-based gamma ray telescopes such as HESS, however, suggest that the universe is more transparent to very high energy photons than had been anticipated. One possible explanation for this is the existence of axion-like-particles (ALPs) which gamma rays can efficiently oscillate into, enabling them to travel cosmological distances without attenuation. In this article, we use data from the Fermi Gamma Ray Space Telescope to calculate the spectra at 1-100 GeV of two gamma ray sources, 1ES1101-232 at redshift z=0.186 and H2356-309 at z=0.165, and use this in conjunction with the measurements of ground-based telescopes to test the ALP hypothesis. We find that the observations can be well-fit by an intrinsic power-law source spectrum with indices of -1.72 and -2.1 for 1ES1101-232 and H2356-309, respectively, and that no ALPs or other exotic physics is necessary to explain the observed degree of attenuation.