Abstract
Abstract We study the implications of ultrahigh-energy cosmic-ray (UHECR) data from the Pierre Auger Observatory for potential accelerator candidates and cosmogenic neutrino fluxes for different combinations of nuclear disintegration and air-shower models. We exploit the most recent spectral and mass composition data (2017) with a new, computationally efficient simulation code, PriNCe. We extend a systematic framework, which has been previously applied in a combined fit by the Pierre Auger Collaboration, with the cosmological source evolution as an additional free parameter. In this framework, an ensemble of generalized UHECR accelerators is characterized by a universal spectral index (equal for all injection species), a maximal rigidity, and the normalizations for five nuclear element groups. We find that the 2017 data favor a small but constrained contribution of heavy elements (iron) at the source. We demonstrate that the results moderately depend on the nuclear disintegration (Puget–Stecker–Bredekamp, Peanut, or Talys) model and more strongly on the air-shower (EPOS-LHC, Sibyll 2.3, or QGSjetII-04) model. Variations of these models result in different source evolution and spectral indices, limiting the interpretation in terms of a particular class of cosmic accelerators. Better-constrained parameters include the maximal rigidity and the mass composition at the source. Hence, the cosmogenic neutrino flux can be robustly predicted. Depending on the source evolution at high redshifts, the flux is likely out of reach of future neutrino observatories in most cases, and a minimal cosmogenic neutrino flux cannot be claimed from data without assuming a cosmological distribution of the sources.
Highlights
The two largest detectors ever built, the Pierre Auger Observatory [1] and the Telescope Array [2], investigate the origin and the nature of Ultra-High Energy Cosmic Rays (UHECRs) above 1018 eV with hybrid detection techniques that combine signals from surface and fluorescence detectors to reconstruct extensive air showers
We revisit the approach of fitting the UHECR spectrum and composition using a density of homogeneously distributed “generic” UHECR sources as in the Auger’s Combined fit (CF) [3]
We study the impact of the model uncertainties on the astrophysical interpretation by performing scans in the three parameters: maximum rigidity Rmax [GV], spectral index γ and cosmological evolution index m, using different combinations of nuclear disintegration and air-shower models
Summary
The two largest detectors ever built, the Pierre Auger Observatory [1] and the Telescope Array [2], investigate the origin and the nature of Ultra-High Energy Cosmic Rays (UHECRs) above 1018 eV with hybrid detection techniques that combine signals from surface and fluorescence detectors to reconstruct extensive air showers. We revisit the approach of fitting the UHECR spectrum and composition using a density of homogeneously distributed “generic” UHECR sources as in the Auger’s Combined fit (CF) [3] In this high-energy approximation, the extragalactic transport is assumed to be purely ballistic, and diffusion due to the presence of magnetic fields is not taken into account. The terms (in order of occurrence) represent adiabatic cooling, pair production, photo-nuclear interactions (interaction and decays; reinjection) and injection from sources This system of PDEs in E and z is solved using a 6th-order stencil operator for the E derivatives and backward differentiation functions (BDF) for the redshift dependence. There are two consequences; the neutrino flux peaks at lower energies ∼ 108 GeV and is significantly lower compared to the protons-on-CMB case
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