Searching for All-scale Anisotropies in the Arrival Directions of Cosmic Rays above the Ankle

Abstract

The Pierre Auger Observatory has recently reported the detection of a dipole anisotropy in the arrival directions of cosmic rays above 8 EeV with a post-trial significance of more than 5.2 σ . This observation has profound consequences for the distribution and composition of candidate sources of cosmic rays above the ankle (3–5 EeV). In this paper we search for the presence of anisotropies on all angular scales in public Auger data. The analysis follows a likelihood-based reconstruction method that automatically accounts for variations in the observatory’s angular acceptance and background rate. Our best-fit dipole anisotropy in the equatorial plane has an amplitude of 5.3% ± 1.3% and R.A. angle of 103° ± 15°, consistent with the results of the Pierre Auger Collaboration. We do not find evidence for the presence of medium- or small-scale anisotropies. The method outlined in this paper is well-suited for future analyses of cosmic ray anisotropies below the ankle, where cosmic ray detection in surface arrays is not fully efficient and dominated by systematic uncertainties.