Search for clustering of ultra high energy cosmic rays from the Pierre Auger Observatory

TL;DR

This study searches for clustering in ultra high energy cosmic rays detected by the Pierre Auger Observatory, finding a potential excess of pairs at energies above 57 EeV that may indicate non-random distribution.

Contribution

It provides the first detailed analysis of clustering among the highest energy cosmic rays using autocorrelation functions with new statistical significance.

Findings

An excess of cosmic ray pairs at energies >57 EeV and angles 9-22 degrees.

Chance probability of this excess being random is about 2%.

Correlation observed at the same energy threshold as with nearby astrophysical objects.

Abstract

We present the results of a search for clustering among the highest energy events detected by the surface detector of the Pierre Auger Observatory between 1 January 2004 and 31 August 2007. We analyse the autocorrelation function, in which the number of pairs with angular separation within a given angle is compared with the expectation from an isotropic distribution. Performing a scan in energy above 30 EeV and in angles smaller than 30 degrees, the most significant excess of pairs appears for E > 57 EeV and for a wide range of separation angles, between 9 and 22 degrees. An excess like this has a chance probability of about 2% to arise from an isotropic distribution and appears at the same energy threshold at which the Pierre Auger Observatory has reported a correlation of the arrival directions of cosmic rays with nearby astrophysical objects.