Probability density distribution for the SPEMD model with two sources (source AB and source C). The marginalized 1D histograms of each parameter are shown along the diagonal, while the other panels show the joint 2D probability distributions of the two parameters reported on the x and y axes. The parameters are those described in Sect. 4.1. The vertical dashed lines in the 1D histograms represent the 16th, 50th and 84th percentiles, which are also reported on top of each 1D histogram, while the solid lines and shaded areas in the 2D distributions represent the 0.68, 0.95, and 0.99 contour levels. In the floating panel, we zoom in the marginalized 1D histogram of the γ′ parameter (in black), and compare it with its distribution for the model with the AB source only (in red). The introduction of source C provides tighter constraints on the value of the γ′ parameter.