Polarized intensity maps for the void G40.9 − 4.2. The upper panel shows the original map, the middle panel shows the map with the zero-level restored with a spectral index of β_PI =  −2.7 according to the restoration model discussed in Sect. 3.1.1. The lower panel shows the map with the zero-level restored using the lowest possible spectral index of β_PI =  −3.1 (see Fig. 3). PI is always shown in grey scale, and bars indicate polarization B-vectors.