Corner plot showing the posterior probability distributions of the best fit MRN grain size model with a power-law parametrisation of the refractive index with 1, 2, and 3σ contours. The key parameters of the MRN maximum grain size a₊ and power-law slope q are shown, as are the refractive index’s real and imaginary power-law indices, n_α, k_α, and values at 1 keV, 1 − n_{1 keV}, k_{1 keV}. The dots indicate the expected refractive indices for different compositions of carbon grains (blue), metallic Fe grains (purple), Fe₂SiO₄ (dark blue), MgFeSiO₄ (red), and MgSiO₃ (orange). The equilibrium distribution found by the walkers displays a convex minima. While not imposing the Kramers-Kronig relations on the refractive indices, the best-fit n and k are close to standard physical compositions expected for ISM dust. The best-fit parameters are provided in Tables 2 and 3.