Silvia Mollerach and the
Pierre Auger Collaboration

Pierre Auger Observatory, av. San Mart'in Norte
304, (5613) Malargüe, Argentina CONICET, Centro Atómico
Bariloche, 8400 Bariloche, Rio Negro, Argentina

**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.

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