Combined fit of UHECR spectrum and composition with two extragalactic source populations

A mixed composition of light-to-heavy nuclei elements (¹H, ⁴He, ¹⁴N, ²⁸Si, ⁵⁶Fe) at injection fits the ultrahigh-energy cosmic ray (UHECR; E > 10¹⁷ eV) spectrum data measured by the Pierre Auger Observatory, beyond the ankle, i.e., E & 5 × 10¹⁸ eV. However, the composition fit can be further improved by the addition of light nuclei at the highest energies. We consider the light nuclei to originate from a discrete source population consisting of protons (¹H). We constrain the maximum allowed proton fraction at the highest-energy bin at 3.5σ statistical significance. Including the redshift evolution of sources as a free parameter further improves the composition fit. We find that low-luminosity gamma-ray bursts match the best-fit evolution index in the case of the one-population model. Active galactic nuclei are the plausible candidates for light nuclei injection in the two-population model, whereas tidal disruption events can inject heavy nuclei composition. We also present the secondary neutrino flux in one- and two-population models, constraining the composition at highest energies.