2D distribution of the spectral index, α (S_ν ∝ ν^α) for region B (4 mas < r < 8 mas) using a beam size of 2.32 × 0.07 mas with a PA of −7° and a pixel size of 0.1 mas. The color map in each panel shows for a given epoch (indicated in the top left corner) the distribution of α and the contours correspond to the 15 GHz VLBI observations, where the lowest contour is plotted at 5× the rms value and increases by factors of 2.