Contour plot of constant photon index, Γ (solid red), and the PD in percent (dashed blue) on the plane electron temperature, kT_e, versus Thomson optical depth, τ_T, computed for Comptonisation in a slab observed at inclination i = 45°. The pink band represents the range Γ = 1.7–1.9, which corresponds to the hard state spectra observed in Swift J1727.8−1613. The blue hatched band corresponds to the PD of 3–4% observed during observations 3, 4, and 8. The two bands cross each other in two regions where τ_T differs by an order of magnitude.