# Is Supernova Remnant Cassiopeia A a PeVatron?

## TL;DR

This paper proposes a two-zone emission model for Cassiopeia A, suggesting it can accelerate particles to PeV energies, challenging previous interpretations of gamma-ray cutoffs and highlighting the potential for future high-energy observations.

## Contribution

The study introduces a novel two-zone emission model for Cas A, explaining multi-wavelength emissions and proposing it remains a PeVatron despite recent gamma-ray spectral cutoffs.

## Key findings

- Zone 1 dominates TeV gamma-ray emission via hadronic processes.
- Zone 2's electron distribution extends to higher energies, explaining radio and X-ray emissions.
- No high-energy cutoff in proton distributions suggests Cas A can still accelerate particles to PeV energies.

## Abstract

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. With a relatively softer particle distribution and a higher cut-off energy for electrons, emissions from zone 2 dominate the low-frequency radio, hard X-ray via the synchrotron process and GeV $\gamma$-ray via hadronic processes. There is no evidence for high-energy cutoffs in the proton distributions implying that Cas A can still be a PeVatron. Hadronic processes from zone 1 dominate very high-energy gamma-ray emission. Future observations in hundreds of TeV range can test this model.