The research investigates how supernovae exploding into dense circumstellar environments, specifically those with dense shells of material, can potentially accelerate particles to energies of a few PeV, thus acting as "PeVatrons" and contributing to the "knee" feature in the cosmic ray spectrum.

Supernova remnants (SNRs) have long been considered prime candidates for the sources of Galactic Cosmic Rays (CRs) up to energies of a few PeV. However, despite decades of gamma-ray astronomy, there hasn't been clear observational proof that standard SNR models can accelerate particles beyond approximately 100 TeV. Young SNRs like Tycho and Casiopeia A, initially expected to be strong accelerators, show even lower cutoff energies.

The presented study explores a different scenario: supernovae that expand into **much denser circumstellar material**, including dense shells ejected by the progenitor star shortly before explosion. These dense shells are thought to be present around massive stars like Luminous Blue Variables (LBVs), which can undergo brief episodes of very high mass-loss rates (up to 1 M⊙/yr). Type IIn supernovae, associated with LBVs, make up about 5% of core-collapse supernovae.

The researchers used spherically symmetric 1D simulations with their time-dependent acceleration code called **RATPaC** (Radiation Acceleration Transport Parallel Code). This code simultaneously solves the transport equations for cosmic rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit. Unlike models that assume a steady state for magnetic turbulence, RATPaC accounts for the time needed for turbulence to build up, which often leads to lower maximum energies in standard scenarios.

**The key finding is that the interaction of the supernova shock front with these dense circumstellar shells can significantly boost the maximum energy** of the accelerated particles.

Specifically, the simulations show that:

* **Interactions with shells that occur earlier post-explosion lead to a greater increase in maximum energy (Emax)**.

* If the interaction happens within the first **5 months (approximately 140 days)** after the explosion, the **Emax can increase to more than 1 PeV**.

* For very early interactions, around **0.1 years**, Emax can even surpass **10 PeV**.

This significant energy boost is attributed to several mechanisms during and after the shock-shell interaction:

1. **Enhanced Particle Escape:** The shock slows down considerably during the interaction with the dense shell, which temporarily enhances the "precursor scale" (the region upstream where particles diffuse back towards the shock, given by D(E)/v_shock). This increased scale provides more time for turbulence to grow. Enhanced particle escape also occurs during the onset of the interaction, boosting the CR current.

2. **Reacceleration in a Pre-amplified Field:** After passing through the shell, the shock propagates into a medium where the magnetic field has been pre-amplified by escaping cosmic rays during the interaction phase. The shock accelerating into this region with an enhanced field boosts Emax.

3. **Interaction with Reflected Shocks:** The collision with the dense shell creates reflected shocks. These can catch up with and interact with the forward shock from behind, leading to sharp increases in the forward shock's velocity and slightly boosting Emax.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: ESO/L. Calçada