Catching the radio flare in CTA102 I. Light curve analysis

Context. The blazar CTA102 (z = 1.037) underwent a historical radio outburst in April 2006. This event offered a unique chance to study the physical properties of the jet. Aims. We used multifrequency radio and mm observations to analyze the evolution of the spectral parameters during the flare as a test of the shock-in-jet model under these extreme conditions. Methods. For the analysis of the flare we took into account that the flaring spectrum is superimposed on a quiescent spectrum. We reconstructed the latter from archival data and fitted a synchrotron self-absorbed distribution of emission. The uncertainties of the derived spectral parameters were calculated using Monte Carlo simulations. The spectral evolution is modeled by the shock-in-jet model, and the derived results are discussed in the context of a geometrical model (varying viewing angle) and shock-shock interaction Results. The evolution of the flare in the turnover frequency-turnover flux density (nu(m) - S(m)) plane shows a double peak structure. The nature of this evolution is dicussed in the frame of shock-in-jet models. We discard the generation of the double peak structure in the nu(m) - S(m) plane purely based on geometrical changes (variation of the Doppler factor). The detailed modeling of the spectral evolution favors a shock-shock interaction as a possible physical mechanism behind the deviations from the standard shock-in-jet model.