Multi-resolution anisotropy studies of ultrahigh-energy cosmic rays detected at the Pierre Auger Observatory

TL;DR

This study conducts a multi-resolution analysis of ultrahigh-energy cosmic ray arrival directions at the Pierre Auger Observatory, finding no significant anisotropy below 8 EeV and a potential dipole signal above, using complementary methods.

Contribution

It introduces a combined use of angular power spectrum and needlet wavelet analysis for multi-scale anisotropy detection in cosmic rays, extending previous studies with thorough angular scale scans.

Findings

No anisotropy detected below 8 EeV.

Potential dipole anisotropy observed above 8 EeV.

Statistically significant indication of anisotropy at high energies.

Abstract

We report a multi-resolution search for anisotropies in the arrival directions of cosmic rays detected at the Pierre Auger Observatory with local zenith angles up to $80^\circ$ and energies in excess of 4 EeV ($4 \times 10^{18}$ eV). This search is conducted by measuring the angular power spectrum and performing a needlet wavelet analysis in two independent energy ranges. Both analyses are complementary since the angular power spectrum achieves a better performance in identifying large-scale patterns while the needlet wavelet analysis, considering the parameters used in this work, presents a higher efficiency in detecting smaller-scale anisotropies, potentially providing directional information on any observed anisotropies. No deviation from isotropy is observed on any angular scale in the energy range between 4 and 8 EeV. Above 8 EeV, an indication for a dipole moment is captured; while no other deviation from isotropy is observed for moments beyond the dipole one. The corresponding $p$-values obtained after accounting for searches blindly performed at several angular scales, are $1.3 \times 10^{-5}$ in the case of the angular power spectrum, and $2.5 \times 10^{-3}$ in the case of the needlet analysis. While these results are consistent with previous reports making use of the same data set, they provide extensions of the previous works through the thorough scans of the angular scales.