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Kilonovae (KNe) are one of the fastest types of optical transients known,cooling rapidly in the first few days following their neutron-star mergerorigin. We show here that KN spectral features go through rapid recombinationtransitions, with features due to elements in the new ionisation state emergingquickly. Due to time-delay effects of the rapidly-expanding KN, a 'wave' ofthese new features passing though the ejecta should be a detectable phenomenon.In particular, isolated line features will emerge as blueshifted absorptionfeatures first, gradually evolving into more pronounced absorption/emission PCygni features and then pure emission features. In this analysis we present theevolution of the individual exposures of the KN AT2017gfo observed withVLT/X-shooter that together comprise X-shooter's first epoch spectrum (1.43days post-merger). We show that the spectra of these 'sub-epochs' show asignificant evolution across the roughly one hour of observations, including adecrease of the blackbody temperature and photospheric velocity. The earlycooling is even more rapid than that inferred from later photospheric epochs,and suggest a fixed power-law relation between temperature and time cannotcapture the data. The cooling constrains the recombination-wave, where a Sr IIinterpretation of the AT2017gfo $\sim1 \mu$m feature predicts both a specifictiming for the feature emergence and its early spectral shape, including thevery weak emission component observed at 1.43 days. This reverberation analysissuggests that temporal modelling is important for interpreting individualspectra and that higher cadence spectral series, especially when concentratedat specific times, can provide strong constraints on KN line identifications.Given the use of such short-timescale information, we lay out improvedobserving strategies for future KN monitoring.

Kilonova evolution -- the rapid emergence of spectral features