Large-scale anisotropies above 0.03 EeV measured by the Pierre Auger Observatory

Abstract

We present analyses of the large-scale anisotropies observed by the Pierre Auger Observatory over more than three decades in energy, considering data since 1 January 2004 up to 31 August 2018. For E ≥ 4 EeV, for which the array with 1500 m separation between detectors is fully efficient, we obtain the dipolar and quadrupolar amplitudes through a combined Fourier analysis of the distribution of events in right ascension and azimuth. A dipolar modulation with an amplitude d = 6.6+1.2-0.8% and pointing ∼ 125◦ away from the Galactic center is observed above 8 EeV. The dipole amplitude shows an indication of an increase with energy above 4 EeV and the quadrupolar components turn out to be not significant. Astrophysical scenarios that could account for these results are briefly discussed. We also extend the study of the equatorial component of the dipolar modulation down to ∼ 0.03 EeV. In the regime in which the efficiencies are small, we use the East-West method, which is largely insensitive to systematic effects. Finally, for the lowest energies we use the data from a subarray with 750 m separation between detectors. The results suggest a change in the phase of the equatorial dipole from values pointing close to the right ascension of the Galactic center below EeV energies towards values indicative of an extragalactic origin for the dipolar anisotropy above a few EeV.