Ultra-high energy cosmic rays detected by the Pierre Auger observatory

First direct evidence, and its implications, that a subset originate in nearby radiogalaxies

Astronomy Department, Universidad de Concepción, Concepción, Chile e-mail: nagar@astro-udec.cl;jmatulich@udec.cl

Received: 1 May 2008
Accepted: 16 June 2008

Abstract

Context. The Pierre Auger Collaboration has reported 27 Ultra-High Energy Cosmic Ray Events (UHECRs) with energies above  eV (56 EeV) and well determined arrival directions as of 2007 August 31. They find that the arrival directions of these UHECRs are not isotropic, but instead appear correlated with the positions of nearby Active Galactic Nuclei (AGNs) from the catalog of Véron-Cetty & Véron.

Aims. Our aim was to determine the sources of these UHECRs by comparing their arrival directions with more complete and/or comprehensive astronomical source catalogs.

Methods. We have cross correlated the arrival directions of the  UHECRs with the positions of supernovae, radio supernovae, galaxies, active galaxies, radiogalaxies, and clusters of galaxies, all at distances within ~200 Mpc.

Results. Four (eight) of the 27 UHECRs with energy greater than 56 EeV detected by the Pierre Auger Observatory have arrival directions within 1.5° (3.5°) of the extended (≥180 kpc) radio structures of nearby radiogalaxies or the single nearby BL Lac with extended radio structure. Conversely the radio structures of three (six) of all ten nearest extended radiogalaxies are within 1.5° (3.5°) of a UHECR; three of the remaining four radiogalaxies are in directions with lower exposure times. This correlation between nearby extended radiogalaxies and a subset of UHECRs is significant at the 99.9% level. For the remaining ~20 UHECRs, an isotropic distribution cannot be ruled out at high significance. The correlation found by the Auger Collaboration between the 27 UHECRs and AGNs in the Véron-Cetty & Véron catalog at  Mpc has a much lower significance when one considers only the ~20 UHECRs not “matched” to nearby extended radiogalaxies. No clear correlation is seen between UHECRs and supernovae, supernova remnants, nearby galaxies, or nearby groups and clusters of galaxies.

Conclusions. Nearby extended radiogalaxies are the most likely source of at least some  UHECRs detected by the Pierre Auger Observatory. The remaining  UHECRs are not inconsistent with an isotropic distribution; their correlation to nearby AGNs is much less significant than earlier estimated. This is the first direct observational proof that radio galaxies are a significant source of UHECRs. The primary difference between the UHECR detections at the Pierre Auger Observatory and previous experiments, e.g. AGASA, may thus primarily be that the Southern Hemisphere is more privileged with respect to nearby radiogalaxies with highly extended radio jets and lobes.