2008A&A...490..197L

The Galactic object G78.2+2.1 is a classical shell-type supernova remnant. Previous radio continuum studies have shown substantial variations in spectral index across the object. Existing studies of the neutral hydrogen are of relatively low resolution and sensitivity. Much higher sensitivity and resolution data from the Canadian Galactic Plane Survey justify a re-examination of the properties of this SNR. In order to improve the accuracy of spectral index determination and its spatial variation, we first remove the underlying thermal emission with the use of a correlation between the 60µm infrared continuum and 1420MHz radio continuum. We use the so-called TT-plot technique to obtain the continuum spectral index between 408 and 1420MHz. We also apply a ``tomography'' technique to investigate the spectral index variations, while keeping the full resolution of the data. Complementary X-ray and infrared data are used to study associated structures in both continuum and in the 21cm line of neutral hydrogen. The spectral index α (S_ν ∝ν^–α) averaged over the SNR is 0.75±0.03, significantly higher than previous studies. The TT-plots and tomography image show spatial variations ranging from as low as 0.40 to as high as 0.80. Two complementary neutral hydrogen structures at LSR velocities of 5.5 and -14km/s, respectively, are shown to circumscribe the continuum emission. A number of scenarios are presented to account for the observed properties. A faint shell to the north-west of the SNR is visible in both radio and infrared continuum as well as in neutral hydrogen, and could be associated with a local blowout of the SNR blast wave into a medium of lower density. An isolated cloudlet probably accelerated by the SNR blast wave shows considerable structure in neutral hydrogen emission, suggesting the cloudlet is either beginning to dissolve within the SNR or is interacting with neighbouring gas. The previously proposed cloudlet at ~-117km/s is not isolated, but is part of a more extended structure, and is thus unlikely an accelerated cloudlet.