Observed polarization of Swift J1727.8−1613 (red star) and the field stars (black circles) on the sky in the R band. The length of the bar corresponds to the PD and is denoted by dashed black lines for the field stars. For Swift J1727.8−1613, the solid red line and the dotted blue line correspond to the PD during SIMS and HIMS, respectively. The directions of the bars denote the PA. The background image is taken from the Aladin sky atlas (https://aladin.cds.unistra.fr/).