Effect of a decaying magnetic field on the synchrotron spectrum. The normalised spectrum ν′u_ν′/u_e is plotted as a function of the normalised frequency $\nu \prime /\nu {\prime }_{\mathrm{m}}$ for a constant magnetic field (dotted line) or a decaying magnetic field on a timescale $t{\prime }_{\mathrm{B}}={10}^{-2}t{\prime }_{\mathrm{dyn}}$ (solid line). The calculation has been done with the numerical radiative code described in Sect. 2.4 either including only the synchrotron process (black) or both the synchrotron radiation and the inverse Compton scatterings (red). The following parameters were adopted: Γ_c, 0 = Γ_m/300, Y_Th = 10, w_m = 10², and p = 2.5. In the synchrotron-only case, the result of the approximate calculation discussed in Sect. 2.3 is plotted in blue for comparison.