2D histogram of the polarized synchrotron spectral indices from Fuskeland et al. (2014) (y-axis) versus the final results in this paper (x-axis). The set of pixels that are common to both studies are the final S-PASS pixels minus pixels around a few bright compact objects. The line y = x is shown. The figure indicates a flattening of about Δβ_s ∼ 0.1 from low to higher frequencies. The histogram column with the darkest blue color corresponds to about 1500 pixels of the total 10 830 pixels.