shock acceleration, including the dynamical effects of the accelerated particles (e.g. Baring et al. 1999), suggest that young and hence fast shocks have a [*larger*]{} compression ratio than older shocks, resulting in a flatter synchrotron spectrum. Duffy, Ball & Kirk (1995) included the effect of accelerated ions on the shock structure and were able to model the observed spectrum with the shock expanding into a terminated stellar wind structure. However, this model predicts a declining brightness and a steepening spectrum for the radio emission, both of which are contrary to observation.

The observed spectral index is also steeper than those for typical supernova remnants; no SNR in the Green (2000) catalogue has a spectral index definitely steeper than $-0.8$ and the mean spectral index for shell-type remnants is $-0.51$. It is interesting to speculate that the current flattening of the spectrum represents an evolution toward the typical index of $-0.5$. At the current rate, it will only take about 50 years to reach $-0.5$.

The clump interaction model of Ball & Kirk (1992) predicts a declining flux density at later times. However, this model was based on only two clumps. The model could be extended to invoke interaction with an 