Average spectral index as a function of distance from the 43 GHz core for different pairs of frequencies. In addition to the VLBA data set here presented, we re-present for comparison the 22–43 GHz spectral index values for the two epochs presented by Ricci et al. (2022). At high frequencies, remarkably steep spectral index values down to α ∼ −2 are observed within one parsec from the core, corresponding to ∼10⁴ R_S. Downstream, a convergence toward flatter values α ∼ −0.8 is observed at all frequencies. The black vertical lines highlight the jet break point, as proposed in Boccardi et al. (2021), while the horizontal dashed line is set at α = −1 as reference.