The low-energy slope α of the fast cooling synchrotron spectrum in the presence of inverse Compton scatterings in Klein-Nishina regime. In the w_m − Y_Th plane, the value of the photon index α of the synchrotron spectrum below the peak of νF_ν is color-coded. All spectra have been computed numerically using a detailed radiative code including synchrotron radiation and inverse Compton scatterings (see text). Black thick lines of constant ratio L_ic/L_syn are plotted on top of this diagram. Cases where L_ic/L_syn > 5 in the top-left corner are not plotted as we focus in this paper on situations where the synchrotron component is dominant. In addition, thin dashed lines show the location of regions IIb and IIc from Nakar et al. (2009), where a slope α → −1 is predicted.