2015A&A...578A.123R

We present a high-frequency very long baseline interferometry (VLBI) kinematical study of the BL Lac object S5 0716+714 over the time period of September 2008 to October 2010. The aim of the study is to investigate the relation of the jet kinematics to the observed broadband flux variability. We find significant non-radial motions in the jet outflow of the source. In the radial direction, the highest measured apparent speed is ∼37c, which is exceptionally high, especially for a BL Lac object. Patterns in the jet flow reveal a roughly stationary feature ∼0.15 mas downstream of the core. The long-term fits to the component trajectories reveal acceleration in the sub-mas region of the jet. The measured brightness temperature, T_B, follows a continuous trend of decline with distance, T_B∝r_jet^{–(2.36±0.41)}, which suggests that there is a gradient in Doppler factor along the jet axis. Our analysis favors that a moving disturbance (or a shock wave) from the base of the jet produces the high-energy (optical to γ-ray) variations upstream of the 7mm core and then later causes an outburst in the core. Repetitive optical/γ-ray flares and the curved trajectories of the associated components suggest that the shock front propagates along a bent trajectory or helical path. Sharper γ-ray flares could be related to the passage of moving disturbances through the stationary feature. Our analysis indicates that the γ-ray and radio emission regions have different Doppler factors.