Modelling the submillimetre-to-radio flaring behaviour of 3C 273

We present a new approach to derive the observed properties of synchrotron outbursts in relativistic jets. The idea is to use the very well sampled submillimetre-to-radio long-term light curves of 3C 273 to extract the spectral and temporal evolution of a typical outburst. The method consists in a decomposition of these light curves into a series of twelve self-similar flares. With a model-independent parameterization, we find that the obtained outburst's evolution is in good qualitative agreement with the expectations of shock models in relativistic jets. We then derive, by a second approach, the relevant parameters of three-stage shock models. We observe for the first time that the optically thin spectral index is steeper during the initial rising phase of the evolution than during the final declining phase as expected by the shock model of Marscher & Gear (1985). We obtain that this index flattens from alpha=-1.1 to alpha=-0.5, in good agreement with what is expected from a power law electron energy distribution with an index of -2.0. The observed flattening gives support to the idea that radiative (synchrotron and/or Compton) losses are the dominant cooling process of the electrons during the initial phase of the shock evolution. Two other results give us confidence in our decomposition: 1) the outbursts that we identify do well correspond to the VLBI components observed in the jet and 2) there is strong evidence that high-frequency peaking outbursts evolve faster than low-frequency peaking outbursts. We propose that this last correlation is related to the distance from the core of the jet at which the shock forms.