Kilonova evolution – the rapid emergence of spectral features
arxiv(2023)
摘要
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 1μm 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]
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