Abstract
We fit the ultrahigh-energy cosmic-ray (UHECR, Egtrsim 0.1 EeV) spectrum and composition data from the Pierre Auger Observatory at energies Egtrsim 5cdot 10^{18} 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, ^{14}N, ^{28}Si, ^{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 3.5sigma 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.
Highlights
The Pierre Auger Observatory (PAO) in Malargüe, Argentina [38] and the Telescope Array (TA) experiment in Utah, United States [39] are attaining unprecedented precision in the measurement of UHECR flux, composition, and arrival directions from 0.3 EeV to beyond 100 EeV using their hybrid detection technique [40,41]
On incidence at the Earth’s atmosphere, these energetic UHECR nuclei initiate hadronic cascades which are intercepted by the surface detector (SD), and the simultaneous fluorescence light emitted by the Nitrogen molecules in the atmosphere is observed using
We study the effect of variation of the proton injection spectral index, which is not done in earlier studies and indicate the maximum allowed proton fraction at the highest-energy bin up to 3.5σ statistical significance
Summary
The Pierre Auger Observatory (PAO) in Malargüe, Argentina [38] and the Telescope Array (TA) experiment in Utah, United States [39] are attaining unprecedented precision in the measurement of UHECR flux, composition, and arrival directions from 0.3 EeV to beyond 100 EeV using their hybrid detection technique [40,41]. On incidence at the Earth’s atmosphere, these energetic UHECR nuclei initiate hadronic cascades which are intercepted by the surface detector (SD), and the simultaneous fluorescence light emitted by the Nitrogen molecules in the atmosphere is observed using
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