Large-scale and multipolar anisotropies of cosmic rays detected at the Pierre Auger Observatory with energies above 4 EeV

Abstract

More than half a century after the discovery of ultra-high energy cosmic rays (UHECRs), their origin is still an open question. The study of anisotropies in the arrival directions of such particles is an essential ingredient to solve this puzzle. We update our previous analysis of large-scale anisotropies observed by the Pierre Auger Observatory using the latest data collected before the AugerPrime upgrade. We select events with zenith angles up to 80 degrees, implying a sky coverage of 85%, and energies above 4 EeV, for which the surface detector of the Observatory is fully efficient. Dipolar and quadrupolar amplitudes are evaluated through a combined Fourier analysis of the event count rate in right ascension and azimuth. The analysis is performed in three energy bins with boundaries at 4, 8, 16 and 32 EeV and two additional cumulative bins with energies above 8 and 32 EeV. The most significant signal is a dipolar modulation in right ascension for energies above 8 EeV, as previously reported, with statistical significance of 6.6 . Additionally, we report the measurements of the angular power spectrum for the same energy bins with the same dataset.

Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers

“…Using relaxed selection criteria, the angular distribution of UHECR arrival directions has been studied with unprecedented statistics at the Pierre Auger Observatory. A modulation in the right ascension distribution of events with E ≥ 8 EeV first discovered in 2017 [37] has now reached a statistical significance of 6.6σ [51]. It can be interpreted as a dipole moment of amplitude d = (5.0 ± 0.7) × (E/10 EeV) 0.98±0.…”

“…The Auger and TA Collaborations have made significant progress both with their individual data sets and in joint efforts. An extremely important milestone achieved by the Pierre Auger Collaboration is the > 6σ measurement of a large scale dipole anisotropy above 8 EeV [37], with an amplitude of d = 0.073 +0.011 −0.009 obtained with the latest published data set [51]. The map showing the cosmic-ray flux, smoothed with a 45 • top-hat function, is illustrated on the upper left panel of Fig.…”

“…The contribution of Auger and TA to the understanding of UHECRs' nature is certainly very remarkable and will be significantly improved in the next decade via the respective upgrades to the observatories. At ultra high energies, cosmic-rays are of extra-galactic origin [51] and there are strong hints of correlation of the arrival directions at intermediate angular scales with some known catalogue of sources, see Sec. 2.4 [39] and of a clustering of events in the northern hemisphere [38].…”

Cosmic Ray Indirect Rapporteur Report (37th ICRC )

“…The dipole anisotropy above 8 EeV with 17 years of data from the Pierre Auger Observatory reaches 6.6 (Rogerio de Almeida) [75]. The data above 32 EeV shows evidence for correlation with catalogues of nearby sources (Jonathan Biteau) [76], with the strongest excess found in the data above 38 EeV in the direction of starburst galaxies.…”

Sensitivity of Cross-Correlation Studies by Using Xmax Information

“…Ultra-high energy cosmic rays (UHECRs), particles with energies above 10 18 eV (1 EeV = 10 18 eV), are messengers of astrophysical sources carrying fundamental information about the universe. Although these high energy particles have been detected since the 1960s, their sources, chemical composition, mechanism of acceleration at the sources, and how they propagate to Earth are still open questions [1][2][3][4][5][6]. The study of anisotropies through the cross-correlation between the arrival directions of ultra high energy cosmic rays (UHECRs) and the positions of astrophysical objects is an important ingredient to determine the sources of these cosmic particles [7].…”