Photon-photon dispersion of TeV gamma rays and its role for photon-ALP conversion

TL;DR

This paper investigates how photon-photon dispersion affects TeV gamma-ray propagation, especially in the context of photon-ALP conversion, highlighting the dominance of background photons and the potential impact on astrophysical observations.

Contribution

It provides a detailed calculation of the photon dispersion effect at TeV energies, emphasizing the role of background photon fields and their influence on photon-ALP oscillations.

Findings

Background photons dominate photon dispersion below pair-production threshold.

Cosmic microwave background imparts an effective photon mass at TeV energies.

Photon-photon dispersion effects are generally small but can be significant in specific scenarios.

Abstract

The propagation of TeV gamma rays can be strongly modified by B-field induced conversion to axionlike particles (ALPs). We show that, at such high energies, photon dispersion is dominated by background photons - the only example where photon-photon dispersion is of practical relevance. We determine the refractive index for all energies and find that, for fixed energy density, background photons below the pair-production threshold dominate. The cosmic microwave background alone provides an "effective photon mass" of (m_gamma)^2 = -(1.01 neV * E/TeV)^2 for E < 1000 TeV. The extragalactic background light is subdominant, but local radiation fields in the galaxy or the source regions provide significant contributions. Photon-photon dispersion is small enough to leave typical scenarios of photon-ALP oscillations unscathed, but big enough to worry about it case by case.