Probing the universe at the highest energies with the Pierre Auger Observatory

Abstract

The Pierre Auger Observatory is currently the largest detector of cosmic rays of ultrahigh energies (from ∼1017 to ∼1020 eV). The size of its accumulated exposure and the hybrid concept of the detector have provided measurements of unprecedented precision on the cosmic ray energy spectrum, mass composition, and anisotropy searches. There is an ankle in the energy spectrum at ∼4 EeV and a steepening at the end of the energy spectrum (above ∼40 EeV). The mass composition around the ankle is mixed and is uncertain at the highest energies due to the lack of statistics and high systematic uncertainties coming from predictions of different models of hadronic interactions. These models have, moreover, serious problems in describing the muon component of air showers. Still, the mean mass number of cosmic rays reaches its lowest value at energy ∼2 EeV and increases gradually above this energy. A dipole in the arrival directions is observed for energies above 8 EeV at more than 5σ significance level. It points ∼125° from the Galactic center, indicating the extra‐Galactic origin of these cosmic rays. The correlation of the arrival directions with starburst galaxies disfavors the isotropy hypothesis at energies above ∼40 EeV at the 4σ level without penalization for the catalog scan. The Observatory is currently being upgraded to improve its sensitivity to the mass of cosmic rays at the highest energies, which is crucial to constrain their sources.