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Astron. Astrophys. 349, 45-54 (1999)

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6. Summary and conclusion

By using most available submillimetre-to-radio observations of 3C 273, we have been able to extract the properties of the spectral and temporal evolution of a typical outburst. The new approach we defined consists in decomposing the light curves into several self-similar outbursts. The main results of our decomposition are the following:

  • It is possible to understand the very different shapes of the submillimetre-to-radio light curves of 3C 273 with only about one outburst every 1.5 year starting simultaneously at all frequencies.

  • There is no need to invoke any underlying quiescent emission apart from the weak contribution of the jet's hot spot 3C 273A.

  • The outbursts that we identify do well correspond to the observed VLBI components in the jet.

  • There is good evidence that short-lived and high-frequency peaking flares are emitted closer to the core of the jet than long-lived and low-frequency peaking outbursts.

  • The spectral and temporal evolution of the outbursts is found to be in good qualitative agreement with the evolution expected by shock models in relativistic jets.

  • We observe a flattening of the optically thin spectral index from the rising to the declining phase of the shock evolution, which supports the idea proposed by MG85 that radiative (synchrotron and/or Compton) losses are the main cooling process of the electrons during the initial phase of the outburst.

We are aware that our decomposition is far from describing the detailed structure of the light curves and that the jet emission is much more complicated than this work tries to show. Nevertheless, the results suggest that the outbursts we identified are closely related to the VLBI knots, and hence that they describe a physical aspect of the jet. The new approach presented here is a powerful tool to derive the observed properties of millimetre and radio outbursts. It allows comparison between shock models and the observations and we are confident that such decompositions are able to further constrain present and future shock models. Finally, we would like to stress the importance of long-term multi-wavelength monitoring campaigns, which turn out to be essential towards a better understanding of the physics involved in relativistic jets.

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999