Clustering properties of ultrahigh energy cosmic rays and the search for their astrophysical sources

TL;DR

This paper investigates the clustering of ultrahigh energy cosmic rays and proposes a robust method to identify their astrophysical sources, suggesting they may originate from a strongly clustered subset of galaxies or active galactic nuclei.

Contribution

It introduces a global auto-correlation comparison method that is less sensitive to magnetic deflections for identifying UHECR sources, with applications to various source catalogs.

Findings

Data suggests UHECR sources are likely a clustered subset of galaxies or AGNs.

The auto-correlation method effectively discriminates potential sources.

Forecasts for source distribution scenarios are provided for upcoming observations.

Abstract

The arrival directions of ultrahigh energy cosmic rays (UHECRs) may show anisotropies on all scales, from just above the experimental angular resolution up to medium scales and dipole anisotropies. We find that a global comparison of the two-point auto-correlation function of the data with the one of catalogues of potential sources is a powerful diagnostic tool. In particular, this method is far less sensitive to unknown deflections in magnetic fields than cross-correlation studies while keeping a strong discrimination power among source candidates. We illustrate these advantages by considering ordinary galaxies, gamma ray bursts and active galactic nuclei as possible sources. Already the sparse publicly available data suggest that the sources of UHECRs may be a strongly clustered sub-sample of galaxies or of active galactic nuclei. We present forecasts for various cases of source distributions which can be checked soon by the Pierre Auger Observatory.