Title: Where Do Ultra-High-Energy Cosmic Particles Come From?

Abstract: Recent data from air shower arrays have transformed our understanding of ultra-high-energy cosmic rays. I will briefly review measurements from the Pierre Auger Observatory and the Telescope Array, which reveal anisotropic arrival directions at large and intermediate angular scales, as well as a cosmic-ray composition that transitions from light to intermediate to heavy nuclei at the highest energies.

As these particles traverse the Galactic magnetic field (GMF), they undergo significant angular deflections, which must be accounted for when tracing them back to their sources. The global structure of the GMF can be inferred from Faraday rotation measures and the synchrotron emission of cosmic-ray electrons in the Galaxy. I will introduce the ensemble of GMF models from Unger & Farrar (2024) and discuss how they are used to determine the arrival directions of cosmic rays at the edge of the Galaxy.

Applying these models to two ultra-high-energy events from the Pierre Auger Observatory and the Telescope Array shows that neither of their back-tracked direction aligns with a compelling candidate for a continuous UHECR accelerator. This strengthens the evidence that at least a fraction of the highest-energy events originate from transient sources.