Hardness–intensity diagram of Swift J1727.8−1613. The gray crosses are from MAXI data. The hardness is taken as the ratio of photon fluxes between 4–10 and 2–4 keV bands. The X-ray photon flux is taken in the 2–20 keV range. The blue circles correspond to the times of optical polarimetric observations. For some optical measurements, the corresponding HR could not be determined, and hence are not shown in the figure. The red triangles denote position of the source during X-ray polarimetric observations by IXPE (Podgorný et al. 2024).