An Indication of Anisotropy in Arrival Directions of Ultra-high-energy Cosmic Rays through Comparison to the Flux Pattern of Extragalactic Gamma-Ray Sources


A. Aab, Radboud University Nijmegen
P. Abreu, Instituto Superior Técnico
M. Aglietta, Istituto Nazionale di Fisica Nucleare, Sezione di Torino
I. F.M. Albuquerque, Universidade de Sao Paulo - USP
I. Allekotte, Instituto Balseiro
A. Almela, Comision Nacional de Energia Atomica Argentina
J. Alvarez Castillo, Universidad Nacional Autónoma de México
J. Alvarez-Muñiz, Universidad de Santiago de Compostela
G. A. Anastasi, Gran Sasso Science Institute
L. Anchordoqui, Lehman College
B. Andrada, Comision Nacional de Energia Atomica Argentina
S. Andringa, Instituto Superior Técnico
C. Aramo, Istituto Nazionale di Fisica Nucleare, Sezione di Napoli
N. Arsene, Institute for Space Sciences, Bucharest
H. Asorey, Instituto Balseiro
P. Assis, Instituto Superior Técnico
G. Avila, Pierre Auger Observatory
A. M. Badescu, University Politehnica of Bucharest
A. Balaceanu, Horia Hulubei National Institute of Physics and Nuclear Engineering
F. Barbato, Università degli Studi di Napoli Federico II
R. J. Barreira Luz, Instituto Superior Técnico
J. J. Beatty, The Ohio State University
K. H. Becker, Bergische Universitat Wuppertal
J. A. Bellido, The University of Adelaide
C. Berat, Universite Grenoble Alpes
M. E. Bertaina, Istituto Nazionale di Fisica Nucleare, Sezione di Torino
X. Bertou, Instituto Balseiro
P. L. Biermann, Max Planck Institute for Radio Astronomy
J. Biteau, Institut de Physique Nucléaire Orsay
S. G. Blaess, The University of Adelaide
A. Blanco, Instituto Superior Técnico
J. Blazek, Institute of Physics of the Czech Academy of Sciences

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© 2018. The American Astronomical Society. All rights reserved. A new analysis of the data set from the Pierre Auger Observatory provides evidence for anisotropy in the arrival directions of ultra-high-energy cosmic rays on an intermediate angular scale, which is indicative of excess arrivals from strong, nearby sources. The data consist of 5514 events above 20 EeV with zenith angles up to 80 recorded before 2017 April 30. Sky models have been created for two distinct populations of extragalactic gamma-ray emitters: active galactic nuclei from the second catalog of hard Fermi-LAT sources (2FHL) and starburst galaxies from a sample that was examined with Fermi-LAT. Flux-limited samples, which include all types of galaxies from the Swift-BAT and 2MASS surveys, have been investigated for comparison. The sky model of cosmic-ray density constructed using each catalog has two free parameters, the fraction of events correlating with astrophysical objects, and an angular scale characterizing the clustering of cosmic rays around extragalactic sources. A maximum-likelihood ratio test is used to evaluate the best values of these parameters and to quantify the strength of each model by contrast with isotropy. It is found that the starburst model fits the data better than the hypothesis of isotropy with a statistical significance of 4.0σ, the highest value of the test statistic being for energies above 39 EeV. The three alternative models are favored against isotropy with 2.7σ-3.2σ significance. The origin of the indicated deviation from isotropy is examined and prospects for more sensitive future studies are discussed.

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Astrophysical Journal Letters