Spatial spectral index variations in galactic shell supernova remnants G39.2-0.3 and G41.1-0.3

Spatial variations are observed in the synchrotron spectra of Galactic shell supernova remnants G39.2-0.3 and G41.1-0.3 (3C 396 and 3C 397). Spectral indices in these remnants, as measured between λ6 and 20 cm, vary on the order of Δα ∼ 0.2. In neither remnant do the spectral variations correlate well with features in total radio intensity. Explanations for these variations are sought in terms of various models of astrophysical particle acceleration. In comparing G39.2-0.3 and G41.1-0.3 with seven other shell remnants exhibiting spatially varying spectral indices, we find that in the older of these remnants, brighter regions tend to be flatter. In the younger remnants, the highest emissivity regions are generally the steepest, but with notable exceptions. We suggest that interactions between the expanding remnant and inhomogeneities in the surrounding medium may be a very important factor in determining the spatial distribution of spectral indices in shell supernova remnants.