Our review on open questions in ultra-high-energy cosmic rays is now out:
Open Questions in Cosmic-Ray Research at Ultrahigh Energies, 1903.06714
We wrote this review following the 2018 workshop at MIAPP: “The High Energy Universe: Gamma Ray, Neutrino, and Cosmic Ray Astronomy”. The review will be published in Frontiers.
The figure above is a Hillas plot by Foteini Oikonomou, Fig. 10 in the paper.