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

Observations of spatial variations in the synchrotron spectra of Galactic shell SNRs G39.2-0.3 and G41.1-0.3 (3C 396 and 3C 397) are reported. Spectral indices in these remnants, as measured between 6 and 20 cm, vary on the order of Delta(alpha) of about 0.2. Comparison of G39.2-0.3 and G41.1-0.3 with seven other shell remnants exhibiting spatially varying spectral indices indicates that in the order 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. It is suggested that interactions between the expanding remnant and inhomogeneities in the surrounding medium may be instrumental to determining the spatial distribution of spectral indices in shell SNRs.