Speaker
Description
The mass composition of ultra-high-energy cosmic rays is usually inferred from the depth of the shower maximum (Xmax) of cosmic-ray showers, which is ambiguously determined by modern hadronic interaction models. We examine a data-driven scenario in which the expectation value of Xmax is considered as a free parameter. We test the hypothesis of whether the cosmic-ray data from the Pierre Auger Observatory can be interpreted in a consistent picture under the assumption that the mass composition of cosmic rays at the highest energies is dominated by high metallicity, resulting in a pure iron composition at energies above ≈40 EeV. We investigate the implications for astrophysical observations and hadronic interactions, and discuss the global consistency of the data assuming this heavy-metal scenario.
We find that the publicly-available data of the Pierre Auger Observatory can be interpreted consistently if the expectation values for Xmax from modern hadronic interaction models are shifted to deeper values. The resulting shifts of the predicted Xmax scale closely match those obtained from joint fits of Xmax and the ground signal distributions in the 3 EeV − 10 EeV range [PRD 109 (2024) 102001] and might be explained by recent improvements in air-shower modelling. Consequently, the changes in the mean and variance of ln A shift the measured data at 3 EeV − 100 EeV well within the region of expected combinations of protons and He, N, and Fe nuclei, contrary to the standard interpretation using unmodified model predictions. The variance of ln A is then consistent with the model-independent constraints on the broadness of the cosmic-ray mass composition at energies 3 EeV − 10 EeV in [PLB 762 (2016) 288].
In the presented scenario, the flux suppression of cosmic rays is consistent with a rigidity cutoff approximately at 2 EV. Consequently, the disappearance of nitrogen and iron nuclei from the cosmic-ray beam at the same rigidity could explain the instep feature of the cosmic-ray energy spectrum. The muon deficit predicted by the QGSJet II-04 and Sibyll 2.3d models is alleviated from ∼30% − 50% to ∼20% − 25% when compared to the direct measurements of the muon signal. There is no indication that the inelastic p-p cross-section or elasticity needs to be modified in the two models within the heavy-metal scenario to describe the tail of the measured Xmax distributions.
Considering the observed dipole anisotropy of cosmic rays above 8 EeV, we confirm that this observation is consistent with a possible extragalactic dipolar distribution of cosmic-ray sources within the heavy-metal scenario at the 2σ level and even at 1σ level for very high extragalactic amplitudes (above ∼40%). Assuming only iron nuclei, the arrival directions of the most energetic Auger events, when backtracked through the Galactic magnetic field, point towards the Galactic anticenter region, consistent with the expectations from isotropic arrival directions at Earth. The estimated luminosity density of the sources in the heavy-metal scenario suggests that only very powerful objects, such as hard X-ray AGNs, could explain the origin of the ultra-high-energy cosmic rays.
