R′ band (0.626 μm) VLT/SPHERE/ZIMPOL images of J1604 (they are not scaled by r²). From left to right: polarised intensity (PI), polar-coordinate Stokes parameters Q_φ and U_φ respectively, such that . The clean U_φ image shows that we had an optimal correction for the instrumental polarisation. The colour scale is the same for the three panels; it is linear and in arbitrary units. The dashed lines in the left panel correspond to 0.35 and 0.48 arcsec, which is the region where the azimuthal profile is calculated in Fig. 3 to distinguish the dip.