Kinematic study of the blazar S5 0716+714

Abstract

Abstract. 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 5 GHz, 8.4 GHz, 15 GHz and 22 GHz. The milliarcsecond source structure is best described by a one-sided core-dominated jet of ∼10 mas length. Embedded jet components move superluminally with speeds ranging from 5 c to 16 c (assuming z = 0.3). Such fast superluminal motion is not typical of BL Lac objects, however it is still in the range of jet speeds typically observed in quasars (10 c to 20 c). 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 (1 mas) 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.

Core shift effect in blazars

“…In addition, high bulk Lorentz factors of 16−21 were estimated based on the inferred jet speeds (Jorstad et al 2001). Radio maps from VLBI reveal a compact core-jet structure as has been suggested for BL Lacertae with different components moving at different velocities (Bach et al 2003). Jet speed > 11-15 h −1 c were estimated from the component proper motions Jorstad et al (2001), who also found quasi-periodic ejection of components every ∼ 0.7 yr. An analysis of the above 26 VLBI observations in 5-22 GHz frequency band gives an apparent velocity in range 5-10 h −1 c. Radio flux density and linear polarization light curves obtained from UMRAO at 4.8, 8.0, and 14.5 GHz were presented by Aller et al (1985) for the 1981-1984 period and those during 1985-1992 were studied by Wagner et al (1996).…”

A synchrotron self-Compton model with low-energy electron cut-off for the blazar S5 0716+714

“…Figure 2 and the bottom panel in Fig. 4 demonstrate that, if the variability of S5 0716+714 is intrinsic and extends down to <32 GHz, the Doppler-boosting factor of the emission region must be much higher than the 20−30 suggested by Bach et al (2005). One possibility for enabling the Doppler factor to fall within this range is to increase the magnetic field strength and the electron density, as shown in Table 1.…”

“…Assuming H 0 = 70 km s −1 Mpc −1 , with Ω λ = 0.7, Ω M = 0.3, and Ω k = 0, and a redshift z > 0.3 based on the non-detection of a host galaxy (e.g. Wagner et al 1996), they derive a variability brightness temperature of T var > (2.1 ± 0.1) × 10 14 K. Bach et al (2005) analysed the data set of VLBI images of 11 jet components of S5 0716+714 at 4.9 GHz, 8.4 GHz, 15.3 GHz, and22.2 GHz, observed between 1992 and2001. Assuming that all the jet components move with the same speed along the jet (i.e. all components have the same bulk Lorentz factor), they propose that the observed wide range (from 5.5c to 16.1c) of apparent component speeds is due to variations in the viewing angle and limit the Lorentz factor and the viewing angle of the VLBI jet to Γ > 15 and θ < 2 • , respectively.…”

“…Even with a viewing angle ϑ ≈ 0, in which case D ≈ 2Γ, a bulk Lorentz factor of Γ > 33 is required, suggesting either that the source was travelling at a much higher speed than during the observations analysed by Bach et al (2005) or that an interpretation of the superluminal motion in the VLBI jet that infers a much higher bulk Lorentz factor should be applied. One such suggestion is a conical jet (Gopal-Krishna et al 2006).…”

“…Alternatively, the jet components may decelerate as they travel down the jet (Marscher 1999;Georganopoulos & Kazanas 2003;Ghisellini et al 2005). The wide range of superluminal velocities shown in Bach et al (2005) would then be a combined result of the variations in speed, as well as of the viewing angle. …”