Update on the searches for anisotropies in UHECR arrival directions with the Pierre Auger Observatory and the Telescope Array

Abstract

The origin of ultra-high-energy cosmic rays (UHECRs), particles from outer space with energies E ≥ 1 EeV, is still unknown, though the near-isotropy of their arrival direction distribution excludes a dominant Galactic contribution, and interactions with background photons prevent them from travelling cosmologically large distances. This suggests that their sources must be searched for in nearby galaxy groups and clusters. Deflections by intergalactic and Galactic magnetic fields are expected to hinder such searches but not preclude them altogether. So far, the only anisotropy detected with statistical significance ≥ 5σ is a modulation in right ascension in the data from the Pierre Auger Observatory at E ≥ 8 EeV interpretable as a 7% dipole moment. Various hints for higher-energy, smaller-scale anisotropies have been reported. UHECR arrival direction data from both the Pierre Auger Observatory and the Telescope Array experiment have been searched for anisotropies by a working group with members from both collaborations; combining the two datasets requires a cross-calibration procedure due to the different systematic uncertainties on energy measurements but allows us to perform analyses that are less model-dependent than what can be done with partial sky coverage. We report a significant dipole pointing away from the Galactic Center and a ∼4.6σ anisotropy found when comparing the directions of UHECRs with a catalog of starburst galaxies.