Fraction of Sr II under LTE conditions as a function of time post-merger. On the upper x-axis, we illustrate the physical time of scattering probed by the 1 μm feature in absorption and emission by the various spectra from ANU, SALT, VLT/X-shooter, and Gemini FLAMINGOS-2. The dotted lines indicate that no feature has clearly emerged (prior to ∼1.4 days in both emission and absorption). We indicate the required fraction of Sr II needed to produce the optical depths observed in each spectrum (assuming M_Sr = 6 × 10⁻⁶ M_⊙ homogeneously distributed across 0.2c < v < 0.4c; see main text). The ANU (X-shooter) and SALT (Gemini) spectra indicate a rapid transition in absorption (emission) from an optically thin to optically thick regime around 1.3–1.5 days. For the feature to emerge around 1.4 days post-merger, the ionisation temperature is required to be nearly identical to the radiation temperature inferred from the blackbody emission. For the feature to emerge at near-contemporaneous physical time in the polar and equatorial ejecta, a near-isotropic temperature is required.