Indication of a mass-dependent anisotropy above 1018.7 eV in the hybrid data of the Pierre Auger Observatory

PIERRE AUGER COLLABORATION; A. MARIAZZI

Berlin

Conferencia; 37th International Cosmic Ray Conference (ICRC2021); 2021

We test the hypothesis of an anisotropy laying along the galactic plane which depends on themass of primary cosmic-rays. The sensitivity to primary mass is provided by the depth of showermaximum, 𝑋max, from hybrid events measured at the Pierre Auger Observatory. The 14 years ofavailable data are split into on- and off-plane regions using the galactic latitude of each event toform two distributions in 𝑋max, which are compared using the Anderson-Darling 2-samples test.A scan over a subset of the data is used to select an optimal threshold energy of 1018.7 eV anda galactic latitude splitting at |𝑏| = 30◦, which are then set as a prescription for the remainingdata. With these thresholds, the distribution of 𝑋max from the on-plane region is found to havea 9.1 ± 1.6+2.1 g/cm2 shallower mean and a 5.9 ± 2.1+3.5 g/cm2 narrower width than that of the −2.2 −2.5off-plane region. These differences are as such to indicate that the mean mass of primary particlesarriving from the on-plane region is greater than that of those coming from the off-plane region.Monte-Carlo studies yield a 4.4 𝜎 post-penalization statistical significance for the independentdata. Including the scanned data results in a 4.9+1.4 𝜎 post-penalization statistical significance, −1.5where the uncertainties are of systematic origin. Accounting for systematic uncertainties leads to an indication for anisotropy in mass composition above 1018.7 eV at a confidence level of 3.3 𝜎. The anisotropy is observed independently at each of the four fluorescence telescope sites. Interpretations of possible causes of the observed effect are discussed.