Direct observational constraint on upstream magnetic field amplification reveals the particle acceleration limit in a supernova remnant

Abstract The origin of Galactic cosmic rays up to the knee energy (PeV) remains a long-standing open question in astrophysics. Supernova remnant shocks are leading candidates for accelerating particles to PeV energies, yet this requires strong magnetic field amplification in the upstream region to confine the highest energy particles. Despite its importance, the upstream magnetic field strength has long lacked direct observational constraints. Here we report the observation of numerous synchrotron X-ray hotspots extending into the upstream of the supernova remnant RX J1713.7−3946 in archival Chandra data. These hotspots show spatial and temporal properties consistent with synchrotron emission from secondary electrons produced by hadronic interactions between accelerated protons and dense molecular clumps. Using the spatial extent of these upstream hotspots as a physical scale, we estimate the proton diffusion length to be ~1pc. This measurement provides a constraint on the upstream magnetic field strength of ~10 μ G, which is consistent with recent simulations of Bell-instability-driven magnetic field amplification. Our results indicate that the current amplification is insufficient to reach PeV energies, explaining the observed TeV gamma-ray spectral cutoff and providing a diagnostic for assessing PeVatron capability in stellar shocks.