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

Bo-Tao Li, Wei Wang, Zhuo Li

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.

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

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 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 at 100 TeV. The energy of accelerated protons in the jet of Cas A could be up to 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.

Paper Structure

This paper contains 10 sections, 16 equations, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Broadband spectrum data
  3. Radiation model
  4. Fitting results
  5. Discussion
  6. Model parameter uncertainties
  7. Comparison with previous studies
  8. Cas A-like SNRs as PeVatrons?
  9. Conclusion
  10. Time scales

Figures (6)

  • Figure 1: Upper panel: The broadband spectrum of Cas A, $E^2dN/dE$ versus energy. Observational data are taken from vinyaikin2014frequency (radio), arnaud2016planck (Planck), de2016dust (IRAC), maeda2009suzaku (Suzaku), wang2016hard (INTEGRAL-IBIS), ahnen2017cut (MAGIC), abeysekara2020evidencehumensky2008veritas (VERITAS), and cao2025broadband (Fermi-LAT and LHAASO). Lower panel: radio spectrum, flux density versus frequency, fitted by power law with free-free absorption.
  • Figure 2: Timescales related to particle acceleration and radiation processes as a function of particle energy in model M3. The red and green lines represent timescales for the shell and jet zones, respectively, while black lines represent those shared timescales.
  • Figure 3: Spectral fit with model M1. Upper panel: broadband spectrum and fitting results and relative error. Lower left panel: hard X-ray zoom-in. Lower right panel: gamma-ray zoom-in.
  • Figure 4: Same as Figure \ref{['fig:M1']}, but for model M2. Red and green lines present contribution from shell and jet zones, respectively.
  • Figure 5: Same as Figure \ref{['fig:M1']}, but for model M3. Red and green lines present contribution from shell and jet zones, respectively.
  • ...and 1 more figures