PoS - Proceedings of Science
Volume 444 - 38th International Cosmic Ray Conference (ICRC2023) - Cosmic-Ray Physics (Indirect, CRI)
Imprints of extragalactic propagation in the UHECR mass composition at the top of the atmosphere
C. Trimarelli* and D.B.S.P. Francesco Salamida
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Pre-published on: July 25, 2023
Published on:
The nuclear species responsible for the flux of the ultra-high-energy cosmic rays (UHECRs)
at Earth can be retrieved from the distributions of the depth in the atmosphere at which the
maximum number of particles in the extensive air showers is reached, i.e. 𝑋max. This is done by
fitting model predictions of four or five mass groups (p, He, N, Si and Fe) to the measured 𝑋max
distributions. The derived mass-fractions-to-energy curves show that different nuclear species
dominate different energy ranges. In this contribution, we investigate this finding by assuming a
parametric model for each elemental spectrum and fitting, at the same time, the energy spectrum
and the 𝑋max distributions measured by the Pierre Auger Observatory without taking into account
the extragalactic propagation in the fit procedure. We find that the peaks of the fractions-to-energy
curves at Earth above 1017.8
eV exhibit a Lorentz-factor dependence that appears to be mainly
driven by the UHECR spectral parameters describing the energy range above the “ankle". Despite
the low maximum rigidity found in current astrophysical scenarios interpreting the UHECR data
at the highest energies, our work confirms the relevance of the photo-hadronic interactions in
shaping the observed cosmic-ray flux at Earth.
DOI: https://doi.org/10.22323/1.444.0316
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