Bulk Lorentz factor as a function of distance z from the black hole. The distance is displayed in units of R_g (top axis) and milliarcsecond (bottom axis). The data includes the decelerating part in Figure 5 (gray data points) and the result inferred from the MERLIN observation at 408 MHz (red data point; see text). Error bars represent ±1σ uncertainties. The black dashed line represents the best-fit broken power-law model in Figure 5, and the red one represents the model obtained by including the red data.