Probing for anisotropies at different angular scales in the arrival directions of the cosmic rays detected at the Pierre Auger Observatory

Abstract

Studies of the arrival-direction distribution of ultra-high-energy cosmic rays are crucial for revealing their origin. In this work, we investigate the distribution of the arrival directions of events detected with the Pierre Auger Observatory over different angular scales. To do this, we adopt a novel method where we use a von Mises--Fisher distribution to model the excesses with respect to isotropic expectations. This method allows us to study the distribution of arrival directions over different angular scales consistently and simultaneously. We consider the events in different energy bins above 4 EeV detected from 1${}^{\text{st}}$ January 2004 to 31${}^{\text{st}}$ December 2022, i.e. during the entire Phase~I of the Pierre Auger Observatory, with a total exposure of $123,000$ km${}^2$ sr yr. The most significant excess in the bins considered is a dipolar modulation in the energy range (8--16) EeV, which is compatible with previous findings of the Pierre Auger Collaboration. A dipolar modulation appears to be present for the higher energies considered as well, and at the highest energies, i.e. above 32 EeV, we find indications of its coexistence with an overdensity of intermediate angular scale, located in the Centaurus region.

arXiv comment19 pages, 7 figures