Indication of anisotropy in arrival directions of ultra-high-energy
  cosmic rays through comparison to the flux pattern of extragalactic gamma-ray
  sources
  A new analysis of the dataset 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 deg 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 4.0 sigma, the
highest value of the test statistic being for energies above 39 EeV. The three
alternative models are favored against isotropy with 2.7-3.2 sigma
significance. The origin of the indicated deviation from isotropy is examined
and prospects for more sensitive future studies are discussed.
