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The temporal and spatial spectral evolution of the jets of AGN can be studiedwith multi-frequency, multi-epoch VLBI observations. The combination of both,morphological and spectral parameters can be used to derive source intrinsicphysical properties such as the magnetic field and the non-thermal particledensity. In the first two papers of this series, we analyzed the single-dishlight curves and the VLBI kinematics of the blazar CTA 102 and suggested ashock-shock interaction between a traveling and a standing shock wave as apossible scenario to explain the observed evolution of the component associatedto the 2006 flare. In this paper we investigate the core-shift and spectralevolution to test our hypothesis of a shock-shock interaction. We used 8multi-frequency VLBA observations to analyze the temporal and spatial evolutionof the spectral parameters during the flare. We observed CTA 102 between May2005 and April 2007 using the VLBA at six different frequencies spanning from 2- 86 GHz. After the calibrated VLBA images were corrected for opacity, weperformed a detailed spectral analysis. From the derived values we estimatedthe magnetic field and the density of the relativistic particles. The detailedanalysis of the opacity shift reveals that the position of the jet core isproportional to nu^-1 with some temporal variations. The value suggestspossible equipartition between magnetic field energy and particle kineticenergy densities at the most compact regions. From the variation of thephysical parameters we deduced that the 2006 flare in CTA 102 is connected tothe ejection of a new traveling feature (t=2005.9) and the interaction betweenthis shock wave and a stationary structure around 0.1 mas from the core. Thesource kinematics together with the spectral and structural variations can bedescribed by helical motions in an over-pressured jet.

Catching the Radio Flare in CTA 102 III. Core-Shift and Spectral Analysis