Modeling the spectrum and composition of ultrahigh-energy cosmic rays with two populations of extragalactic sources

Abstract

We fit the ultrahigh-energy cosmic-ray (UHECR,E 0.1EeV) spectrum and composition data from the Pierre Auger Observatory at energies E 5 • 1018 eV, i.e., beyond the ankle using two populations of astrophysical sources.One population, accelerating dominantly protons (1H), extends up to the highest observed energies with maximum energy close to the GZK cutoff and injection spectral index near the Fermi acceleration model; while another population accelerates light-to-heavy nuclei (4He, 14N, 28Si, 56Fe) with a relatively low rigidity cutoff and hard injection spectrum. A significant improvement in the combined fit is noted as we go from a one-population to two-population model. For the latter, we constrain the maximum allowed proton fraction at the highest-energy bin within 3.5σ statistical significance. In the single-population model, low-luminosity gamma-ray bursts turn out to match the best-fit evolution parameter. In the twopopulationmodel, the active galactic nuclei is consistent with the best-fit redshift evolution parameter of the pure protonemitting sources, while the tidal disruption events could be responsible for emitting heavier nuclei. We also compute expected cosmogenic neutrino flux in such a hybrid source population scenario and discuss possibilities to detect these neutrinos by upcoming detectors to shed light on the sources of UHECRs.