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A new analysis of the dataset from the Pierre Auger Observatory providesevidence for anisotropy in the arrival directions of ultra-high-energy cosmicrays on an intermediate angular scale, which is indicative of excess arrivalsfrom strong, nearby sources. The data consist of 5514 events above 20 EeV withzenith angles up to 80 deg recorded before 2017 April 30. Sky models have beencreated 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-BATand 2MASS surveys, have been investigated for comparison. The sky model ofcosmic-ray density constructed using each catalog has two free parameters, thefraction of events correlating with astrophysical objects and an angular scalecharacterizing the clustering of cosmic rays around extragalactic sources. Amaximum-likelihood ratio test is used to evaluate the best values of theseparameters and to quantify the strength of each model by contrast withisotropy. It is found that the starburst model fits the data better than thehypothesis of isotropy with a statistical significance of 4.0 sigma, thehighest value of the test statistic being for energies above 39 EeV. The threealternative models are favored against isotropy with 2.7-3.2 sigmasignificance. The origin of the indicated deviation from isotropy is examinedand prospects for more sensitive future studies are discussed.

the astrophysical journal. letters

Indication of anisotropy in arrival directions of ultra-high-energy  cosmic rays through comparison to the flux pattern of extragalactic gamma-ray  sources