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

Abstract

The chemical composition of the highest-energy cosmic rays, namely the atomic number $Z$ of rays with energies $E\gtrsim 40\mathrm{EeV}$, 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 $f_{\mathrm{Fe}}\ge \mathcal{O}(10\%)$ 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 $f_{\mathrm{H}}\approx 10\%$ 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.

arXiv comment9 pages, 3 figures + appendix. Version matching publication at journal

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