Gravitational waves and mass ejecta from binary neutron star mergers: Effect of large eccentricities (1807.06857v2)

Abstract: As current gravitational wave (GW) detectors increase in sensitivity, and particularly as new instruments are being planned, there is the possibility that ground-based GW detectors will observe GWs from highly eccentric neutron star binaries. We present the first detailed study of highly eccentric BNS systems with full (3+1)D numerical relativity simulations using consistent initial conditions, i.e., setups which are in agreement with the Einstein equations and with the equations of general relativistic hydrodynamics in equilibrium. Overall, our simulations cover two different equations of state (EOSs), two different spin configurations, and three to four different initial eccentricities for each pairing of EOS and spin. We extract from the simulated waveforms the frequency of the f-mode oscillations induced during close encounters before the merger of the two stars. The extracted frequency is in good agreement with f-mode oscillations of individual stars for the irrotational cases, which allows an independent measure of the supranuclear equation of state not accessible for binaries on quasi-circular orbits. The energy stored in these f-mode oscillations can be as large as $10^{-3}M_\odot \sim 10^{51}$ erg, even with a soft EOS. In order to estimate the stored energy, we also examine the effects of mode mixing due to the stars' offset from the origin on the f-mode contribution to the GW signal. While in general (eccentric) neutron star mergers produce bright electromagnetic counterparts, we find that the luminosity decreases when the eccentricity becomes too large, due to a decrease of the ejecta mass. Finally, the use of consistent initial configurations also allows us to produce high-quality waveforms for different eccentricities which can be used as a testbed for waveform model development of highly eccentric binary neutron star systems.

• The paper demonstrates that f-mode oscillations in eccentric NS mergers match isolated neutron star frequencies, offering an independent probe of the dense matter equation of state.
• The paper quantifies energy storage up to 10^51 erg in f-mode oscillations and finds that increasing orbital eccentricity reduces both ejecta mass and luminosity.
• The paper provides high-quality gravitational wave waveforms from highly eccentric mergers, setting the stage for advanced modeling and enhanced multi-messenger astronomy.

Exploring Gravitational Waves from Highly Eccentric Binary Neutron Star Mergers

This paper presents a meticulous paper of gravitational waves (GWs) and mass ejecta resulting from binary neutron star (BNS) mergers that exhibit high eccentricities. The research leverages full 3D numerical relativity simulations to examine how varying eccentric orbits affect waveforms and mass emissions from BNS mergers, providing novel insights into this complex astrophysical phenomenon as GW detectors increase in sensitivity.

Simulation Methodology

The authors employ high-fidelity, (3+1)D numerical simulations consistent with the Einstein equations and general relativistic hydrodynamics to explore the effects of these eccentricities on the BNS systems. They investigate scenarios with two distinct equations of state (EOSs) and various spin configurations, applying three to four initial eccentricities per EOS-spin configuration pair. This comprehensive approach allows the paper of heretofore inaccessible states of supranuclear matter.

Key Findings

1. Frequency of f-mode Oscillations: The paper identifies that the frequencies of f-mode oscillations induced during close encounters display remarkable agreement with f-mode oscillations of individual neutron stars, particularly for irrotational cases. This finding provides an independent method to probe the supranuclear EOS of dense matter, which is otherwise challenging to measure in quasicircular orbits.
2. Energy and Ejecta Dynamics: The energy stored in the f-mode oscillations can reach magnitudes as large as $10^{-3}M_\odot \sim 10^{51}$ erg. The paper also meticulously quantifies the mode mixing due to positional offsets of the stars and its influence on GW signals. Notably, the paper reports a decrease in luminosity and ejecta mass with increasing eccentricity due to closer periapsis passages.
3. Implications for Waveform Modeling: The simulations yield high-quality waveforms for varying eccentricities, presenting an invaluable testbed for developing future waveform models of highly eccentric BNS systems. Currently, such explicit waveform models for highly eccentric BNS systems are not available, presenting an opportunity for further research and model development.

Theoretical and Practical Implications

The research contributes significantly to the theoretical understanding of BNS mergers, specifically addressing configurations and interactions that involve high eccentricity. The insights into frequency-domain characteristics could enhance multi-messenger astronomy by synergizing electromagnetic observations with GW data. Practically, this work informs the design and calibration of GW detectors poised to explore these interactions, such as the upcoming 3G detectors like the Einstein Telescope and Cosmic Explorer. These findings are crucial for refining GW models for precise source parameter estimation and potentially capturing GWs from systems on highly non-circular orbits.

Future Implications in AI and Data Analysis

The precision and complexity of the simulations illustrate the potential of leveraging advanced computational techniques and AI-enhanced analysis methods. As more data from GW detectors become available, AI models could optimize data processing pipelines to efficiently handle the voluminous, intricate data from such detailed simulations.

Additionally, future research could explore the use of machine learning-based frameworks to automate the derivation of EOS characteristics from GW signals—exploiting the unique signatures highlighted in this research. This opens a promising path wherein AI facilitates the direct extraction of physical phenomena parameters for dense astrophysical objects from high-dimensional observational data.

Conclusion

This research advances the understanding of the dynamics of highly eccentric BNS mergers, contributing to the body of knowledge essential for capturing and analyzing such mergers as GW detectors grow more sensitive. The synthesis of advanced numerical simulations with theoretical insights will serve as a foundation for enhancing GW astronomy and understanding neutron star physics under extreme conditions. Future research will undoubtedly expand upon these findings to further elucidate the complex nature of highly eccentric BNS mergers and their associated GM signatures.