Azimuthal profile of the postshock ISM density (see regions in Fig. 1). Error bars are shown at the 68% (crosses) and 90% (shaded areas) confidence levels. The blue curves show the profiles predicted by Völk et al. (2003) with δB/B = 1 (dot) and 0.8 (dashed), and derived by the radio flux (solid curve, see text) by assuming the relationship between the injection efficiency and shock compression ratio adopted by Ferrand et al. (2010). The green line indicates the expected trend in the case of no shock modification.