Spectral index maps of T1 and T2 computed with a threshold of S ≥ 5σ. In all the panels, the contour levels are [±5, 10, 20, ...] × σ of the lowest-frequency image in each combination. Top: spectral index maps at 30 between 50–143 MHz (left) and 143–323 MHz (right). Bottom: spectral index maps at 14″ between 143–323 MHz (left) and 608–1284 MHz (right). The corresponding error maps are shown in Fig. A.1.