Spectral evolution in GRB 060218. The blue, green, and red points are taken from table 1 in Toma et al. (2007), fig. 1 in Ghisellini et al. (2007a), and Margutti et al. (2015), respectively. The lower solid black line indicates the typical high-energy spectral index of the Band function, F_ν ∝ ν^−1.5. The upper solid and dashed black lines shows the two-point XRT flux spectral index, log (F(10 keV)/F(0.3 keV))/log (10/0.3), as a function of time for the same pair of models as in Fig. 2. The time when the echo flux first exceeds the prompt flux in our model is shown by the vertical dashed line.