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

Abstract

Abstract We fit the ultrahigh-energy cosmic-ray (UHECR, $$E\gtrsim 0.1$$ E ≳ 0.1 EeV) spectrum and composition data from the Pierre Auger Observatory at energies $$E\gtrsim 5\cdot 10^{18}$$ E ≳ 5 · 10 18 eV, i.e., beyond the ankle using two populations of astrophysical sources. One population, accelerating dominantly protons ( $$^1$$ 1 H), 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 ( $$^4$$ 4 He, $$^{14}$$ 14 N, $$^{28}$$ 28 Si, $$^{56}$$ 56 Fe) 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 $$\sigma $$ σ 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.