Kavli Affiliate: Li Xin Li
| First 5 Authors: Zhen Cao, F. Aharonian, Y. X. Bai, Y. W. Bao, D. Bastieri
| Summary:
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.
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