Super-knee cosmic rays from interacting supernovae

There is increasing evidence that, in the very late phase of stellar evolution before core collapse, massive stars have winds with large mass loss rates that give rise to a dense circumstellar medium (CSM) surrounding the progenitor star. After core collapse, a shock wave forms when the supernova ejecta interacts with this CSM. In such an interaction, the nuclei in the CSM can undergo diffusive shock acceleration and reach very high energies. We consider such a model, which includes magnetic field amplification from the non-resonant streaming instability, enhancement to the abundance of heavy-ions, and composition-dependent acceleration. Applying this to several supernova subclasses, we find that IIn supernovae can supply a dominant fraction of the observed super-knee cosmic-ray (CR) flux from $\sim{\rm few}\times10^{15},{\rm eV}$ to $\sim{\rm few}\times10^{17},{\rm eV}$ and is consistent with recent LHAASO measurements above the CR knee. This systematic model also explains the increasingly heavy nuclear composition in this energy range.

💡 Research Summary

The paper investigates the origin of cosmic rays (CRs) in the “super‑knee” energy range (approximately 10¹⁵–10¹⁷ eV), a region where the Galactic contribution is still debated. The authors focus on interacting supernovae (ISNe), a class of core‑collapse supernovae that explode into a dense circum‑stellar medium (CSM) produced by intense mass loss in the final stages of massive star evolution. When the fast supernova ejecta collides with this CSM, a forward shock forms. Initially the shock is radiation‑mediated, but as the optical depth drops it becomes collisionless, allowing diffusive shock acceleration (DSA) of particles.

A central element of the model is magnetic‑field amplification by the non‑resonant streaming (NRS) instability (also known as the Bell instability). Accelerated CRs escaping upstream generate a return current that drives rapid growth of magnetic perturbations on scales smaller than the particle Larmor radius. The amplified field saturates at a level proportional to the shock ram pressure, expressed as B_sat ≈