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

We fit the ultrahigh-energy cosmic-ray (UHECR, E greater than or similar to 0.1 EeV) spectrum and composition data from the Pierre Auger Observatory at energies E greater than or similar to 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 3.5 sigma statistical significance. In the single-population model, low-luminosity gamma-ray bursts turn out to match the best-fit evolution parameter. In the two-population model, the active galactic nuclei is consistent with the best-fit redshift evolution parameter of the pure proton-emitting 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.

Automated summary

This study fits the spectrum and composition data of ultrahigh-energy cosmic rays from the Pierre Auger Observatory using two populations of astrophysical sources, with the two-population model showing significant improvement in the fit compared to the one-population model. It also discusses the possibility of detecting cosmogenic neutrinos to shed light on the sources of UHECRs.