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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
The electromagnetic transient following a binary neutron star merger is known
as a kilonova (KN). Owing to rapid expansion velocities and small ejecta
masses, KNe rapidly transition into the Non-Local Thermodynamic Equilibrium
(NLTE) regime. In this study, we present synthetic NLTE spectra of KNe from 5
to 20 days after merger using the \texttt{SUMO} spectral synthesis code. We
study three homogeneous composition, 1D multi-zone models with characteristic
electron fractions of $Y_e \sim 0.35, 0.25$ and $0.15$. We find that emission
features in the spectra tend to emerge in windows of reduced line blocking, as
the ejecta are still only partially transparent even at 20 days. For the $Y_e
\sim 0.35$ (lanthanide-free) ejecta, we find that the neutral and singly
ionised species of Rb, Sr, Y and Zr dominate the spectra, all with good
potential for identification. We directly test and confirm an impact of Sr on
the 10000 angstrom spectral region in lanthanide-free ejecta, but also see that
its signatures may be complex. We suggest the Rb I $\rm{5p^{1}}$- $\rm{5s^{1}}$
7900 angstrom transition as a candidate for the $\lambda_0 \sim$ 7500--7900
angstrom P-Cygni feature in AT2017gfo. For the $Y_e \sim 0.25$ and $0.15$
compositions, lanthanides are dominant in the spectral formation, in particular
Nd, Sm, and Dy. We identify key processes in KN spectral formation, notably
that scattering and fluorescence play important roles even up to 20 days after
merger, implying that the KN ejecta are not yet optically thin at this time.
