Kilonova from post-merger ejecta as an optical and near-infrared counterpart of GW170817

Abstract: Recent detection of gravitational waves from a neutron star (NS) merger event GW170817 and identification of an electromagnetic counterpart provide a unique opportunity to study the physical processes in NS mergers. To derive properties of ejected material from the NS merger, we perform radiative transfer simulations of kilonova, optical and near-infrared emissions powered by radioactive decays of r-process nuclei synthesized in the merger. We find that the observed near-infrared emission lasting for > 10 days is explained by 0.03 Msun of ejecta containing lanthanide elements. However, the blue optical component observed at the initial phases requires an ejecta component with a relatively high electron fraction (Ye). We show that both optical and near-infrared emissions are simultaneously reproduced by the ejecta with a medium Ye of ~ 0.25. We suggest that a dominant component powering the emission is post-merger ejecta, which exhibits that mass ejection after the first dynamical ejection is quite efficient. Our results indicate that NS mergers synthesize a wide range of r-process elements and strengthen the hypothesis that NS mergers are the origin of r-process elements in the Universe.

• The paper demonstrates that radiative transfer simulations reconcile kilonova optical and near-infrared observations with r-process decay models.
• It shows that a post-merger ejecta mass of 0.03 solar masses with a medium electron fraction (Yₑ ≈ 0.25) reproduces the observed prolonged near-infrared emission.
• Findings underline the crucial impact of ejecta composition and lanthanide-driven opacity in producing both early blue and late infrared light curve features.

Analysis of Kilonova Emissions from Post-Merger Ejecta in GW170817

The paper under scrutiny presents a meticulous investigation into the kilonova emissions observed as an optical and near-infrared counterpart to the neutron star merger event GW170817. This analysis is achieved through radiative transfer simulations aimed at understanding the properties of material ejected post-merger. The primary focus is to reconcile the electromagnetic observations with theoretical models of kilonovae powered by the radioactive decay of rapid neutron capture (r-process) nuclei.

Observational Context and Methodology

GW170817 provided a rare multi-messenger astronomical observation opportunity, combining gravitational wave (GW) detection with a wide range of electromagnetic (EM) follow-ups, including optical and near-infrared wavelengths. The paper identifies the kilonova, labeled SSS17a/DLT17ck in galaxy NGC 4993, as a result of r-process nucleosynthesis in the post-merger ejecta. The radiative transfer simulations conducted utilized abundance patterns indicative of r-process nuclei and took into account the opacity effects predominantly arising from lanthanide elements.

Key Findings

1. Ejecta Characterization: The simulations indicate that an ejecta mass of 0.03 solar masses with a medium electron fraction (Y_e ~ 0.25) reproduces the observed near-infrared light curves extending beyond 10 days post-merger. The presence of lanthanide elements within this emixture is critical in achieving the observed opacity-driven light curve characteristics.
2. Emission Spectrum: The early blue optical component observed required an ejecta component with relatively high electron fractions to explain the lower opacity and resultant spectrum. This highlights the need for a stratified ejecta composition and variability in electron fraction across the ejecta, which was addressed within the simulated models.
3. Post-Merger Ejecta Dominance: The results point towards a significant post-merger ejecta contribution, which appears to be more efficient than previously expected. Specifically, factors such as viscous heating and neutrino irradiation are indicated as integral in setting the ejecta's electron fraction and enabling the observed mass ejection efficiency.

Implications and Future Prospects

This paper substantiates the hypothesis that neutron star mergers are pivotal sites for the synthesis of r-process elements, a significant implication for the field of nuclear astrophysics. It signifies that mergers can produce a wide spectrum of these elements, corroborating observed Galactic abundance patterns.

The theoretical implications are substantial, suggesting revisions in the treatment of post-merger nucleosynthesis processes and the need for further in-depth simulations. Observationally, this calls for future kilonova observations to focus on detailed spectral analyses and multi-band follow-ups to further constrain ejection mechanisms and opacities.

Continued development of radiative transfer models, incorporating more sophisticated treatments of ejecta composition and opacity, will enhance our understanding of the nuanced processes governing these enigmatic astronomical events. The observational benchmarks set by GW170817 serve as a guidepost for future detections, driving a more comprehensive synthesis of theoretical and empirical astrophysics.