data corresponding to the model from the observed data, and then image the resulting visibilities to produce the residual image shown in the third column.

In the first row of Figure \[fig\_models\], we show the results of fitting a thin spherical shell to the 1998.0 data (cf. Gaensler et al. 1997) — the corresponding best-fit flux and radius are 18.4 mJy and 073. The resulting image successfully approximates the limb-brightened morphology seen in Figure \[fig\_movie\_super\], but lacks the enhancements in brightness seen on either side of the shell. The residual image, while having small amplitudes in an absolute sense, shows clear and systematic differences between this simple model and the data (maximum fractional residual of $\sim$20%).

Given that the main difference between a thin shell model and the data seems to be the presence of the bright lobes on either side, we next generate a model involving a thin shell (of arbitrary flux, position and radius), and two point-sources of emission (each of arbitrary flux and position). For the 1998.0 data set, the best-fit parameters for this model are a shell of flux 15.0 mJy and radius 079, and point-sources of fluxes 3.1 and 0.4 mJy, one projected near the 