Spectral-index radial profiles derived from the GMVA 2018 and EHT 2018 images for different assumptions of compact flux density. All images have been convolved with a 30 μas beam. Left panel: profiles with a fixed compact flux density of F_cpct,230 = 0.58 Jy and varying F_cpct,86 from 0.5 Jy (black dashed line) to 0.6 Jy (black dash–dotted line); the spanned range is indicated by the magenta shaded region. The case with F_cpct,86 = 0.56 Jy and F_cpct,230 = 0.58 Jy, adopted in this work, is shown by the thick black solid line. Right panel: same as the left panel, but for profiles with a fixed F_cpct,86 = 0.56 Jy and varying F_cpct,230 from 0.5 Jy (black dashed line) to 0.7 Jy (black dash–dotted line).