Broadband $\gamma$-ray spectrum of supernova remnant Cassiopeia A

TL;DR

This study combines multi-instrument gamma-ray data to reveal a complex, broken power-law spectrum of supernova remnant Cassiopeia A, indicating diverse particle acceleration processes and spatial emission contributions.

Contribution

It provides the first combined analysis of LHAASO and Fermi-LAT data showing a broken power-law spectrum in Cas A, highlighting the importance of spatial and spectral complexity in SNR gamma-ray emission.

Findings

Spectrum is softer than IACTs at TeV energies.

Flux near 1 TeV is about twice higher than previous measurements.

Spectrum above 30 GeV shows a significant deviation from a single power-law.

Abstract

The core-collapse supernova remnant (SNR) Cassiopeia A (Cas A) is one of the brightest galactic radio sources with an angular radius of $\sim$ 2.5 $\arcmin$. Although no extension of this source has been detected in the $\gamma$-ray band, using more than 1000 days of LHAASO data above $\sim 0.8$ TeV, we find that its spectrum is significantly softer than those obtained with Imaging Air Cherenkov Telescopes (IACTs) and its flux near $\sim 1$ TeV is about two times higher. In combination with analyses of more than 16 years of \textit{Fermi}-LAT data covering $0.1 \, \mathrm{GeV} - 1 \, \mathrm{TeV}$, we find that the spectrum above 30 GeV deviates significantly from a single power-law, and is best described by a smoothly broken power-law with a spectral index of $1.90 \pm 0.15_\mathrm{stat}$ ($3.41 \pm 0.19_\mathrm{stat}$) below (above) a break energy of $0.63 \pm 0.21_\mathrm{stat} \, \mathrm{TeV}$. Given differences in the angular resolution of LHAASO-WCDA and IACTs, TeV $\gamma$-ray emission detected with LHAASO may have a significant contribution from regions surrounding the SNR illuminated by particles accelerated earlier, which, however, are treated as background by IACTs. Detailed modelling can be used to constrain acceleration processes of TeV particles in the early stage of SNR evolution.