Detecting gamma rays above 100 TeV from an astrophysical source has been widely interpreted as evidence that the source accelerates protons to PeV energies — a “PeVatron.” The logic: photons at these energies must come from pion decay, pions come from proton collisions, and protons energetic enough to produce such pions must be at PeV scales. The hadronic interpretation has shaped the field's understanding of cosmic ray origins.
Curtis-Ginsberg, Hooper, and Vandenbroucke (arXiv:2603.17012) show the logic is flawed. Leptonic models — where electrons, not protons, produce the gamma rays via inverse Compton scattering off background radiation fields — can fully account for the observed emission above 100 TeV from sources including SS 433 and the Galactic Center.
The key is that the photon energy depends on both the electron energy and the target photon energy. Low-energy background photons (infrared, microwave) scattered by multi-TeV electrons can produce 100+ TeV gamma rays without requiring PeV-scale particles of any kind. The electron energies needed are high but not extraordinary — well within the range of known acceleration mechanisms in these environments.
The implication reverses a major observational claim. Sources previously identified as PeVatrons based on their ultrahigh-energy gamma ray emission may not be accelerating protons to PeV at all. The gamma rays are real. The inference about protons is not required by the data. Detecting high-energy photons tells you about the photon production mechanism, not directly about the parent particle species.
The PeVatron label was always an interpretation, not a measurement. The measurement is the photon spectrum. The interpretation requires excluding leptonic models, and the exclusion fails.