Is Supernova Remnant Cassiopeia A a PeVatron?

Understand

Cassiopeia A, a well-observed young core-collapse supernova remnant (SNR), is considered as one of the best candidates for studying very high-energy particle acceleration up to PeV via the diffusive shock mechanism.

• Recently, MAGIC observations revealed a $\gamma$-ray spectral cutoff at $\sim3.5$ TeV, suggesting that if the TeV $\gamma$-rays have a hadronic origin, SNR Cas A can only accelerate particles to tens of TeV.
• Here, we propose a two-zone emission model for regions associated with the forward (zone 1) and inward/reverse shocks (zone 2).
• Given the low density in zone 1, it dominates the high-frequency radio emission, soft X-ray rim via the synchrotron process and TeV $\gamma$-ray via the inverse Comptonization.