I_R − I_X slope as a function of the radius (i.e. distance from the halo centre) for the five presented targets. The k values are computed using Eq. (6) among the consequent annuli. To reduce possible dependencies on the annulus width, we report the results for three different sizes of the annuli (1,  1.5,  2 × beam size). The red dotted horizontal line indicates the best-fit slope found by the point-to-point analysis (with its error) up to the radius considered for that analysis.