Time dependence of injection term. Blue circles correspond to the days of the radio observations. A red solid line traces the function Q_t(t) derived as described in Sect. 2 from the observations during the period between 1517 and 8014 days after explosion and from the extrapolation of α_r after the 8014th day. As for the behaviour of the injection term before day 1517, we adopt a simple approximation as described in the text, with β_x = 0.8 and d_x = 100. Green dashed line represents the case of the steady-state injection. The thin black line is the function Q_t = lg(t/ 950).