Intrinsic evolution of a superluminal knot in 3C 345

In the quasar 3C 345 jet components move superluminally along bent trajectories, which differ for each component. Qian et al., (1991) & (1992) derived a geometric helical motion model for the jet components C4 & C5. From this helical geometry and the observed motion, the time-dependence of the Doppler boosting factor is derived and used to relativistically correct the observed flux density and size variability of C4. Very different from the observed behaviour and not consistent with expectations from canonical models, the intrinsic (Doppler-corrected) flux density evolution of C4 shows a nearly rectangular outburst-like shape with rising and decay times of similar to 5 - 10 yrs in the comoving frame. At the same time C4 expands subluminally with an intrinsic velocity of upsilon(int) similar to 0.09 c. Both, variability and subluminal expansion, are discussed in a model involving field amplification and 'in situ' particle acceleration followed by adiabatic expansion.