The hunt for PeVatrons as the origin of the most energetic photons observed in the Galaxy

Ultrarelativistic particles called cosmic rays permeate the Milky Way, propagating through the turbulent Galactic magnetic fields. The mechanisms under which these particles increase their energy can be reasonably described by current theories of acceleration and propagation of cosmic rays. There are, however, still many open questions as to how to reach petaelectronvolt (PeV) energies, the maximum energy believed to be attained in our Galaxy, and in which astrophysical sources (dubbed PeVatrons) this ultrahigh-energy acceleration happens. In this Review, we describe the theoretical conditions for plasma acceleration to these energies and the Galactic sources in which these conditions are possible. These theoretical predictions are then compared to the latest experimental results, summarizing the state of the art of our current knowledge of PeVatrons. We finally describe the prospects to keep advancing the understanding of these elusive objects, still unidentified more than 100 years after the discovery of cosmic rays. Cosmic rays at petaelectronvolt energies permeate the Milky Way, but their origin is unknown. This Review Article summarizes the physics required to accelerate particles to these ultrahigh energies, and their potential astrophysical sources (‘PeVatrons’).