Emergence of a Sr II P Cygni given different electron densities (left) and time post-merger (right). Left: X-shooter epoch 1 spectrum with a model blackbody + Sr II P Cygni feature overlaid. The wavelength-dependent strength of the Sr II feature is modulated by the fractional abundance of singly ionised Sr at the time of observation, t_obs = 1.43 days (see Fig. 4). There is only a weak dependence on the electron density as the transition time is a relatively robust prediction in time. This means that the electron density is not tightly constrained, with the majority of the feature indicating a moderate to high electron density of n_e ≈ 10⁸ − 10⁹ cm⁻³. Right: given an electron density n_e = 10⁹ cm⁻³, the corresponding Sr II P Cygni from 0.9 to 1.8 days post-merger in intervals of 0.1 days. The feature rapidly emerges with the reverberation wave, moving from the blueshifted (and nearer) ejecta towards the red. At the time of observation the absorption is fully formed, while the emission is weak due to the decreased fraction of Sr II in the emitting region at the time of emission. This means that the Sr II interpretation can be used to make strong predictions for the timing of the emergence of the feature and the early spectral shape and its evolution.