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

Abstract

We investigate the effect of the presence of lanthanides (Z = 57–71) on the kilonova at t ∼ 1 hr after the neutron star merger for the first time. For this purpose, we calculate the atomic structures and the opacities for selected lanthanides: Nd (Z = 60), Sm (Z = 62), and Eu (Z = 63). We consider the ionization degree up to 10th (XI), applicable for the ejecta at t ∼ a few hours after the merger, when the temperature is T ∼ 105 K. We find that the opacities for the highly ionized lanthanides are exceptionally high, reaching κ exp ∼ 1000 cm 2 g − 1 for Eu, due to the highly dense energy levels. Using the new opacity, we perform radiative transfer simulations to show that the early light curves become fainter by a (maximum) factor of four, in comparison to lanthanide-free ejecta at t ∼ 0.1 days. However, the period at which the light curves are affected is relatively brief owing to the rapid time evolution of the opacity in the outermost layer of the ejecta. We predict that for a source at a distance of ∼100 Mpc, UV brightness for lanthanide-rich ejecta shows a drop to ∼21–22 mag at t ∼ 0.1 days and the UV peaks around t ∼ 0.2 days with a magnitude of ∼19 mag. Future detection of such a kilonova by an existing UV satellite like Swift or the upcoming UV satellite ULTRASAT will provide useful constraints on the abundance in the outer ejecta and the corresponding nucleosynthesis conditions in the neutron star mergers.

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

“…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.…”

“…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.…”

“…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.…”

“…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.…”

“…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).…”