Honing cross-correlation tools for inference on ultra-high-energy cosmic-ray composition

TL;DR

This paper introduces an improved cross-correlation method to analyze ultra-high-energy cosmic-ray anisotropies, enabling better constraints on cosmic-ray composition, particularly iron fractions, by leveraging harmonic multipole sensitivities.

Contribution

The work presents a novel test statistic for cross-correlation analysis that enhances sensitivity to harmonic multipoles, improving composition constraints over previous methods.

Findings

Can exclude iron fractions above ~10% at 2σ level

Achieves smaller hydrogen fractions below 10% with Galactic Centre masking

Method improves sensitivity by a factor of a few over previous approaches

Abstract

The chemical composition of the highest-energy cosmic rays, namely the atomic number $Z$ of rays with energies $E\gtrsim40~\mathrm{EeV}$, remains to date largely unknown. Some information on the composition can be inferred from the deflections that charged ultra-high-energy cosmic rays experience while they traverse intervening magnetic fields. Indeed, such deflections distort and suppress the original anisotropy in the cosmic ray arrival directions; thus, given a source model, a measure of the anisotropy is also a measurement of the deflections, which in turn informs us on the chemical composition. In this work, we show that, by quantifying ultra-high-energy cosmic ray anisotropies through the angular cross-correlation between cosmic rays and galaxies, we would be able to exclude iron fractions $f_{\rm Fe}\geq{\cal O}(10\%)$ assuming a fiducial hydrogen map at $2\,\sigma$ level, and even smaller fractions in the reverse case of hydrogen on an iron map, going well below $f_{\rm H}\approx10\%$ when we mask the Galactic Centre up to latitudes of $40^\circ$. This is an improvement of a factor of a few compared to our previous method, and is mostly ascribable to a new test statistics which is sensitive to each harmonic multipole individually. Our method can be applied to real data as an independent test of the recent claim that current cosmic-ray data can not be reproduced by any existing model of the Galactic magnetic field, as well as an additional handle to compare any realistic, competing, data-driven composition models.