Correlation of the highest-energy cosmic rays with the positions of nearby active galactic nuclei


J. Abraham, Max Planck Institute for Radio Astronomy
P. Abreu, University of California, Los Angeles
M. Aglietta, Argonne National Laboratory
C. Aguirre, Universität Siegen
D. Allard, Centro de Investigacion y de Estudios Avanzados
I. Allekotte, APC - AstroParticule et Cosmologie
J. Allen, Universidad Nacional de La Plata
P. Allison, Pierre Auger Observatory
J. Alvarez-Muñiz, University of Pennsylvania
M. Ambrosio, Colorado State University
L. Anchordoqui, Instituto de Astronomía y Física del Espacio
S. Andringa, University of California, Los Angeles
A. Anzalone, University of Leeds
C. Aramo, Colorado State University
S. Argirò, University of Oxford
K. Arisaka, Universidad Tecnologica Nacional
E. Armengaud, Centro de Investigacion y de Estudios Avanzados
F. Arneodo, Case Western Reserve University
F. Arqueros, University of Minnesota Twin Cities
T. Asch, Netherlands Foundation for Research in Astronomy
H. Asorey, Institute of Physics of the Czech Academy of Sciences
P. Assis, University of California, Los Angeles
B. S. Atulugama, Pierre Auger Observatory
J. Aublin, Universidad Nacional Autónoma de México
M. Ave, The University of Adelaide
G. Avila, Karlsruhe Institute of Technology, Campus North
T. Bäcker, Jozef Stefan Institute
D. Badagnani, Laboratoire de Physique Subatomique et de Cosmologie de Grenoble
A. F. Barbosa, Università degli Studi di Milano
D. Barnhill, Universidad Tecnologica Nacional
S. L.C. Barroso, Università degli Studi di Torino
P. Bauleo, Centro Atomico Bariloche

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Data collected by the Pierre Auger Observatory provide evidence for anisotropy in the arrival directions of the cosmic rays with the highest-energies, which are correlated with the positions of relatively nearby active galactic nuclei (AGN) [Pierre Auger Collaboration, Science 318 (2007) 938]. The correlation has maximum significance for cosmic rays with energy greater than ∼6 × 1019 eV and AGN at a distance less than ∼75 Mpc. We have confirmed the anisotropy at a confidence level of more than 99% through a test with parameters specified a priori, using an independent data set. The observed correlation is compatible with the hypothesis that cosmic rays with the highest-energies originate from extra-galactic sources close enough so that their flux is not significantly attenuated by interaction with the cosmic background radiation (the Greisen-Zatsepin-Kuz'min effect). The angular scale of the correlation observed is a few degrees, which suggests a predominantly light composition unless the magnetic fields are very weak outside the thin disk of our galaxy. Our present data do not identify AGN as the sources of cosmic rays unambiguously, and other candidate sources which are distributed as nearby AGN are not ruled out. We discuss the prospect of unequivocal identification of individual sources of the highest-energy cosmic rays within a few years of continued operation of the Pierre Auger Observatory. © 2008 Elsevier B.V. All rights reserved.

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Astroparticle Physics