Fractional polarization versus Mach number obtained from the injection spectral index (upper panel) and from the integrated spectral index (bottom panel). Each marker represents a single relic. Blue circles are average fractional polarization computed integrating over the polarized regions of the relic, orange downward triangles are the maximum values (therefore one relic can have both measurements in the plot). Arrows are upper limits computed for relics where we did not detect polarization.