Upper Limit on the Cosmological Gamma-ray Background

TL;DR

This paper establishes an upper limit on the extragalactic gamma-ray background above 100 GeV based on current measurements below 100 GeV, constraining models and new physics scenarios.

Contribution

It introduces a conservative upper limit on the VHE EGB using cascade emission modeling, impacting understanding of gamma-ray sources and potential new physics.

Findings

The cascade emission fits the VHE EGB spectrum well.

Known sources may exceed the established upper limit.

The limit constrains theories involving axion-like particles and Lorentz-invariance violation.

Abstract

We show that the current extragalactic gamma-ray background (EGB) measurement below 100 GeV sets an upper limit on EGB itself at very high energy (VHE) above 100 GeV. The limit is conservative for the electromagnetic cascade emission from VHE EGB interacting with the cosmic microwave-to-optical background radiation not to exceed the current EGB measurement. The cascade component fits the measured VHE EGB spectrum rather well. However, once we add the contribution from known source classes, the Fermi VHE EGB observation exceeds or even violates the limit, which is approximated as E^2dN/dE < 4.5x10^-5 (E/100 GeV)^-0.7 MeV/cm^2/s/sr. The upper limit above 100 GeV is useful in the future to probe the EGB origin and the new physics like axion-like particles and Lorentz-invariance violation.