Time-space plot for the variation of the axial emission at 43 GHz for the OP jet in the observer’s frame at a viewing angle of 3°. The top panel shows the emission taking opacity effects into account, whereas the bottom panel shows the optically thin emission. The solid black line indicates propagation at the speed of light, so any structure that propagates with a flatter slope is superluminal. The emission values are normalized to their maximum value (given by the optically thin map).