Top panel: Same as in Fig. 5, but for the RM signal obtained from synchrotron radiation without any background source (setup D). Middle panel: Difference between the RM reference map (see Fig. 5) and the one presented in the top panel of this figure. Bottom panel: Difference between the RM map obtained by considering only the volume enclosed by the bubble walls (see Fig. 9) and the one presented in the top panel of this figure.