Gravitational-Wave Luminosity of Binary Neutron Stars Mergers

Constraining the equation of state in neutron-star cores via the long-ringdown signal

Multimessenger signals from binary neutron star (BNS) mergers are promising tools to infer the properties of nuclear matter at densities inaccessible to laboratory experiments. Gravitational waves (GWs) from BNS merger remnants can constrain the neutron-star equation of state (EOS) complementing constraints from late inspiral, direct mass-radius measurements, and ab-initio calculations. We perform a series of general-relativistic simulations of BNS systems with EOSs constructed to comprehensively cover the high-density regime. We identify a tight correlation between the ratio of the energy and angular-momentum losses in the late-time portion of the post-merger signal, called the long ringdown, and the EOS at the highest pressures and densities in neutron-star cores. Applying this correlation to post-merger GW signals significantly reduces EOS uncertainty at densities several times the nuclear saturation density, where no direct constraints are currently available. Hence, the long ringdown can provide stringent constraints on material properties of neutron stars cores.

Constraints on the Maximum Densities of Neutron Stars from Postmerger Gravitational Waves with Third-Generation Observations

Using data from 289 numerical relativity simulations of binary neutron star mergers, we identify, for the first time, a robust quasiuniversal relation connecting the postmerger peak gravitational-wave frequency and the value of the density at the center of the maximum mass nonrotating neutron star. This relation offers a new possibility for precision equation-of-state constraints with next-generation ground-based gravitational-wave interferometers. Mock Einstein Telescope observations of fiducial events indicate that Bayesian inferences can constrain the maximum density to ∼15% (90% credibility level) for a single signal at the minimum sensitivity threshold for a detection. If the postmerger signal is included in a full-spectrum (inspiral-merger-postmerger) analysis of such a signal, the pressure-density function can be tightly constrained up to the maximum density, and the maximum neutron star mass can be measured with an accuracy better than 12% (90% credibility level).

NICER view on holographic QCD

The holographic models for dense QCD matter work surprisingly well. A general implication seems that the deconfinement phase transition dictates the maximum mass of neutron stars. The nuclear matter phase turns out to be rather stiff which, if continuously merged with nuclear matter models based on effective field theories, leads to the conclusion that neutron stars do not have quark matter cores in the light of all current astrophysical data. We comment that as the perturbative QCD results are in stark contrast with strong coupling results, any future simulations of neutron star mergers incorporating corrections beyond ideal fluid should proceed cautiously. For this purpose, we provide a model which treats nuclear and quark matter phases in a unified framework at strong coupling.

Neutron Star Mergers and the Quark Matter Equation of State

As neutron stars merge they can approach very high nuclear density. Here, we summarized recent results for the evolution and gravitational wave emission from binary-neutron star mergers using a a variety of nuclear equations of state with and without a crossover transition to quark matter. We discuss how the late time gravitational wave emission from binary neutron star mergers may possibly reveal the existence of a crossover transition to quark matter.

Neutron star merger remnants

Binary neutron star mergers observations are a unique way to constrain fundamental physics and astrophysics at the extreme. The interpretation of gravitational-wave events and their electromagnetic counterparts crucially relies on general-relativistic models of the merger remnants. Quantitative models can be obtained only by means of numerical relativity simulations in 3 + 1 dimensions including detailed input physics for the nuclear matter, electromagnetic and weak interactions. This review summarizes the current understanding of merger remnants focusing on some of the aspects that are relevant for multimessenger observations.

Equation of State Constraints from the Threshold Binary Mass for Prompt Collapse of Neutron Star Mergers

Using hydrodynamical simulations for a large set of high-density matter equations of state (EOSs), we systematically determine the threshold mass M_{thres} for prompt black-hole formation in equal-mass and asymmetric neutron star (NS) mergers. We devise the so far most direct, general, and accurate method to determine the unknown maximum mass of nonrotating NSs from merger observations revealing M_{thres}. Considering hybrid EOSs with hadron-quark phase transition, we identify a new, observable signature of quark matter in NS mergers. Furthermore, our findings have direct applications in gravitational wave searches, kilonova interpretations, and multimessenger constraints on NS properties.

Binary Neutron Star Merger Simulations with a Calibrated Turbulence Model

Magnetohydrodynamic (MHD) turbulence in neutron star (NS) merger remnants can impact their evolution and multi-messenger signatures, complicating the interpretation of present and future observations. Due to the high Reynolds numbers and the large computational costs of numerical relativity simulations, resolving all the relevant scales of the turbulence will be impossible for the foreseeable future. Here, we adopt a method to include subgrid-scale turbulence in moderate resolution simulations by extending the large-eddy simulation (LES) method to general relativity (GR). We calibrate our subgrid turbulence model with results from very-high-resolution GRMHD simulations, and we use it to perform NS merger simulations and study the impact of turbulence. We find that turbulence has a quantitative, but not qualitative, impact on the evolution of NS merger remnants, on their gravitational wave signatures, and on the outflows generated in binary NS mergers. Our approach provides a viable path to quantify uncertainties due to turbulence in NS mergers.

Black-Hole Remnants from Black-Hole-Neutron-Star Mergers

Observations of gravitational waves and their electromagnetic counterparts may soon uncover the existence of coalescing compact binary systems formed by a stellar-mass black hole and a neutron star. These mergers result in a remnant black hole, possibly surrounded by an accretion disk. The mass and spin of the remnant black hole depend on the properties of the coalescing binary. We construct a map from the binary components to the remnant black hole using a sample of numerical-relativity simulations of different mass ratios q, (anti)aligned dimensionless spins of the black hole a_{BH}, and several neutron star equations of state. Given the binary total mass, the mass and spin of the remnant black hole can therefore be determined from the three parameters (q,a_{BH},Λ), where Λ is the tidal deformability of the neutron star. Our models also incorporate the binary black hole and test-mass limit cases and we discuss a simple extension for generic black-hole spins. We combine the remnant characterization with recent population synthesis simulations for various metallicities of the progenitor stars that generated the binary system. We predict that black-hole-neutron-star mergers produce a population of remnant black holes with masses distributed around 7  M_{⊙} and 9  M_{⊙}. For isotropic spin distributions, nonmassive accretion disks are favored: no bright electromagnetic counterparts are expected in such mergers.