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Non-Thermal Radio and Gamma-Ray Emission from a Supernova Remnant by the Blast Wave Breaking Out of the Circumstellar Matter

We calculate synchrotron radio emission and gamma-ray emission due to bremsstrahlung, inverse-Compton scattering and $pi^0$-decay from the remnant of supernova which exploded in the circumstellar matter (CSM) formed by the progenitor's stellar wind. This sort of situation is a possible origin of mixed-morphology supernova remnants (SNRs) like W49B, which exhibit recombination-radiation spectra in X-ray emission. We assume that the CSM of 1.5 $M_{odot}$ exists at 0.07--3 pc away from the supernova in the interstellar medium (ISM) of density 0.016 cm$^{-3}$. When the blast wave breaks out of the CSM into the ISM, its velocity rapidly increases and hence particle acceleration is enhanced. The maximum energy of protons reaches $sim$ 1300 TeV just after the break-out with $sim$ 0.5% of the explosion energy. We consider the non-thermal emission from the blast-shocked ISM shell after the break-out. Synchrotron radio flux at 1 GHz is tens Jy, comparable to the observed from mixed-morphology SNRs. Because of low density, the gamma-ray luminosity is dominated by inverse-Compton scattering, which is higher than the $pi^0$-decay luminosity by an order of magnitude. The total gamma-ray luminosity including bremsstrahlung is of the order of $10^{33}$ erg s$^{-1}$ lower than the typical value $10^{35}$--$10^{36}$ erg s$^{-1}$ observed from mixed-morphology SNRs. However, if, e.g., $sim$10% of accelerated protons interact with some matter of density $sim$100 cm$^{-3}$, $pi^0$-decay gamma-ray luminosity would be enhanced to be comparable with the observed value.