Cosmogenic photons strongly constrain UHECR source models

TL;DR

This paper uses advanced simulations and recent gamma-ray data to place strong constraints on models of ultra-high-energy cosmic ray sources, especially those involving pure protons, by analyzing secondary photon fluxes.

Contribution

It introduces updated Monte Carlo simulations and combines them with Fermi LAT data to significantly constrain UHECR source models based on secondary photon predictions.

Findings

Pure proton UHECR models are strongly constrained by gamma-ray observations.

Source evolution significantly impacts the expected secondary photon flux.

Current data limits the viability of certain UHECR source scenarios.

Abstract

With the newest version of our Monte Carlo code for ultra-high-energy cosmic ray (UHECR) propagation, CRPropa 3, the flux of neutrinos and photons due to interactions of UHECRs with extragalactic background light can be predicted. Together with the recently updated data for the isotropic diffuse gamma-ray background (IGRB) by Fermi LAT, it is now possible to severely constrain UHECR source models. The evolution of the UHECR sources especially plays an important role in the determination of the expected secondary photon spectrum. Pure proton UHECR models are already strongly constrained, primarily by the highest energy bins of Fermi LAT's IGRB, as long as their number density is not strongly peaked at recent times.