Emergence hour-by-hour of r-process features in the kilonova AT2017gfo
arxiv(2024)
摘要
The spectral features in the optical/near-infrared counterparts of neutron
star mergers (kilonovae, KNe), evolve dramatically on hour timescales. To
examine the spectral evolution we compile a temporal series complete at all
observed epochs from 0.5 to 9.4 days of the best optical/near-infrared (NIR)
spectra of the gravitational-wave detected kilonova AT2017gfo. Using our
analysis of this spectral series, we show that the emergence times of spectral
features place strong constraints on line identifications and ejecta
properties, while their subsequent evolution probes the structure of the
ejecta. We find that the most prominent spectral feature, the 1μm
P Cygni line, appears suddenly, with the earliest detection at 1.17 days. We
find evidence in this earliest feature for the fastest kilonova ejecta
component yet discovered, at 0.40-0.45c; while across the observed epochs and
wavelengths, the velocities of the line-forming regions span nearly an order of
magnitude, down to as low as 0.04-0.07c. The time of emergence closely
follows the predictions for Sr II, due to the rapid recombination of Sr III
under local thermal equilibrium (LTE) conditions. The time of transition
between the doubly and singly ionised states provides the first direct
measurement of the ionisation temperature, This temperature is highly
consistent, at the level of a few percent, with the temperature of the emitted
blackbody radiation field. Further, we find the KN to be isotropic in
temperature, i.e. the polar and equatorial ejecta differ by less than a few
hundred Kelvin or within 5
measurements of the reverberation time-delay effect. This suggests that a model
with very simple assumptions, with single-temperature LTE conditions,
reproduces the early kilonova properties surprisingly well.
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