Binary Neutron Star Mergers: Mass Ejection, Electromagnetic Counterparts and Nucleosynthesis (1809.11161v3)

Abstract: We present a systematic numerical relativity study of the mass ejection and the associated electromagnetic transients and nucleosynthesis from binary neutron star (NS) mergers. We find that a few $10^{-3}\, M_\odot$ of material are ejected dynamically during the mergers. The amount and the properties of these outflow depend on binary parameters and on the NS equation of state (EOS). A small fraction of these ejecta, typically ${\sim}10^{-6}\, M_\odot$, is accelerated by shocks formed shortly after merger to velocities larger than $0.6\, {\rm c}$ and produces bright radio flares on timescales of weeks, months, or years after merger. Their observation could constrain the strength with which the NSs bounce after merger and, consequently, the EOS of matter at extreme densities. The dynamical ejecta robustly produce second and third $r$-process peak nuclei with relative isotopic abundances close to solar. The production of light $r$-process elements is instead sensitive to the binary mass ratio and the neutrino radiation treatment. Accretion disks of up to ${\sim}0.2\, M_\odot$ are formed after merger, depending on the lifetime of the remnant. In most cases, neutrino- and viscously-driven winds from these disks dominate the overall outflow. Finally, we generate synthetic kilonova light curves and find that kilonovae depend on the merger outcome and could be used to constrain the NS EOS.

• The paper presents an extensive numerical study using 59 high-resolution simulations to provide detailed insights into mass ejection, electromagnetic counterparts, and nucleosynthesis during binary neutron star mergers.
• Key findings show dynamical ejecta are neutron-rich ($Y_e < 0.25$), exhibit broad angular distributions, and robustly produce heavy r-process elements, while secular ejecta also contribute significantly to kilonova signals.
• The research underscores the potential for using kilonova observations to constrain the neutron star equation of state and confirms binary neutron star mergers as major contributors to the cosmic inventory of heavy r-process elements.

Overview of "Binary Neutron Star Mergers: Mass Ejection, Electromagnetic Counterparts, and Nucleosynthesis"

This paper presents an extensive numerical relativity paper focusing on binary neutron star (NS) mergers. The research addresses crucial aspects such as mass ejection, electromagnetic (EM) counterparts, and nucleosynthesis. The authors employ a suite of 59 high-resolution simulations incorporating a range of NS equations of state (EOS) and treatments for neutrino effects. The primary focus centers on understanding the mass ejection dynamics and the resultant signatures that these mergers produce, both in terms of observable EM transients and nucleosynthetic yields. The systematic approach taken in this paper involves full general-relativistic hydrodynamical simulations, shedding light on various outcomes based on initial conditions like total binary mass, mass ratios, and EOS.

Mass Ejection

The mass ejected during a binary neutron star merger is categorized into primarily dynamical and secular ejecta:

• Dynamical Ejecta: This component is ejected on a timescale of milliseconds around the merger and is strongly influenced by factors such as binary parameters and EOS. The paper finds that the dynamical ejecta are generally neutron-rich, with an average electron fraction, $Y_e$, well below 0.25. Concerning its spatial distribution, the dynamical ejecta exhibit broad angular distributions and tend to produce robust $r$-process nucleosynthetic yields, primarily second and third peak elements, while the production of light $r$-process elements is sensitive to various parameters.
• Secular Ejecta: Resulting from longer-term processes such as disk winds driven by neutrino and viscous interactions, the secular ejecta become dominant especially in merger remnants that do not immediately form a black hole (BH). This component is noted for potentially contributing significantly to the observed kilonova signatures.

Numerical Methods and Simulation Design

The simulations explore the effects of different EOSs, thermal transport effects, and the role of neutrino physics on ejecta composition. The authors utilize high-resolution shock-capturing schemes incorporating microphysical EOS and treatments for general-relativistic magnetohydrodynamics (GRMHD), thus enabling more comprehensive modeling of post-merger configurations. The paper accounts for neutrino losses and utilizes state-of-the-art leakage schemes for modeling intricate matter-radiation interactions.

Electromagnetic Counterparts

The paper explores kilonovae, the EM counterparts powered by the radioactive decay of freshly synthesized material in merger ejecta. It proposes that kilonova light curves depend significantly on the merger remnants' lifetimes and binary parameters. The comprehensive calculations indicate kilonovae could serve as observational probes for constraining the EOS by examining their color evolution and peak luminosities.

Additionally, an outlook for synchrotron radiation, which could emerge from the interaction of fast ejecta components with the interstellar medium (ISM), is given. The paper suggests that observing these signals in the radio and X-ray could provide insights into the fast ejecta's properties and thus indirect information on the EOS of neutron star matter.

Nucleosynthesis

The robust production of heavy $r$-process elements is a haLLMark conclusion, with these mergers confirmed as significant contributors to the cosmic inventory of such elements. The dependence of the nucleosynthetic yields on the binary's mass ratio, total mass, and the properties of the EOS is critically examined. Uncertainties due to neutrino absorption mechanisms underscore the need for further detailed spectral treatment in future simulations.

Implications and Future Directions

Overall, this paper provides comprehensive insights into the dynamics of binary neutron star mergers, emphasizing the role of neutron-rich ejecta in both electromagnetic signals and nucleosynthesis. It underscores the potential to leverage EM observations to constrain the NS EOS, with implications reaching to the modeling of kilonovae and further observables in multi-messenger astrophysics.

Future developments in AI-algorithmic approaches could further refine the estimates on ejecta properties and transient evolution, potentially aiding in more robust predictions and analyses of NS mergers. Enhanced computational techniques might also allow higher fidelity studies that resolve the noted discrepancies in dynamical ejecta mass and yield further insights into multi-physics interactions during and after NS mergers. The advancement in GRMHD simulations could, therefore, provide a richer understanding of the processes that produce observable astrophysical phenomena following these cosmic events.