PD and PA averaged in the 2–8 keV band as a function of X-ray hardness for all IXPE observations of Swift J1727.8−1613, as measured by IXPE. This plot is an update of Fig. 5 from Svoboda et al. (2024). The red arrow signifies the 1.2% upper limit on the PD (with the PA unconstrained) at the 99% confidence level, as obtained in the soft state (observations 6 and 7 combined; Svoboda et al. 2024). Observation 8, of the dim hard state, is shown in blue. Observations 1–5, of the peak outburst (Veledina et al. 2023; Ingram et al. 2024), are shown in black.