Variability and dust filtration in the transition disk J160421.7-213028 observed in optical scattered light

Context. Protoplanetary disks around young stars are the birth-sites of planets. Spectral energy distributions and direct images of a subset of disks known as transition disks reveal dust-depleted inner cavities. Some of these disks show asymmetric structures in thermal submillimetre emission and optical scattered light. These structures can be the result of planet(s) or companions embedded in the disk. Aims. We aim to detect and analyse the scattered light of the transition disk J160421.7-213028, identify disk structures, and compare the results with previous observations of this disk at other wavelengths. Methods. We obtained and analysed new polarised intensity observations of the transition disk J160421.7-213028 with VLT/SPHERE using the visible light instrument ZIMPOL at R'-band (0.626 mu m). We probed the disk gap down to a radius of confidence of 0.1 '' (similar to 15 AU at 145 pc). We interpret the results in the context of dust evolution when planets interact with the parental disk. Results. We observe a gap from 0.1 to 0.3 '' (similar to 15 to 40 AU) and a bright annulus as previously detected by HiCIAO H-band observations at 1.65 mu m. The radial width of the annulus is around 40 AU, and its centre is at similar to 61 AU from the central star. The peak of the reflected light at 0.626 mu m is located 20 AU inward of the cavity detected in the submillimetre. In addition, we detect a dip at a position angle of similar to 46.2 +/- 5.4 degrees. A dip was also detected with HiCIAO, but located at similar to 85 degrees. If the dip observed with HiCIAO is the same, this suggests an average dip rotation of similar to 12 degrees/year, which is inconsistent with the local Keplerian angular velocity of similar to 0.8 degrees/yr at similar to 61 AU. Conclusions. The spatial discrepancy in the radial emission in J160421.7-213028 at different wavelengths is consistent with dust filtration at the outer edge of a gap carved by a massive planet. The dip rotation can be interpreted as fast variability of the inner disk and/or the presence of a warp or circumplanetary material of a planet at similar to 9.6 AU.