Linear fitting of the polarization angle of the core, ϕ = ϕ₀ + RMλ², as a function of the observation wavelength squared, λ². The ϕ values are determined from the grid-search exploration of the χ² parametric space, by fitting the closure-trace model to the simulated data, with the polarization of knots A and B fixed to their values at spw0 (see Sect. 2.1). We obtain a fitted value of RM = (0.32 ± 0.04) × 10⁷ rad m⁻², compatible with the expected value of (0.35 ± 0.04) × 10⁷ rad m⁻² obtained from fitting the simulated polarization angles.