Two pictures displaying the maps of the Lorentz factor (right panel) and of the tracer of the medium material (left panel) that is accelerated above a threshold of v_z ≥ 0.1 in the x − z plane, at y = 0. The white curve on the left is a contour of the 2D stationary jet. White horizontal lines on the right indicate four different distances z^* at which we evaluate the x − y cuts in Fig. 3.