Sub-epoch evolution of the power-law index of cooling, see Eq. (1), of the X-shooter spectra at 1.43 days post-merger (red histogram). The temperature evolution is highly significant (> 10σ from α = 0). The inferred cooling between exposures, α = −0.77 ± 0.06, is more rapid than inferred from the average epoch 1–2 evolution (α = −0.62 ± 0.01, Sneppen et al. 2023a), which is itself more rapid than the cross-epoch analysis over the period 1–6 days post-merger (Waxman et al. 2018).