Multi - frequency radio spectral studies of the supernova remnant G 78 . 2 + 2 . 1

A new map of the SNR G78.2+2.1 at 232 MHz is presented. This map is used with other images at 408, 1420, 2695 and 4800 MHz to investigate the total spectral index of the SNR and the variation of spectral index across it. The integrated spectral index is α = 0.54 ± 0.02 (where S ν ∝ ν −α). Maps of the spectral-index variation across the source are derived using a method which calculates a differential spectral index. This method is independent of offset errors and large-scale background variations. Systematic spectral-index variations of ∼ ± 0.15 are found, consistently in the many independent pairs of data sets. Regions of higher spectral index (steeper spectra) appear around the periphery of the remnant and in its central area. Regions of lower spectral index (flatter spectra) are also found within the remnant. The latter may result from a change in the nature of the non-thermal emission or may represent " contamination " from underlying patches of thermal emission. There is no clear correlation of spectral index with intensity across the SNR. Theoretical reasons for spectral-index variations have been explored but no clear interpretation of the apparent spectral steepening near the shock front has been found nor has an explanation of the steep-spectrum central region.