Binary neutron-star mergers: a review of Einstein's richest laboratory (1607.03540v2)

Abstract: The merger of binary neutron-stars systems combines in a single process: extreme gravity, copious emission of gravitational waves, complex microphysics, and electromagnetic processes that can lead to astrophysical signatures observable at the largest redshifts. We review here the recent progress in understanding what could be considered Einstein's richest laboratory, highlighting in particular the numerous significant advances of the last decade. Although special attention is paid to the status of models, techniques, and results for fully general-relativistic dynamical simulations, a review is also offered on initial data and advanced simulations with approximate treatments of gravity. Finally, we review the considerable amount of work carried out on the post-merger phase, including: black-hole formation, torus accretion onto the merged compact object, connection with gamma-ray burst engines, ejected material, and its nucleosynthesis.

• The paper presents a comprehensive review of binary neutron-star mergers, emphasizing gravitational-wave signatures and simulations that constrain the neutron-star equation of state.
• It details the methodology behind modeling inspiral dynamics, post-merger outcomes, and the formation of hypermassive neutron stars or black holes with associated accretion processes.
• It establishes key implications for multi-messenger astronomy by correlating gravitational waves and electromagnetic emissions to advance our understanding of extreme matter states.

Understanding Binary Neutron-Star Mergers: Insights from a Comprehensive Review

Binary neutron-star (BNS) mergers present an intricate tapestry of physical phenomena, offering critical insights at the intersection of astrophysics, nuclear physics, and general relativity. Luca Baiotti and Luciano Rezzolla meticulously explore these mergers, often dubbed "Einstein's richest laboratory," presenting a synthesized review of significant advancements within the past decade.

Key Highlights and Numerical Insights

The authors begin by describing BNS mergers' multifaceted nature, which encapsulates extreme gravitational fields, high-energy electromagnetic emissions, and complex microphysics, all occurring alongside copious gravitational wave emission. The review provides a detailed analysis of fully general-relativistic dynamical simulations, discussing diverse facets such as initial data, simulation techniques, and post-merger dynamics, including black-hole formation and torus accretion.

Inspiral and Gravitational-Wave Emission

The paper emphasizes the crucial role of gravitational wave astronomy in observing BNS mergers, aiding in discerning the equation of state (EOS) of neutron-star matter. Gravitational waves emitted during inspiral phases offer several measurable parameters like the tidal deformability, which links back to neutron-star properties, significantly informing us about the dense matter EOS. Numerical results suggest precise EOS dependences in the inspiral gravitational-wave signal, highlighting that measurement of these parameters with advanced detectors could constrain neutron star radius to 10% accuracy.

Post-Merger Dynamics and Black Hole Formation

Post-merger dynamics are particularly complex, with several pathways involving the formation of hypermassive neutron stars (HMNSs) or direct black hole formation depending on the initial mass and EOS. The authors report that the gravitational waveform spectrum post-merger carries distinct peaks that could potentially unlock detailed EOS characteristics of the merging stars. Advanced numerical simulations have confirmed that a hot accretion torus often remains, with potential to further gravitational wave, neutrino, and electromagnetic emissions—elements valuable for multi-messenger astronomy.

Implications and Future Prospects

Baiotti and Rezzolla extend their discussion to the broader implications of BNS mergers. They explore the prospects for detecting electromagnetic counterparts, which, combined with gravitational waves, could offer unparalleled insights into SGRB production and heavy element nucleosynthesis through rapid neutron capture processes (r-process). Additionally, the interplay between magnetic fields and differential rotation presents a mechanism to drive relativistic jets, although achieving ultrarelativistic speeds seen in SGRBs remains a complex challenge not yet resolved definitively in simulations.

The review speculates on future advancements, particularly concerning the precision of EOS constraints and the lifetime of HMNSs pre-collapse, which still present notable uncertainties. Further improvements in numerical techniques, including high-order schemes and sub-grid modeling for magnetic field dynamics, are likely pivotal in enhancing simulation accuracy.

Conclusion

Binary neutron-star mergers embody a pinnacle of complexity in astrophysics, demanding sophisticated numerical and analytical methods to unravel their secrets. This review by Baiotti and Rezzolla compiles extensive advances, underscoring both our current understanding and the gaps that drive future research. As we stand at the brink of enhanced observational capabilities via next-generation gravitational-wave detectors, the synthesis of theoretical prediction and observational data promises to unlock new frontiers in our understanding of the universe's most extreme matter states.