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

TL;DR

This paper models gamma-ray and radio emissions from supernova remnants interacting with gas clouds, comparing re-acceleration and nonlinear shock acceleration scenarios to observations of SNR W44.

Contribution

It introduces a hydrodynamical model to analyze non-thermal emissions in evolved SNRs, contrasting two particle acceleration mechanisms with observational data.

Findings

Re-acceleration predicts enhanced radio and GeV emission during the radiative phase.

Re-acceleration with strong magnetic turbulence can explain broadband spectra.

NLDSA explains gamma-ray spectra but not radio spectral index.

Abstract

Recent observations by the Large Area Telescope (LAT) onboard the Fermi satellite have revealed bright gamma-ray emission from middle-aged supernova remnants (SNRs) inside our Galaxy. These remnants, which also possess bright non-thermal radio shells, are often found to be interacting directly with surrounding gas clouds. We explore the non-thermal emission mechanism at these dynamically evolved SNRs by constructing a hydrodynamical model. Two scenarios of particle acceleration, either a re-acceleration of Galactic cosmic rays (CRs) or an efficient nonlinear diffusive shock acceleration (NLDSA) of particles injected from downstream, are considered. Using parameters inferred from observations, our models are contrasted with the observed spectra of SNR W44. For the re-acceleration case, we predict a significant enhancement of radio and GeV emission as the SNR undergoes a transition into the radiative phase. If sufficiently strong magnetic turbulence is present in the molecular cloud, the re-acceleration scenario can explain the observed broadband spectral properties. The NLDSA scenario also succeeds in explaining the $\gamma$-ray spectrum but fails to reproduce the radio spectral index. Efficient NLDSA also results in a significant post-shock non-thermal pressure that limits the compression during cooling and prevents the formation of a prominent dense shell. Some other interesting differences between the two models in hydrodynamical behavior and resulting spectral features are illustrated.