| Abstract: | Puppis A is an interesting ~4 kyr-old supernova remnant (SNR) that shows
strong evidence of interaction between the forward shock and a molecular cloud.
It has been studied in detail from radio frequencies to high-energy (HE,
0.1-100 GeV) gamma-rays. An analysis of the Fermi-LAT data has shown an
extended HE gamma-ray emission with a 0.2-100 GeV spectrum exhibiting no
significant deviation from a power law, unlike most of the GeV-emitting SNRs
known to be interacting with molecular clouds. This makes it a promising target
for imaging atmospheric Cherenkov telescopes (IACTs) to probe the gamma-ray
emission above 100 GeV. Very-high-energy (VHE, E >= 0.1 TeV) gamma-ray emission
from Puppis A is for the first time searched for with the High Energy
Stereoscopic System (H.E.S.S.). The analysis of the H.E.S.S. data does not
reveal any significant emission towards Puppis A. The derived upper limits on
the differential photon flux imply that its broadband gamma-ray spectrum must
exhibit a spectral break or cutoff. By combining Fermi-LAT and H.E.S.S.
measurements, the 99% confidence level upper limits on such a cutoff are found
to be 450 and 280 GeV, assuming a power law with a simple exponential and a
sub-exponential cutoff, respectively. It is concluded that none of the standard
limitations (age, size, radiative losses) on the particle acceleration
mechanism, assumed to be still on-going at present, can explain the lack of VHE
signal. The scenario in which particle acceleration has ceased some time ago is
considered as an alternative explanation. The HE/VHE spectrum of Puppis A could
then exhibit a break of non-radiative origin, (as observed in several other
interacting SNRs, albeit at somewhat higher energies) owing to the interaction
with dense and neutral material in particular towards the northeastern region. |
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