Is Supernova Remnant Cassiopeia A a PeVatron?

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 $\sim 3.5$ 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.

arXiv comment15 pages, 5 figures, 2 tables, accepted for publication in ApJ

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• TeV cosmic ray nuclei acceleration in shell-type supernova remnants with hard $\gamma$-ray spectra
• Electron Acceleration in Middle-age Shell-type $\gamma$-Ray Supernova Remnants
• New estimation of the nuclear de-excitation line emission from the supernova remnant Cassiopeia A
• Trans-Alfvenic Magnetohydrodynamic Turbulence in the Vicinity of Supernova Remnant Cassiopeia-A Shocks
• SNR G54.1+0.3, a PeVatron candidate unveiled by LHAASO
• An Asymmetrical Model for High Energy Radiation of Cassiopeia A