Parameter spaces of k-index for the spherical MHs. The red lines locate the 1-σ confidence interval for k measured for each MH. For RBS 797 we report the result obtained with α = 1.1. For Abell 3444 we report in red the result at 610 MHz and in green the result at 1.4 GHz. The horizontal, dashed line points out the level η = 0.5 that reproduces the equilibrium between thermal and non-thermal energy. For RXC J1504.1−0248 we report the η = 1 level with the black dash-dotted line and the lower limit derived from the Fermi-LAT observation presented in Dutson et al. (2013). The lower limits for B₀ for the other clusters are below 1 μG and they are reported in Table 6.