Evolution of LX over the time t since the SN explosion according to Eq. (3) with L₀ = (2.9 ± 0.4) × 10³⁸ erg s⁻¹ (T_e ≤ 2.24 keV) and L₀ = (6.9 ± 0.7) × 10³⁸ erg s⁻¹ (T_e > 2.24keV), assuming s = 2 and n = 10. The data used for the fit in the ED phase in Fig. 3 were over-plotted. Evidently, a power law index of s = 2 is not sufficient to describe the steep decline of the data. The parameter L₀ in each case was estimated by fitting Eq. (3) to the first eight data points (solid markers), where the decline is lower compared to the rest of the data. Additionally, a broken power law (blue dashed line) was fitted according to Eq. (8), assuming α₁ = −1, to all data points (solid and transparent markers) to better describe the steep decline of the data towards the end of the ED phase. From the fit we obtain t_break = 39.9 ± 2.5 yr.