Probing the Intergalactic Magnetic Field with the Anisotropy of the Extragalactic Gamma-ray Background

TL;DR

This paper explores how the anisotropy in the extragalactic gamma-ray background can be used to probe the strength of the intergalactic magnetic field, showing that current data already suggest non-negligible IGMF values.

Contribution

It introduces a simple calculation demonstrating the potential of gamma-ray anisotropy measurements to constrain the intergalactic magnetic field.

Findings

Current Fermi data favor non-zero IGMF values.

Gamma-ray anisotropy spectrum can serve as a probe for IGMF strength.

Abstract

The intergalactic magnetic field (IGMF) may leave an imprint on the angular anisotropy of the extragalactic gamma-ray background through its effect on electromagnetic cascades triggered by interactions between very high energy photons and the extragalactic background light. A strong IGMF will deflect secondary particles produced in these cascades and will thus tend to isotropize lower energy cascade photons, thereby inducing a modulation in the anisotropy energy spectrum of the gamma-ray background. Here we present a simple, proof-of-concept calculation of the magnitude of this effect and demonstrate that current Fermi data already seem to prefer non-negligible IGMF values. The anisotropy energy spectrum of the Fermi gamma-ray background could thus be used as a probe of the IGMF strength.