Sum of chi-square values from Fig. 8 vs. density (black dots). The minimum chi-square value of 128.76 is obtained for a density of 10^13.4 m⁻³ (green line). The chi-square value for fits with the pion model is shown as a solid horizontal line, which is higher than the tpho value at the 3σ confidence level (orange line). The vertical dashed lines indicate the densities of the outflow corresponding to a recombination time scale of 10⁵ s for oxygen and iron.