Figure 1: Flux above 8 EeV in Equatorial coordinates. The position of the Galactic Center is shown with a star and the Galactic Plane is indicated with a dashed line.
The Pierre Auger Collaboration has published an update in the Astrophysical Journal on large-angular-scale analyses using data from phase 1 of the Auger Observatory, which was the phase prior to AugerPrime, the upgrade of the Observatory. Studying the arrival directions of ultra-high energy cosmic rays is key to gaining insight into their possible sources, a mystery since the first detection of such energetic particles more than 60 years ago. Regarding large-angular scales, a dipolar modulation in right ascension at energies above 8 EeV (1 EeV = 1018 eV) has been determined with a significance exceeding the 5-sigma discovery level (a result published in Science in 2017). The direction of this dipole, lying ~115 degrees away from the Galactic Center, suggests an extragalactic origin for the cosmic rays above this energy threshold.
Figure 2: Map with the directions of the 3D dipole for different energy bins, in Galactic coordinates
In this work, the statistics of 19 years of data from the Pierre Auger Observatory are analyzed, representing an increase of ~55% and ~30% compared to previous studies. The 3D dipole amplitude and direction are reconstructed above 4 EeV in four energy bins. Besides the established dipolar anisotropy in right ascension above 8 EeV, the Fourier amplitude of the 8 to 16 EeV energy bin now also exceeds the 5-sigma discovery level. Additionally, the results for the angular power spectrum are shown, demonstrating no other statistically significant multipoles. The results for the equatorial dipole component down to 0.03 EeV are presented, using for the first time a data set obtained with a trigger that has been optimized for lower energies. Finally, model predictions are discussed and compared with observations, based on two source emission scenarios obtained in the combined fit of spectrum and composition published by the Auger Collaboration.
Figure 3: Equatorial dipole amplitude (left) and phase (right) for the energy bins for the analysis with events with energies above 0.03 EeV.
Related paper:
Large-scale cosmic ray anisotropies with 19 years of data from the Pierre Auger Observatory
The Pierre Auger Collaboration, ApJ 976(2024)48
[arxiv.org/abs/2408.05292] [doi: 10.3847/1538-4357/ad843b]
