Temporal evolution of synchrotron SEDs calculated at selected radii with the utility function discussed in Sect. 2.6. We refer to the text for further details. Coloured dashed lines refer to an evolution in which synchrotron cooling is assumed to be negligible, and this would be equivalent to the traditional van der Laan (1966) formalism. Solid lines represent the case in which synchrotron cooling is enabled. The black line is the expected evolution of the self-absorbed peak according to the van der Laan (1966) model.