Mass composition of ultra-high energy cosmic rays from distribution of their arrival directions with the Telescope Array

TL;DR

This study introduces a new method to estimate the mass composition of ultra-high energy cosmic rays by comparing their arrival directions with large-scale structure models, revealing a trend towards heavier elements at higher energies.

Contribution

The paper presents a novel approach that links cosmic ray arrival directions with large-scale structure to infer their mass composition, accounting for magnetic field uncertainties.

Findings

Data suggests a transition to heavier elements at higher energies.

Results are robust against uncertainties in injection spectra and magnetic fields.

Isotropy at energies above 100 EeV indicates a heavy composition regardless of magnetic field strength.

Abstract

We use a new method to estimate the injected mass composition of ultrahigh cosmic rays (UHECRs) at energies higher than 10 EeV. The method is based on comparison of the energy-dependent distribution of cosmic ray arrival directions as measured by the Telescope Array experiment (TA) with that calculated in a given putative model of UHECR under the assumption that sources trace the large-scale structure (LSS) of the Universe. As we report in the companion letter, the TA data show large deflections with respect to the LSS which can be explained, assuming small extra-galactic magnetic fields (EGMF), by an intermediate composition changing to a heavy one (iron) in the highest energy bin. Here we show that these results are robust to uncertainties in UHECR injection spectra, the energy scale of the experiment and galactic magnetic fields (GMF). The assumption of weak EGMF, however, strongly affects this interpretation at all but the highest energies E > 100 EeV, where the remarkable isotropy of the data implies a heavy injected composition even in the case of strong EGMF. This result also holds if UHECR sources are as rare as $2 \times 10^{-5}$ Mpc$^{-3}$, that is the conservative lower limit for the source number density.