Expected α_M values as a function of B_M′ (y-axis) and b (x-axis). In particular, the {B_M′, b} range shown (in orange) is the one allowed by the α_M slope obtained from our analysis made in Sect. 3.2 (in orange). The blue box and brown point are the best-fit values from the fit of Eq. (16). Red lines represent literature results of estimates of the average cluster magnetic field from polarisation studies (Murgia et al. 2004; Govoni et al. 2006; Guidetti et al. 2008; Bonafede et al. 2010; Vacca et al. 2010, 2012; Govoni et al. 2017; Stuardi et al. 2021; De Rubeis et al. 2024).