Particle Acceleration in Cassiopeia A Revealed by Broadband High-Energy Spectrum

TL;DR

This study revisits Cassiopeia A's broadband spectrum, modeling particle acceleration to explain gamma-ray and X-ray emissions, and suggests Cas A-like supernova remnants could be Galactic PeVatrons.

Contribution

It introduces a shell-plus-jet asymmetric model to explain multi-wavelength emissions and constrains proton acceleration and PeV cosmic ray production in Cas A.

Findings

Gamma-ray spectrum explained by proton-proton collisions and inverse Compton scattering.

Synchrotron emission from a jet accounts for hard X-ray emission up to 220 keV.

Protons in the jet could reach energies of 5×10^47 erg, supporting Cas A as a potential PeVatron.

Abstract

Recently, the GeV--sub-PeV spectrum of supernova remnant (SNR) Cassiopeia A (Cas A), one of the youngest and most well-studied SNRs in our Galaxy, has been updated by observations of Fermi-LAT and LHAASO. We revisit Cas A with our previous shell-plus-jet asymmetric model and investigate its particle acceleration ability. The broadband fitting results suggest that the double-peaked gamma-ray spectrum can be well attributed to proton-proton (PP) collisions and inverse Compton scattering within the SNR shell, while the synchrotron emission from a jet component with velocity of $\sim0.1c$ can account for the hard X-ray emission up to 220 keV. Furthermore, the PP collisions in the jet can produce a sub-PeV emission, but constrained by the LHAASO-KM2A limit to a flux below $\sim 1\times10^{-14}\rm erg/(cm^2s)$ at 100 TeV. The energy of accelerated protons in the jet of Cas A could be up to $5\times10^{47}$ erg, which, assuming that the PeV cosmic ray distribution is clumpy in the Galaxy with the clump size comparable to the thickness of the Galactic plane, derives a proton flux consistent with the observed one at 1 PeV, implying that the Cas A-like SNRs can still be PeVatrons in the Galaxy. It is encouraging for LHAASO and future telescopes to detect or constrain Cas A spectrum above 100-TeV more precisely.