Cosmogenic gamma-rays and neutrinos constrain UHECR source models

TL;DR

This paper uses simulations to explore how cosmogenic neutrino and gamma-ray spectra depend on UHECR source properties, showing current gamma-ray data already constrains some models and future neutrino experiments can limit proton fractions.

Contribution

It demonstrates the dependence of cosmogenic spectra on source parameters and assesses how upcoming neutrino observations can constrain UHECR composition and source evolution models.

Findings

Fermi/LAT gamma-ray background constrains certain UHECR source models.

Future neutrino experiments can significantly limit the proton fraction in UHECRs.

Cosmogenic spectra are sensitive to source maximum energy, spectral index, and composition.

Abstract

We use CRPropa 3 to show how the expected cosmogenic neutrino and gamma-ray spectra depend on the maximum energy of ultra-high energy cosmic rays (UHECRs) at their sources, on the spectral index at injection and on the chemical composition of UHECRs. The isotropic diffuse gamma-ray background measured by Fermi/LAT is already close to touching upon a model with co-moving source evolution and with the chemical composition, spectral index and maximum acceleration energy optimized to provide the best fit to the UHECR spectrum and composition measured by the Pierre Auger Collaboration. Additionally, the detectable fraction of protons present at the highest energies in UHECRs, for experiments with sensitivities to the single-flavor neutrino flux at $\sim1$ EeV in the range of $\sim 10^{-8}$ - $10^{-10}$ GeV cm$^{-2}$ s$^{-1}$ sr$^{-1}$, is shown as a function of the evolution of UHECR sources. Experiments that reach this sensitivity will be able to significantly constrain the proton fraction for realistic source evolution models.