Modelling 20 years of synchrotron flaring in the jet of 3C 273

We present a phenomenological jet model which is able to reproduce well the observed variations of the submillimetre-to-radio emission of the bright q uasar 3C 273 during the last 20 years. It is a generalization of the origin al shock model of Marscher & Gear (1985), which is now able to describe an accelerating or decelerating shock wave, in a curved, non-conical and non-a diabatic jet. The model defines the properties of a synchrotron outburst wh ich is expected to be emitted by the jet material in a small region just be hind the shock front. By a proper parameterization of the average outburst' s evolution and of the peculiarities of individual outbursts, we are able t o decompose simultaneously thirteen long-term light-curves of 3C 273 in a s eries of seventeen distinct outbursts. It is the first time that a model is so closely confronted to the long-term multi-wavelength variability proper ties of a quasar. The ability of the model to reproduce the very different shapes of the subm illimetre-to-radio light curves of 3C 273 gives strong support to the shock model of Marscher & Gear (1985). Indirectly, it also reinforces the idea t hat the outbursts seen in the light-curves are physically linked to the dis tinct features observed to move along the jet with apparently superluminal velocities. The more than 5000 submillimetre-to-radio observations in the d ifferent light-curves are able to constrain the physical properties of the jet. The results suggest, for instance, that the magnetic field behind the shock front is rather turbulent. There is also some evidence that the jet r adius does not increase linearly with distance down the jet or, alternative ly, that the synchrotron emitting material decelerates with distance and/or bends away from the line-of-sight.