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

TL;DR

This paper investigates cosmic ray arrival directions above the ankle energy, confirming large-scale anisotropy detection and developing a method suitable for analyzing anisotropies at lower energies with complex detection efficiencies.

Contribution

It introduces a likelihood-based analysis method that accounts for observatory acceptance variations, confirming large-scale anisotropy and enabling future studies at lower energies.

Findings

Confirmed a dipole anisotropy with 5.3% amplitude

No evidence found for medium- or small-scale anisotropies

Method is suitable for future anisotropy analyses below 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$\sigma$. 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 $\pm$ 1.3 percent and right ascension angle of 103 $\pm$ 15 degrees, 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 the 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.