2005A&A...433..815B

We present the results of a multi-frequency study of the structural evolution of the VLBI jet in the BL Lac object 0716+714 over the last 10 years. We show VLBI images obtained at 5GHz, 8.4GHz, 15GHz and 22GHz. The milliarcsecond source structure is best described by a one-sided core-dominated jet of ∼10mas length. Embedded jet components move superluminally with speeds ranging from 5c to 16c (assuming z=0.3). Such fast superluminal motion is not typical of BLLac objects, however it is still in the range of jet speeds typically observed in quasars (10c to 20c). In 0716+714, younger components that were ejected more recently seem to move systematically slower than the older components. This and a systematic position angle variation of the inner (1mas) portion of the VLBI jet suggests an at least partly geometric origin of the observed velocity variations. The observed rapid motion and the derived Lorentz factors are discussed with regard to the rapid Intra-Day Variability (IDV) and the γ-ray observations, from which very high Doppler factors are inferred.