Secondary neutrino and gamma-ray fluxes from SimProp and CRPropa

The interactions of ultra-high energy cosmic rays (UHECRs) with background photons in extragalactic space generate high-energy neutrinos and photons. Simulating UHECR propagation requires assumptions about physical quantities such as the spectrum of the extragalactic background light (EBL) and photodisintegration cross sections. These assumptions, as well as the approximations used in the codes, may influence the computed predictions both of cosmic-ray spectra and composition, and of cosmogenic neutrino and photon fluxes. Following up on our previous work where we studied the resulting uncertainties on cosmic-ray simulations, here we quantify those on neutrinos and photons, using the Monte Carlo codes CRPropa and SimProp in various source scenarios. We discuss the results in the light of the constraining power of the neutrino and photon spectra on the origin of the UHECRs. We show that cosmogenic neutrino fluxes are more sensitive to the parametrization of the EBL than UHECR spectra, whereas the overall cosmogenic gamma-ray production rates are relatively independent on details of the propagation. We also find large differences between neutrino fluxes predicted by the latest released versions of CRPropa and SimProp, and discuss their causes and possible improvements in future versions of the codes.