Astrophysical interpretation of the medium scale clustering in the ultra-high energy sky

TL;DR

This study analyzes the clustering of ultra-high energy cosmic rays and compares it with local universe galaxy distributions, suggesting magnetic fields and heavy nuclei influence their propagation.

Contribution

It provides the first detailed comparison of UHECR clustering with local galaxy structures and discusses the impact of magnetic deflections on these correlations.

Findings

Clustering matches local universe structures within z<0.02

No significant cross-correlation found overall

Magnetic fields and heavy nuclei likely influence UHECRs

Abstract

We compare the clustering properties of the combined dataset of ultra-high energy cosmic rays events, reported by the AGASA, HiRes, Yakutsk and Sugar collaborations, with a catalogue of galaxies of the local universe (redshift z<~0.06). We find that the data reproduce particularly well the clustering properties of the nearby universe within z <~0.02. There is no statistically significant cross-correlation between data and structures, although intriguingly the nominal cross-correlation chance probability drops from ~50% to ~10% using the catalogue with a smaller horizon. Also, we discuss the impact on the robustness of the results of deflections in some galactic magnetic field models used in the literature. These results suggest a relevant role of magnetic fields (possibly extragalactic ones, too) and/or possibly some heavy nuclei fraction in the UHECRs. The importance of a confirmation of these hints by Auger data is emphasized.