Effect of the magnetar on the velocity, density and temperature profiles, as a function of the Lagrangian mass coordinate. Right panels: same pre-SN model as on the left but including a magnetar with E_rot = 10 foe and t_p = 1 d, and all other parameters fixed. The color code of the epochs is preserved in each column, and is measured starting at the moment of E_k injection. With this powerful magnetar (right panels) the maximum velocity reaches one third of the speed of light at the edge of the ejecta. We cut the velocity axis in order to show the most relevant interval.