Surface mass density of our fiducial model composed of a Hernquist (dashed red) + generalized NFW (dotted black) model. Two power-law models which reproduce equally well an ensemble of lensed images whose positions range typically in x ∈  [0.5,1.5] (vertical dashed lines) are shown. The solid blue curve is for a singular power-law model, whereas the green curve is for a power law with a core (see Sect. 4.2).