Left panel: spatially resolved spectral-index map within a 200 μas field of view, derived from horizon-scale images observed by the GMVA in 2018 (R.-S. Lu et al. 2023) and the EHT in 2018 (Event Horizon Telescope Collaboration et al. 2024). Both images have been convolved with a 30 μas beam, indicated by the circle in the lower-left corner. Right panel: corresponding spatial distribution of the spectral-index uncertainty. In both panels, pixels are masked to include only regions where the intensity in both bands exceeds 3 σ_rms,ν.