Evolution of the radio spectral index as observed by Zanardo et al. (2010) during the period between 1517 and 8014 days after explosion (filled circles with errors), extrapolation (black circles without errors) of the linear fit (black line; from the same reference) up to the value α_r = 0.5; then the constant α_r is assumed. The red solid line represents the spectral index α_r calculated by Eq. (8) with the dependence Q_t(t) derived from the observed α_r with the simple approximate formula (9); this Q_t(t) is shown in Fig. 3. Green dashed line represents the case of the steady-state injection.