Kilonova evolution -- the rapid emergence of spectral features

Kilonovae (KNe) are one of the fastest types of optical transients known, cooling rapidly in the first few days following their neutron-star merger origin. We show here that KN spectral features go through rapid recombination transitions, with features due to elements in the new ionisation state emerging quickly. Due to time-delay effects of the rapidly-expanding KN, a 'wave' of these new features passing though the ejecta is a detectable phenomenon. In particular, isolated line features will emerge as blueshifted absorption features first, gradually evolving into more pronounced absorption/emission P Cygni features and then pure emission features. In this analysis, we present the evolution of the individual exposures of the KN AT2017gfo observed with VLT/X-shooter that together comprise X-shooter's first epoch spectrum (1.43 days post-merger). We show that the spectra of these 'sub-epochs' show a significant evolution across the roughly one hour of observations, including a decrease of the blackbody temperature and photospheric velocity. The cooling blackbody constrains the recombination-wave, where a Sr II interpretation of the AT2017gfo $1\mu$m feature predicts both a specific timing for the feature emergence and its early spectral shape, including the very weak emission component observed at about 1.43 days. This empirically indicates a strong correspondence between the radiation temperature and the ejecta's electron temperature. Furthermore, this reverberation suggests that temporal modelling is important for interpreting individual spectra and that higher cadence spectral series, especially when concentrated at specific times, can provide strong constraints on KN line identifications and the ejecta physics. Given the use of such short-timescale information, we lay out improved observing strategies for future KN monitoring. [abridged]