Synchrotron emission within a decaying magnetic field: Parameter space. In the $t{\prime }_{\mathrm{B}}/t{\prime }_{\mathrm{dyn}}$ vs Γ_c, 0/Γ_m plane, the value of the low-energy photon index α of the synchrotron spectrum is colour-coded. In this plane, the standard synchrotron spectrum with a constant magnetic field is at the top ($t{\prime }_{\mathrm{B}}\to \infty$). In this case the fast cooling regime is on the left (Γ_c, 0 ≪ Γ_m), while the marginally fast cooling regime is on the right (Γ_c, 0 ≃ Γ_m). Black solid lines indicate the limits of the high radiative efficiency region (thick: f_rad = 0.9; thin: f_rad = 0.5; very thin: f_dad = 0.1). Contours of the inverse Compton/synchrotron ratio L_ic/L_syn are plotted with black dashed lines. Each panel corresponds to a set of values for the parameters Y_Th, 0 and w_m that govern the importance of inverse Compton scatterings and Klein-Nishina corrections. (a) Y_Th, 0 = 0.1; w_m = 10⁻². (b) Y_Th, 0 = 0.1; w_m = 10². (c) Y_Th, 0 = 10²; w_m = 10². (d) Y_Th, 0 = 10²; w_m = 10⁴. (e) Y_Th, 0 = 10⁴; w_m = 10⁴.