Galactic vs. intrinsic column densities contour plot for the six bursts of our samples. Vertical lines mark the Galactic column density derived from different estimates: dotted lines indicates the value adopted in the present paper (Kalberla et al. 2005, the value includes a 10% uncertainty); dashed lines the Kalberla et al. (2005) estimates, dot-dashed lines the Dickey & Lockman (1990) estimates, and dot-dot-dashed lines the Willingale et al. (2013) estimates. The cross in each contour plot marks the best-fit value. In the first panel the dot-dot-dashed line and the dotted line coincide.