Measured and predicted radius of the extended component of HESS J1809−193 (component A). As measured radius, we used the 1-σ extent of the semi-major axis of the elongated Gaussian spatial model for component A (cf. Table 1). The solid blue curve has been obtained by fitting the electron diffusion parameters (D₀, δ; cf. Eq. (4)) to the measured data points; the dashed orange and dotted green curves show results for fixed values of δ as indicated in the legend.