Opacity of the Highly Ionized Lanthanides and the Effect on the Early Kilonova

Banerjee, Smaranika; Tanaka, Masaomi; Kato, Daiji; Gaigalas, Gediminas; Kawaguchi, Kyohei; Domoto, Nanae

doi:10.3847/1538-4357/ac7565

Cited by 30 publications

(37 citation statements)

References 79 publications

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“…8. The ejecta structure in our simulations suggests that in typical merger systems such as those investigated here, an opacity boost from highly ionized lanthanide nuclei arising at temperatures 70,000 K as recently reported by Banerjee et al (2022) is unlikely to affect the early UV/ optical emission on timescales ≈1 hr (Section 5.3, Figure 16). 9.…”

Section: Discussionsupporting

confidence: 75%

“…where a is the radiation constant, 〈A〉 is the average mass number as obtained from the nucleosynthesis calculations (Section 4), and m u is the atomic mass unit (see, also, Just et al 2022). In agreement with the model calculations of Banerjee et al (2022), we find a 70,000 K temperature regime at lowvelocity coordinates v ∼ 0.2c and early times 1 hr, in which high ionization states of lanthanides can be realized. However, the photosphere in our merger ejecta resides at much lower gas temperatures at all times, leading us to conclude that the opacity boost from highly ionized lanthanides is unlikely to significantly affect the early kilonova emission in practice.…”

Section: Impact Of Highly Ionized Lanthanides On Early Kilonova Emissionsupporting

confidence: 81%

“…Since we focus on the early light curve evolution from the outermost (dynamical) ejecta, the photospheric temperatures satisfy T > 4000-5000 K, at which opacities are largely insensitive to temperature (e.g., Kasen et al 2013;Banerjee et al 2020;Tanaka et al 2020). Recent opacity calculations by Banerjee et al (2022) found a strong enhancement in the opacity of lanthanide elements when highly ionized at temperatures ≈8 × 10 4 K. We discuss in Section 5.3 that this temperature regime is not of concern for the ejecta structure of our binary simulations presented here, as the photosphere never reaches this temperature regime. The opacity is largely controlled by the lanthanide content, which, in our scenario, mainly depends on the electron fraction of the ejecta.…”

Section: Kilonova Semianalytical Modelmentioning

confidence: 99%

“…Recent opacity calculations by Banerjee et al (2022) point out that high ionization states of lanthanide elements reached at temperatures ≈80,000 K can boost the opacity by a factor of 10-100, up to ≈10 3 cm 2 g −1 . If this temperature regime is realized in the early evolution of merger ejecta ∼1 hr that contains only a small fraction of lanthanides, Banerjee et al (2022) showed that the boosted opacity can significantly dim the early ∼1 hr kilonova light curve. This affecting, in particular, the early UV brightness is a potential concern for detection prospects of the neutron-precursor emission as discussed in the previous sections.…”

Section: Impact Of Highly Ionized Lanthanides On Early Kilonova Emissionmentioning

confidence: 99%

“…In Section 5, we present models of the kilonova emission including the contribution of free-neutron heating (Sections 5.1 and 5.2), of the nonthermal kilonova afterglow (Section 5.5), and discuss their application in the context of GW170817 (Sections 5.2.2 and 5.5.3). We also discuss the role of exceptionally high opacities associated with high ionization states of lanthanides recently calculated by Banerjee et al (2022) with respect to early kilonova and precursor emission (Section 5.3). We also compute high-energy gamma-ray emission from inverse-Compton and synchrotron self-Compton processes associated with the kilonova afterglow (Section 5.5.4).…”

Section: Introductionmentioning

confidence: 99%

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GRMHD Simulations of Neutron-star Mergers with Weak Interactions: r-process Nucleosynthesis and Electromagnetic Signatures of Dynamical Ejecta

Combi

Siegel

2023

ApJ

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Fast neutron-rich material ejected dynamically over ≲10 ms during the merger of a binary neutron star (BNS) can give rise to distinctive electromagnetic counterparts to the system’s gravitational-wave emission that serve as a “smoking gun” to distinguish between a BNS and an NS–black hole merger. We present novel ab initio modeling of the kilonova precursor and kilonova afterglow based on 3D general-relativistic magnetohydrodynamic simulations of BNS mergers with nuclear, tabulated, finite-temperature equations of state (EOSs), weak interactions, and approximate neutrino transport. We analyze dynamical mass ejection from 1.35–1.35 M ⊙ binaries, consistent with properties of the first observed BNS merger GW170817, using three nuclear EOSs that span the range of allowed compactness of 1.35 M ⊙-neutron stars. Nuclear reaction network calculations yield a robust second-to-third-peak r-process. We find few ×10−6 M ⊙ of fast (v > 0.6c) ejecta that give rise to broadband synchrotron emission on ∼years timescales, consistent with tentative evidence for excess X-ray/radio emission following GW170817. We find ≈2 × 10−5 M ⊙ of free neutrons that power a kilonova precursor on ≲ hours timescale. A boost in early UV/optical brightness by a factor of a few due to previously neglected relativistic effects, with enhancements up to ≲10 hr post-merger, is promising for future detection with UV/optical telescopes like Swift or ULTRASAT. We find that a recently predicted opacity boost due to highly ionized lanthanides at ≳70,000 K is unlikely to affect the early kilonova based on the obtained ejecta structures. Azimuthal inhomogeneities in dynamical ejecta composition for soft EOSs found here (“lanthanide/actinide pockets”) may have observable consequences for both early kilonova and late-time nebular emission.

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Section: Discussionsupporting

confidence: 75%

Section: Impact Of Highly Ionized Lanthanides On Early Kilonova Emissionsupporting

confidence: 81%

Section: Kilonova Semianalytical Modelmentioning

confidence: 99%

Section: Impact Of Highly Ionized Lanthanides On Early Kilonova Emissionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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GRMHD Simulations of Neutron-star Mergers with Weak Interactions: r-process Nucleosynthesis and Electromagnetic Signatures of Dynamical Ejecta

Combi

Siegel

2023

ApJ

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Atomic data and expansion opacity calculations in two representative 4d transition elements, niobium and silver, of interest for kilonovae studies

Ben Nasr,

Carvajal Gallego,

Deprince

et al. 2023

A&A

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Aims. Neutron star (NS) mergers are thought to be a source of heavy trans-iron element production. The latter can be detected in the spectra of the ejected materials, from which bright electromagnetic radiation is emitted. This latter is due to the radioactive decay of the produced heavy r-process nuclei and is known as kilonova. Because of their complex atomic structures – characterized by configurations involving unfilled nd or nf subshells – the heavy elements of the kilonova ejecta often give rise to numerous absorption lines generating significant opacities. The determination of the latter, which are of paramount importance for the analysis of kilonova light curves, requires knowledge of the radiative parameters of the spectral lines belonging to the ions expected to be present in the kilonova ejecta. The aim of the present work is to provide new atomic opacity data for two representative 4d elements, niobium (Nb) and silver (Ag), in their first four charge states, namely for Nb I–IV and Ag I–IV. Methods. Large-scale calculations based on the pseudo-relativistic Hartree-Fock (HFR) method were performed to obtain the atomic structure and radiative parameters while the expansion formalism was used to estimate the opacities. Results. Wavelengths and oscillator strengths were computed for several million spectral lines in Nb I–IV and Ag I–IV ions. The reliability of these parameters was estimated by comparison with the few previously published experimental and theoretical results. The newly obtained atomic data were then used to calculate expansion opacities for typical kilonova conditions expected one day after NS merger, a density of ρ = 10−13 g cm−3, and temperatures ranging from T = 5000 K to T =15 000 K.

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Overview of the contributions from all lanthanide elements to kilonova opacity in the temperature range from 25 000 to 40 000 K

Carvajal Gallego,

Deprince,

Maison

et al. 2024

A&A

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It is now well established that the neutron star (NS) merger is at the origin of the production of trans-iron heavy elements in the universe. These elements are therefore present in large quantities in the ejected matter, whose electromagnetic radiation, called kilonova, is characterized by a significant opacity due to the high density of spectral lines belonging to many heavy ions. Among these, the lanthanide ions play an essential role since, with their open 4f subshell, they have a considerable number of transitions that can absorb emitted light. The knowledge of the atomic structure and the radiative parameters of these ions as well as the determination of the corresponding opacities is therefore of paramount importance for the spectral analysis of kilonovae. The main goal of the present work is to determine the relative contributions of the different lanthanide elements to the opacity of the emission spectrum of a kilonova in its early phase, that is, a few hours after the NS merger, where the conditions are such that the temperature is between 25000 and 40000 K. At these temperatures, the lanthanide ions whose charge states are between V and VII are predominant. We used the pseudo-relativistic Hartree-Fock (HFR) method extensively to calculate the relevant atomic data (energy levels, wavelengths, and oscillator strengths) in La-Lu V-VII ions. The corresponding monochromatic opacities were estimated from the expansion formalism. We calculated the spectroscopic parameters for a total of more than 800 million radiative transitions in all the ions considered. These data were used to estimate the expansion opacities and Planck mean opacities for all the lanthanide elements at early-phase kilonova conditions between 25000 and 40000 K, making it possible to deduce the respective contributions of each element as a function of temperature. Atomic calculations were also carried out with the fully relativistic Multiconfiguration Dirac-Hartree-Fock (MCDHF) method in the specific case of the Yb V ion, as the available experimental data had not yet been compared with the theoretical calculations in our previous studies on lanthanide ions.

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Opacity of the Highly Ionized Lanthanides and the Effect on the Early Kilonova

Cited by 30 publications

References 79 publications

GRMHD Simulations of Neutron-star Mergers with Weak Interactions: r-process Nucleosynthesis and Electromagnetic Signatures of Dynamical Ejecta

GRMHD Simulations of Neutron-star Mergers with Weak Interactions: r-process Nucleosynthesis and Electromagnetic Signatures of Dynamical Ejecta

Atomic data and expansion opacity calculations in two representative 4d transition elements, niobium and silver, of interest for kilonovae studies

Overview of the contributions from all lanthanide elements to kilonova opacity in the temperature range from 25 000 to 40 000 K

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