Empirical Ml diffuse spectral index map posterior mean, obtained by power-law fits to the diffuse emission component samples provided by the Ml reconstruction and subsequent averaging. Because we reconstruct fluxes integrated over logarithmically equidistant energy bins, spectral index values are offset by +1 with respect to un-integrated energy spectra (see Eqs. (3) and (4) in Sect. 2.1 for details).