2005MNRAS.362..966B

The BL Lacertae object S5 1803+784 has been monitored with very long baseline interferometry at λ= 3.6 cm in 43 epochs between 1986.21 and 1993.95. The motivation of this work is to obtain statistically meaningful data with which to study the short-term structural variability in the source on monthly time-scales. We present a detailed analysis of the structural evolution seen in the milliarcsecond-scale jet by way of model-fitting results. Within the simplest identification scenario, the jet can be described as a bent chain of seven jet components within 0.2- and 3-mas separation from the core. New components seem to emerge almost regularly (every 2 yr) from the core. Three jet components approach the brightest and so-called `stationary' component (at ∼1.4 mas at 8.4 GHz) with apparent superluminal motion of 8-11c. In this paper, we show that the `stationary' component (under the assumption that this is the brightest jet component in each epoch) oscillates, and we discuss several possible explanations. We show that a reconfinement shock can easily reproduce the observed most recent oscillation of the `stationary' component. We present evidence that the average jet ridge line is significantly curved and discuss the possibility that the jet components follow a helically bent path.