Spectral index map (top) and corresponding error map (bottom) at three frequencies pairs: 43.12−86.25 GHz (left), 15.35−43.12 GHz (middle), and 1.68−15.35 GHz (right). The color scale indicates the spectral index, α, and its error, σ_α, as shown at right of top and bottom panels, respectively. The contours of the source structure at each frequency are shown in each panel with gray, broken line for the lower frequency and black, solid line for the higher one. The contours are set to 2, 4, 8, 16, 32, 64% of the peak intensity. Note that the relative position of each image has been aligned according to cross correlation of core-masked images, as described in Sect. 3.1.