The low-energy slope α of the synchrotron spectrum in the presence of inverse Compton scatterings in Klein-Nishina regime, including the effect of adiabatic cooling. Same as Fig. 3, now including adiabatic cooling for Γ_c/Γ_m = 10^-2, 10^-1, 1 and 10. Black solid lines of constant radiative efficiency f_rad are plotted on top of the three last diagrams. In the first panel, the radiative efficiency is always close to 100%. For larger ratio Γ_c/Γ_m, the radiative efficiency remains larger than respectively 96% (Γ_c/Γ_m = 10^-1), 65% (Γ_c/Γ_m = 1) and 28% (Γ_c/Γ_m = 10).