Large scale distribution of ultra high energy cosmic rays detected at the Pierre Auger Observatory with zenith angles up to 80$^\circ$

Abstract: We present the results of an analysis of the large angular scale distribution of the arrival directions of cosmic rays with energy above 4 EeV detected at the Pierre Auger Observatory including for the first time events with zenith angle between $60^\circ$ and $80^\circ$. We perform two Rayleigh analyses, one in the right ascension and one in the azimuth angle distributions, that are sensitive to modulations in right ascension and declination, respectively. The largest departure from isotropy appears in the $E > 8$ EeV energy bin, with an amplitude for the first harmonic in right ascension $r_1^\alpha =(4.4 \pm 1.0){\times}10^{-2}$, that has a chance probability $P(\ge r_1^\alpha)=6.4{\times}10^{-5}$, reinforcing the hint previously reported with vertical events alone.

• The paper reports a significant first harmonic in right ascension for energies above 8 EeV, with an amplitude of (4.4 ± 1.0) × 10⁻².
• It employs Rayleigh methods on nearly a decade of data, incorporating events up to 80° zenith to enhance sky coverage.
• Implications suggest that detected dipole anisotropies point to extragalactic sources in regions of higher matter density.

Large Scale Anisotropies of Ultra-High Energy Cosmic Rays

The paper conducts an in-depth analysis of the angular distribution of ultra-high energy cosmic rays (UHECRs) using data from the Pierre Auger Observatory. This study extends previous analyses by incorporating events detected with zenith angles up to 80 degrees. The research aims to discern potential anisotropies in the arrival directions of cosmic rays, which could provide insights into their origin and the transition from galactic to extragalactic sources.

Methodology

The Pierre Auger Observatory, with its extensive array of surface detectors and fluorescence telescopes, serves as a pivotal apparatus in this study. The dataset spans nearly a decade, from January 2004 to December 2013, and includes events with zenith angles up to 80 degrees, enhancing coverage of the sky to 85%. The analysis employed Rayleigh methods in both right ascension and azimuthal distributions, which are sensitive to flux modulations along right ascension and declination, respectively.

To ensure robustness against systematic biases, the study meticulously accounts for variations in detector efficiency, atmospheric conditions, and geomagnetic effects. Energy thresholds of 4 EeV were set to ensure full detector efficiency, with a focus on two distinct energy bands: 4 to 8 EeV and above 8 EeV.

Findings

The analysis revealed several notable results:

1. Right Ascension Analysis: For energies exceeding 8 EeV, a prominent first harmonic was detected in right ascension, with an amplitude of $r_1^\alpha = (4.4 \pm 1.0) \times 10^{-2}$ and a chance probability of $6.4 \times 10^{-5}$. This indicates a significant deviation from isotropy, consistent with prior findings from the Pierre Auger Observatory.
2. Azimuth Analysis: The first harmonic in azimuth for both energy bins presented larger amplitudes than expected for an isotropic distribution, though these were only mildly significant statistically. This suggests potential north-south asymmetries in the cosmic ray flux.
3. Overall Dipole and Quadrupole Patterns: The analysis of both dipole and quadrupole contributions to the UHECR distribution was performed. For energies above 8 EeV, the reconstructed dipole points toward a declination of approximately -39 degrees, with a total amplitude of $d = 0.073 \pm 0.015$. Quadrupole contributions, while evaluated, did not exhibit conclusive significance.

Implications

These findings corroborate the hypothesis of anisotropic UHECR distributions potentially linked to the large-scale structure of matter in the universe. A confirmed dipolar pattern at the highest energies suggests extragalactic sources, aligned in regions of higher matter density, could be the primary contributors to UHE cosmic rays. Continued study could elucidate the interaction of cosmic rays with cosmic magnetic fields and refine models of their propagation.

Future Directions

The insights garnered from this expanded zenith angle range open avenues for further research into the large-scale cosmic ray distribution. Future work could focus on refining the directional mapping of cosmic ray arrival directions, employing longer observation periods to improve the statistical significance of detected anisotropies. Such research could also explore the interaction dynamics between cosmic rays and the galactic and intergalactic environments.

Overall, the study offers a substantial progression in understanding UHECR anisotropy, providing pivotal data for the astrophysics community in unravelling the mysteries of cosmic ray origins and propagation.