Analysed sources in the spectral indices α_ox−α_ro plane. Lower limits are indicated with an arrow. Numbers refer to the values presented in Table 1, while letters L, M, and R refer to the objects in Landi et al. (2015b,a) associated by Landoni et al. (2015), Massaro et al. (2015b), and Ricci et al. (2015), respectively. Sources 5, 6, 11 and 13 are shown as points for the sake of clarity. The red shaded area indicates the region where the relationship describing synchrotron peak values changes its functional form, as explained in Abdo et al. (2010).