| Abstract: | Although their astrophysical sources remain a mystery, there is a lot to learn from the study of the transition between the Galactic cosmic rays, which dominate the flux up to energies around 10^{18} eV, and the cosmic rays of extragalactic origin, at the highest energies. In the past few years new measurements (brought by experiments such as KASCADE-Grande, the Pierre Auger Observatory and Telescope Array) have radically improved our knowledge of the highest energy domain of the cosmic-ray spectrum. Surprisingly, recent Auger analyses suggest a composition getting gradually heavier above a few 10^18 eV. In this talk, I will show that the observational data across the Galactic to extragalactic cosmic-ray transition up to the highest energies can be described with minimal assumptions within a generic model putting together only one Galactic component and one extragalactic component. The key to the success of this model is that the protons emitted by the extragalactic sources have a different spectrum from the other types of nuclei, as we recently showed to be expected in highly magnetized and photon-rich environments, such as the relativistic plasma jets associated with GRBs or AGNs. I will finally discuss multi-messenger constraints brought by neutrino and gamma-ray experiments, and show that our mixed-composition scenario is compatible with both the most recent Fermi-LAT measurements and with current IceCube limits.
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