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Recent observations by the Large Area Telescope (LAT) onboard the Fermisatellite have revealed bright gamma-ray emission from middle-aged supernovaremnants (SNRs) inside our Galaxy. These remnants, which also possess brightnon-thermal radio shells, are often found to be interacting directly withsurrounding gas clouds. We explore the non-thermal emission mechanism at thesedynamically evolved SNRs by constructing a hydrodynamical model. Two scenariosof particle acceleration, either a re-acceleration of Galactic cosmic rays(CRs) or an efficient nonlinear diffusive shock acceleration (NLDSA) ofparticles injected from downstream, are considered. Using parameters inferredfrom observations, our models are contrasted with the observed spectra of SNRW44. For the re-acceleration case, we predict a significant enhancement ofradio and GeV emission as the SNR undergoes a transition into the radiativephase. If sufficiently strong magnetic turbulence is present in the molecularcloud, the re-acceleration scenario can explain the observed broadband spectralproperties. The NLDSA scenario also succeeds in explaining the $\gamma$-rayspectrum but fails to reproduce the radio spectral index. Efficient NLDSA alsoresults in a significant post-shock non-thermal pressure that limits thecompression during cooling and prevents the formation of a prominent denseshell. Some other interesting differences between the two models inhydrodynamical behavior and resulting spectral features are illustrated.

Modeling Bright Gamma-ray and Radio Emission at Fast Cloud Shocks