Isotropy of cosmic rays beyond $10^{20}$ eV favors their heavy mass composition

TL;DR

This study analyzes the isotropy of ultra-high energy cosmic rays above 10 EeV, suggesting their composition is heavy at the highest energies, influenced by magnetic fields and cosmic structure.

Contribution

It provides the first estimation of the energy-dependent mass composition of UHECRs based on sky distribution and large-scale structure comparisons.

Findings

Composition is heavy at >100 EeV even with strong extragalactic magnetic fields.

At ~10 EeV, composition appears lighter, influenced by magnetic field assumptions.

Results are robust against uncertainties in galactic magnetic field modeling.

Abstract

We report an estimation of the injected mass composition of ultra-high energy cosmic rays (UHECRs) at energies higher than 10 EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extra-galactic magnetic fields the results are consistent with a relatively heavy injected composition at E ~ 10 EeV that becomes lighter up to E ~ 100 EeV, while the composition at E > 100 EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extra-galactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.