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The present models for compact radio source variability, with reference to the early 1983 mm-to-IR flare of the quasar 3C 273, indicate that the outburst spectrum's early evolution is most easily explained if the flaring component is expanding. The models encompass the effects of synchrotron, Compton, and expansion losses, as well as variable injection of relativistic electrons and magnetic field. A model based on a uniform expanding source requires comparatively artificial variations of particle injection with source radius, in order to explain the 3C 273 flare data; superior results are noted for a second model in which the outburst is due to a shock wave passing through an adiabatic, conical, relativistic jet.
Marscher et al. (Fri,) studied this question.