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

We report a multi-resolution search for anisotropies in the arrival directions ofcosmic rays detected at the Pierre Auger Observatory with local zenith angles up to 80◦ andenergies in excess of 4EeV (4 × 1018 eV). This search is conducted by measuring the angularpower spectrum and performing a needlet wavelet analysis in two independent energyranges. Both analyses are complementary since the angular power spectrum achieves a betterperformance in identifying large-scale patterns while the needlet wavelet analysis, consideringthe parameters used in this work, presents a higher efficiency in detecting smaller-scaleanisotropies, potentially providing directional information on any observed anisotropies. Nodeviation from isotropy is observed on any angular scale in the energy range between 4 and8EeV. Above 8EeV, an indication for a dipole moment is captured; while no other deviationfrom isotropy is observed for moments beyond the dipole one. The corresponding p-values obtainedafter accounting for searches blindly performed at several angular scales, are 1.3×10−5in the case of the angular power spectrum, and 2.5×10−3 in the case of the needlet analysis.While these results are consistent with previous reports making use of the same data set, theyprovide extensions of the previous works through the thorough scans of the angular scales.