Speaker
Description
We compare the $\gamma$-ray spectrum available in literature from middle aged supernova remnants (SNRs) interacting with molecular clouds (MCs). We demonstrate the similarity in the shape of $\gamma$-ray spectra and then clarify a few points about the $\pi^0$-decay signatures claimed in a few SNRs. Next, we discuss the escaping scenario and direct interaction scenario, which have been proposed to interpret the observed $\gamma$-ray emission. We show that the similarity presented in $\gamma$-ray spectra is inconsistent with the prediction from escaping scenario statistically. It may imply that the widely used free escape boundary is not a good prescription to study escaping CRs and illuminated MCs. In the direct interaction scenario involving re-acceleration of pre-existing CRs, the similarity in $\gamma$-ray spectra can be understood as a reflection of almost uniform CR background in our Galaxy. However, the model suggests a transition in seed particles for diffusive shock acceleration during the SNR evolution. Whether such transition indeed exists and how does it affect SNRs’ contribution to Galactic CRs have to be investigated by future observation and theoretical modeling. In the end, we discuss the contribution of SNRs to the Galactic diffuse $\gamma$-ray emission.
