Contour plots for the three regions showing external absorption in Figure 6: north (shown in red over Tycho in Figure 6), center (in blue), and rim (in red). Plotted are the 1σ, 2σ, and 3σ confidence intervals for the parameters α and τ_40,ext for each of the regions. Only for one region, center, τ_40,ext = 0 (no absorption) is not excluded at the 3σ. For the two other regions, in particular for the northern region that we base our analysis on, we require the presence of absorption along the line of sight at the 3σ level.