NLTE spectra of kilonovae

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 d after merger using the sumo spectral synthesis code. We study three homogeneous composition, 1D multizone models with characteristic electron fractions of Ye ∼ 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 d. For the Ye ∼ 0.35 (lanthanide-free) ejecta, we find that the neutral and singly ionized 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 10 000 Å 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 Å transition as a candidate for the λ0 ∼ 7500–7900 Å P-Cygni feature in AT2017gfo. For the Ye ∼ 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 d after merger, implying that the KN ejecta are not yet optically thin at this time.

KilonovAE: Exploring Kilonova Spectral Features with Autoencoders

“…The kilonova associated with GW170817 (called AT2017gfo) was observed with photometry (Andreoni et al 2017;Arcavi et al 2017;Coulter et al 2017;Cowperthwaite et al 2017;Díaz et al 2017;Drout et al 2017;Evans et al 2017;Hu et al 2017;Kasliwal et al 2017Kasliwal et al , 2022Lipunov et al 2017;Soares-Santos et al 2017;Tanvir et al 2017;Troja et al 2017;Utsumi et al 2017;Valenti et al 2017;Villar et al 2018) and spectroscopy (Chornock et al 2017;Nicholl et al 2017;Pian et al 2017;Shappee et al 2017;Smartt et al 2017). Since then, significant attention has been devoted to inferring key properties of the merger ejecta by modeling the observed kilonova light curve (see, e.g., Chornock et al 2017;Cowperthwaite et al 2017;Drout et al 2017;Villar et al 2017;Waxman et al 2018;Kawaguchi et al 2020;Almualla et al 2021;Ristic et al 2022;Kedia et al 2023) and spectra (e.g., Kasen et al 2017;Watson et al 2019;Gillanders et al 2022Gillanders et al , 2023bHotokezaka et al 2023;Pognan et al 2023;Ristic et al 2023;Shingles et al 2023;Vieira et...…”

Targeted optimization in small-scale atomic structure calculations: application to Au I

“…SCI atomic structure models are of particular current interest as they are the predominant foundation in the atomic data used for computations of atomic processes and, ultimately, multielement modelling of r-process dominated kilonovae plasmas (e.g. Kasen et al 2017, Fontes et al 2020, Tanaka et al 2020, Pognan et al 2023.…”

“…resonance lines and thermodynamics in plasma modelling (e.g. in kilonova modelling; Pognan et al 2023).…”

On a Spectral Method for β-particle Bound Excitation Collisions in Kilonovae

“…While the basic picture of KNe has remained unchanged for several decades since the semi-analytic work of Li & Paczyński (1998; for a review of KN physics, see also Metzger 2019), simulating KNe at high fidelity is an ever-developing field. Recent studies have explored detailed radiative transfer, atomic physics, non-local thermodynamic equilibrium (non-LTE), and multidimensional spatial ejecta; see, for instance, Fontes et al (2020), Tanaka et al (2020), Fontes et al (2023) on detailed LTE opacity, Hotokezaka et al (2021), Pognan et al (2022a), Pognan et al (2022bPognan et al ( , 2023 on detailed non-LTE opacity, and Heinzel et al (2021), Korobkin et al (2021), Bulla (2023), and Fryer et al (2024) on spatial distribution and multidimensional spatial effects (and see references therein). Each of these aspects brings a level of uncertainty into the simulations that otherwise might be encapsulated in free parameters (for example, gray opacity).…”