How to turn a supernova into a PeVatron

Context. It is important to determine which Galactic cosmic-ray (CR) sources have the ability to accelerate particles to the knee of the CR spectrum at a few peta-electronvolt (PeV). In particular, we need to consider whether supernova remnants (SNRs) could also be contributors to this process. Current models for particle acceleration in very young remnants assume the circumstellar material (CSM) consists of smooth, freely expanding winds. There is strong evidence that some supernovae (SNs) expand into much denser CSM, including dense shells ejected by eruptions shortly before explosion. Aims. We investigate the effects of dense circumstellar shells on particle acceleration in SN shocks during the first few years post-explosion to quantify whether SNs resulting from interactions may act as PeVatrons. Methods. We used the PION code to model the CSM around luminous blue variables (LBVs) after having a brief episode with a mass-loss rate of up to Ṁ = 2 M ⊙ yr. Consequently, we performed spherically symmetric 1D simulations using our time-dependent acceleration code RATP A C, where we simultaneously solved the transport equations for CRs, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit. Results. We find that the interaction with the circumstellar shells can significantly boost the maximum energy by enhancing particle escape during the onset of the shock-shell interaction, followed by the reacceleration of the shock propagating into a medium with a preamplified field. Early interactions boost the maximum energy to a greater degree and interactions within the first five months after explosion can increase E max to levels over 1 PeV.