Connection between inner jet kinematics and broadband flux variability in the BL Lac object S5 0716+714

TL;DR

This study investigates the connection between jet kinematics and broadband flux variability in the BL Lac object S5 0716+714, revealing high apparent speeds, jet acceleration, and the influence of shock waves on multi-wavelength emission.

Contribution

It provides the first detailed VLBI kinematic analysis linking jet motions and flux variability in S5 0716+714, highlighting high apparent speeds and jet acceleration.

Findings

Detected superluminal jet speeds up to ~37c.

Identified a stationary feature 0.15 mas downstream of the core.

Found a decline in brightness temperature with distance, indicating a Doppler factor gradient.

Abstract

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 \sim37 c, which is exceptionally high, especially for a BL Lac object. Patterns in the jet flow reveal a roughly stationary feature \sim0.15 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 \propto r_{jet}^{-(2.36\pm0.41)}, which suggests a gradient in Doppler factor along the jet axis. Our analysis suggest that a moving disturbance (or a shock wave) from the base of the jet produces the high-energy (optical to \gamma-ray) variations upstream of the 7 mm core, and then later causes an outburst in the core. Repetitive optical/\gamma-ray flares and the curved trajectories of the associated components suggest that the shock front propagates along a bent trajectory or helical path. Sharper \gamma-ray flares could be related to the passage of moving disturbances through the stationary feature. Our analysis suggests that the \gamma-ray and radio emission regions have different Doppler factors.