HBA map with overlaid regions of analysis. The values of f, τ₄₀, and α are unitless. For all regions, the errors were rescaled in such a way that the best-fit power law has a reduced χ² of 1. The top plots and the bottom right plot (corresponding to the green, red, and blue regions as overlaid on Tycho) are fitted including external absorption (in blue, the best-fit unabsorbed power law is in green), and in all cases including the absorption term improves the fit: with a Δχ² = 16 for “EXT ABS NORTH”, a Δχ² = 4 for “EXT ABS CENTRE”, and a Δχ² = 10.5 for “RIM” (in all cases, for an additional degree of freedom). The bottom left plot corresponds to the region of possible internal absorption. The mask of the reverse shock radius is plotted in yellow over the map of Tycho. In the legends, “UL” stands for upper limit and “PL” stands for power law.