Simulations Of Binary Neutron Stars Research Articles

Hybrid approach to long-term binary neutron-star simulations

Hybrid approach to long-term binary neutron-star simulations

Formula omitted]-mode oscillations of compact stars with realistic equations of state in dynamical spacetime

In this study, we perform full three dimensional numerical relativity simulations of non-rotating general relativistic stars. Extending the studies for polytropic equation of state, we investigate the accuracy and robustness of numerical scheme on measuring fundamental (f)-mode frequency for realistic equations of state (EoS). We use various EoS with varying range of stiffness and numerically evolve perturbed stellar models for several mass configurations (in the range of 1.2−2.0M⊙) for each of these EoS. Using the gravitational waveform obtained from the simulations we extract the f-modes of the stars. The obtained results are tested against the pre-existing perturbation methods and find good agreement. Validity and deviation of universal relations have been carried out and are compared with earlier results under Cowling approximation as well as perturbative approaches. We also show that even using perturbed single star simulations can provide good agreement with f-modes extracted from inspiral phase of binary neutron star simulations which are computationally more expensive.

Handing off the outcome of binary neutron star mergers for accurate and long-term postmerger simulations

We perform binary neutron star (BNS) merger simulations in full dynamical general relativity with IllinoisGRMHD, on a Cartesian grid with adaptive-mesh refinement. After the remnant black hole has become nearly stationary, the evolution of the surrounding accretion disk on Cartesian grids over long timescales (1s) is suboptimal, as Cartesian coordinates over-resolve the angular coordinates at large distances, and the accreting plasma flows obliquely across coordinate lines dissipating angular momentum artificially from the disk. To address this, we present the Handoff, a set of computational tools that enables the transfer of general relativistic magnetohydrodynamic (GRMHD) and spacetime data from IllinoisGRMHD to HARM3D, a GRMHD code that specializes in modeling black hole accretion disks in static spacetimes over long timescales, making use of general coordinate systems with spherical topology. We demonstrate that the Handoff allows for a smooth and reliable transition of GRMHD fields and spacetime data, enabling us to efficiently and reliably evolve BNS dynamics well beyond merger. We also discuss future plans, which involve incorporating advanced equations of state and neutrino physics into BNS simulations using the \handoff approach.

Numerical Simulations of Dark Matter Admixed Neutron Star Binaries

Multi-messenger observations of compact binary mergers provide a new way to constrain the nature of dark matter that may accumulate in and around neutron stars. In this article, we extend the infrastructure of our numerical-relativity code BAM to enable the simulation of neutron stars that contain an additional mirror dark matter component. We perform single star tests to verify our code and the first binary neutron star simulations of this kind. We find that the presence of dark matter reduces the lifetime of the merger remnant and favors a prompt collapse to a black hole. Furthermore, we find differences in the merger time for systems with the same total mass and mass ratio, but different amounts of dark matter. Finally, we find that electromagnetic signals produced by the merger of binary neutron stars admixed with dark matter are very unlikely to be as bright as their dark matter-free counterparts. Given the increased sensitivity of multi-messenger facilities, our analysis gives a new perspective on how to probe the presence of dark matter.

Can a binary neutron star merger in the vicinity of a supermassive black hole enable a detection of a post-merger gravitational wave signal?

ABSTRACT The post-merger gravitational-wave (GW) signal of a binary neutron star (BNS) merger is expected to contain valuable information that could shed light on the equation of state (EOS) of NSs, the properties of the matter produced during the merger, as well as the nature of any potential intermediate merger product such as hypermassive or supramassive NSs. However, the post-merger lies in the high frequency regime (≳1000 Hz) where current LIGO-Virgo detectors are insensitive. While proposed detectors such as NEMO, Cosmic Explorer and Einstein Telescope could potentially detect the post-merger for BNSs within $\mathcal {O}(10~\mathrm{Mpc})$, such events are likely to be rare. In this work, we speculate on the possibility of detecting the post-merger from BNSs coalescing in the vicinity of supermassive black holes (SMBHs). The redshift produced by the gravitational field of the SMBH, as well as the BNS’s proper motion around the SMBH, could effectively ‘stretch’ the post-merger signal into the band of the detectors. We demonstrate, using a phenomenological model, that such BNS coalescences would enable constraints on the peak of the post-merger signal that would otherwise have not been possible, provided the degree of redshifting due to the SMBH can be independently acquired. Further, using numerical simulations of binary neutron stars, we show how such mergers would improve EOS model selection using the post-merger signal. We discuss the mechanisms that might deliver such events and the limitations of this work.

High-accuracy simulations of highly spinning binary neutron star systems

With an increasing number of expected gravitational-wave detections of binary neutron star mergers, it is essential that gravitational-wave models employed for the analysis of observational data are able to describe generic compact binary systems. This includes systems in which the individual neutron stars are millisecond pulsars for which spin effects become essential. In this work, we perform numerical-relativity simulations of binary neutron stars with aligned and anti-aligned spins within a range of dimensionless spins of $\chi \sim [-0.28,0.58]$. The simulations are performed with multiple resolutions, show a clear convergence order and, consequently, can be used to test existing waveform approximants. We find that for very high spins gravitational-wave models that have been employed for the interpretation of GW170817 and GW190425 are not capable of describing our numerical-relativity dataset. We verify through a full parameter estimation study in which clear biases in the estimate of the tidal deformability and effective spin are present. We hope that in preparation of the next gravitational-wave observing run of the Advanced LIGO and Advanced Virgo detectors our new set of numerical-relativity data can be used to support future developments of new gravitational-wave models.

Jet launching from merging magnetized binary neutron stars with realistic equations of state

We perform general relativistic, magnetohydrodynamic (GRMHD) simulations of binary neutron stars in quasi-circular orbit that merge and undergo delayed or prompt collapse to a black hole (BH). The stars are irrotational and modeled using an SLy or an H4 nuclear equation of state. To assess the impact of the initial magnetic field configuration on jet launching, we endow the stars with a purely poloidal magnetic field that is initially unimportant dynamically and is either confined to the stellar interior or extends from the interior into the exterior as in typical pulsars. Consistent with our previous results, we find that only the BH + disk remnants originating from binaries that form hypermassive neutron stars (HMNSs) and undergo delayed collapse can drive magnetically-powered jets. We find that the closer the total mass of the binary is to the threshold value for prompt collapse, the shorter is the time delay between the gravitational wave peak amplitude and jet launching. This time delay also strongly depends on the initial magnetic field configuration. We also find that seed magnetic fields confined to the stellar interior can launch a jet over $\sim 25\,\rm ms$ later than those with pulsar-like magnetic fields. The lifetime of the jet [$\Delta t\lesssim 150\,\rm ms$] and its outgoing Poynting luminosity [$L_{\rm EM}\sim 10^{52\pm 1}\rm erg/s$] are consistent with typical short gamma-ray burst central engine lifetimes, as well as with the Blandford--Znajek mechanism for launching jets and their associated Poynting luminosities. Our numerical results also suggest that the dynamical ejection of matter can be enhanced by the magnetic field. Therefore, GRMHD studies are required to fully understand kilonova signals from GW170818-like events.

Machine Learning for Conservative-to-Primitive in Relativistic Hydrodynamics

The numerical solution of relativistic hydrodynamics equations in conservative form requires root-finding algorithms that invert the conservative-to-primitive variables map. These algorithms employ the equation of state of the fluid and can be computationally demanding for applications involving sophisticated microphysics models, such as those required to calculate accurate gravitational wave signals in numerical relativity simulations of binary neutron stars. This work explores the use of machine learning methods to speed up the recovery of primitives in relativistic hydrodynamics. Artificial neural networks are trained to replace either the interpolations of a tabulated equation of state or directly the conservative-to-primitive map. The application of these neural networks to simple benchmark problems shows that both approaches improve over traditional root finders with tabular equation-of-state and multi-dimensional interpolations. In particular, the neural networks for the conservative-to-primitive map accelerate the variable recovery by more than an order of magnitude over standard methods while maintaining accuracy. Neural networks are thus an interesting option to improve the speed and robustness of relativistic hydrodynamics algorithms.

Increasing the accuracy of binary neutron star simulations with an improved vacuum treatment

Numerical-relativity simulations are essential for studying the last stages of the binary neutron star coalescence. Unfortunately, for stable simulations there is the need to add an artificial low-density atmosphere. Here we discuss a new framework in which we can effectively set the density surrounding the neutron stars to zero to ensure a more accurate simulation. We test our method with a number of single star test cases and for an equal-mass binary neutron star simulation. While the bulk motion of the system is not influenced, and hence there is no improvement with respect to the emitted gravitational-wave signal, we find that the new approach is superior with respect to mass conservation and it allows a much better tracking of outward moving material. This will allow a more accurate simulation of the ejected material and supports the interpretation of present and future multimessenger observations with more accurate numerical-relativity simulations.

Hybrid equation of state approach in binary neutron-star merger simulations

We investigate the use of hybrid equations of state in binary neutron-star simulations in full general relativity, where thermal effects are included in an approximate way through the adiabatic index $\Gamma_{\rm{th}}$. We employ a newly developed finite-temperature equation of state derived in the Brueckner-Hartree-Fock approach and carry out comparisons with the corresponding hybrid versions of the same equation of state, investigating how different choices of $\Gamma_{\rm{th}}$ affect the gravitational-wave signal and the hydrodynamical properties of the remnant. We also perform comparisons with the widely used SFHo equation of state, detailing the differences between the two cases. Overall, we determine that when using a hybrid equation of state in binary neutron-star simulations, the value of thermal adiabatic index $\Gamma_{\rm{th}} \approx 1.7$ best approximates the dynamical and thermodynamical behaviour of matter computed using complete, finite-temperature equations of state.

Advancing Computational Methods to Understand the Dynamics of Ejection, Accretion, Winds, and Jets in Binary Neutron Star Mergers

AbstractWe introduce the HandOff, a set of computational tools to transfer general relativistic magnetohydrodynamic and spacetime data from the numerical code IllinoisGRMHD to Harm3d. While the former simulates binary neutron star (BNS) mergers in full dynamical General Relativity, the latter specializes in modeling accretion disks around black holes (BH) over long timescales. The HandOff, then, enables us to transfer the outcome of BNS mergers after BH formation from IllinoisGRMHD to Harm3d and to evolve the post-merger system efficiently over long timescales. We show our latest results in this respect, for matter modeled as a magnetized ideal-fluid, and discuss our future plans which involve incorporating advanced equations of state and neutrino physics into BNS simulations using the HandOff approach.

On the use of tracer particles in simulations of binary neutron stars

In grid-based codes that provide the combined solution of the Einstein equations and of relativistic hydrodynamics, the history of the fluid is not simple to track, especially when compared with particle-based codes. The use of tracers, namely massless particles that are advected with the flow, represents a simple and effective way to solve this problem. Yet, the use of tracers in numerical relativity is far from being settled and several issues, such as the impact of different placements in time and space of the tracers, or the relation between the placement and the description of the underlying fluid, have not yet been addressed. In this paper we present the first detailed discussion of the use tracers in numerical-relativity simulations focussing on both unbound material—such as the one needed as input for nuclear-reaction networks calculating r-process nucleosynthesis in mergers of neutron stars—and on bound material—such as the one in the core of the object produced from the merger of two neutron stars. In particular, when interested in unbound matter, we have evaluated different placement schemes that could be used to initially distribute the tracers and how well their predictions match those obtained when using information from the actual fluid flow. Countering our naive expectations, we found that the most effective method does not rely on the rest-mass density distribution nor on the fluid that is unbound, but simply distributes tracers uniformly in rest-mass density. This prescription leads to the closest matching with the information obtained from the hydrodynamical solution. When considering bound matter, we demonstrate that tracers can provide insight into the fine details of the fluid motion as they can be used to track the evolution of fluid elements or to calculate the variation of quantities that are conserved along streamlines of adiabatic flows.

Gravitational Waveforms from Multiple-Orbit Simulations of Binary Neutron Stars

We study the gravitational wave emission of equal-mass neutron stars in binary orbits as the stars approach the inner most last stable circular orbit. We illustrate the extraction of gravitational wave forms in a sequence of quasi-circular orbit simulations including the general relativistic hydrodynamic response of the stars. We compare the computed results with the Newtonian and post Newtonian results and show that substantial differences can arise as the orbits approach the final inspiral.

General-relativistic resistive-magnetohydrodynamic simulations of binary neutron stars

We have studied the dynamics of an equal-mass magnetized neutron-star binary within a resistive magnetohydrodynamic (RMHD) approach in which the highly conducting stellar interior is matched to an electrovacuum exterior. Because our analysis is aimed at assessing the modifications introduced by resistive effects on the dynamics of the binary after the merger and through to collapse, we have carried out a close comparison with an equivalent simulation performed within the traditional ideal magnetohydrodynamic approximation. We have found that there are many similarities between the two evolutions but also one important difference: the survival time of the hyper massive neutron star increases in a RMHD simulation. This difference is due to a less efficient magnetic-braking mechanism in the resistive regime, in which matter can move across magnetic-field lines, thus reducing the outward transport of angular momentum. Both the RMHD and the ideal magnetohydrodynamic simulations carried here have been performed at higher resolutions and with a different grid structure than those in previous work of ours [L. Rezzolla, B. Giacomazzo, L. Baiotti, J. Granot, C. Kouveliotou, and M. A. Aloy, Astrophys. J. Letters 732, L6 (2011)], but confirm the formation of a low-density funnel with an ordered magnetic field produced by the black hole--torus system. In both regimes the magnetic field is predominantly toroidal in the highly conducting torus and predominantly poloidal in the nearly evacuated funnel. Reconnection processes or neutrino annihilation occurring in the funnel, none of which we model, could potentially increase the internal energy in the funnel and launch a relativistic outflow, which, however, is not produced in these simulations.

Mergers of binary neutron stars with realistic spin

Simulations of binary neutron stars have seen great advances in terms of physical detail and numerical quality. However, the spin of the neutron stars, one of the simplest global parameters of binaries, remains mostly unstudied. We present the first fully nonlinear general relativistic dynamical evolutions of the last three orbits for constraint-satisfying initial data of spinning neutron star binaries, with astrophysically realistic spins aligned and antialigned to the orbital angular momentum. The initial data are computed with the constant rotational velocity approach. The dynamics of the systems is analyzed in terms of gauge-invariant binding energy vs orbital angular momentum curves. By comparing to a binary black hole configuration, we can estimate the different tidal and spin contributions to the binding energy for the first time. First results on the gravitational waveforms are presented. The phase evolution during the orbital motion is significantly affected by spin-orbit interactions, leading to delayed or early mergers. Furthermore, a frequency shift in the main emission mode of the hypermassive neutron star is observed. Our results suggest that a detailed modeling of merger waveforms requires the inclusion of spin, even for the moderate magnitudes observed in binary neutron star systems.

Beyond 2nd order in the simulations of binary neutron stars in general relativity

Beyond 2nd order in the simulations of binary neutron stars in general relativity

Beyond second-order convergence in simulations of binary neutron stars in full general relativity

Abstract Despite the recent rapid progress in numerical relativity, a convergence order less than the second has so far plagued codes solving the Einstein–Euler system of equations. We report simulations of the inspiral of binary neutron stars in quasi-circular orbits computed with a new code employing high-order, high-resolution shock-capturing, finite-differencing schemes that, for the first time, go beyond the second-order barrier. In particular, without any tuning or alignment, we measure a convergence order above three both in the phase and in the amplitude of the gravitational waves. Because the new code is already able to calculate waveforms with very small phase errors at modest resolutions, we are able to obtain accurate estimates of tidal effects in the inspiral that are essentially free from the large numerical viscosity typical of lower order methods, and even for the challenging large compactness and small-deformability binary considered here. We find a remarkable agreement between our Richardson-extrapolated waveform and the one from the tidally corrected post-Newtonian (PN) Taylor-T4 model, with a de-phasing smaller than 0.4 rad during the seven orbits of the inspiral and up to the contact point. Because our results can be used reliably to assess the validity of the PN or other approximations at frequencies significantly larger than those considered so far in the literature, at these compactnesses, they seem to exclude significant tidal amplifications from next to next-to-leading-order terms in the PN expansion.

Constraint damping of the conformal and covariant formulation of the Z4 system in simulations of binary neutron stars

Following previous work in vacuum spacetimes, we investigate the constraint-damping properties in the presence of matter of the recently developed traceless, conformal and covariant Z4 (CCZ4) formulation of the Einstein equations. First, we evolve an isolated neutron star with an ideal gas equation of state and subject to a constraint-violating perturbation. We compare the evolution of the constraints using the CCZ4 and Baumgarte-Shibata-Shapiro-Nakamura-Oohara-Kojima (BSSNOK) systems. Second, we study the collapse of an unstable spherical star to a black hole. Finally, we evolve binary neutron star systems over several orbits until the merger, the formation of a black hole, and up to the ringdown. We show that the CCZ4 formulation is stable in the presence of matter and that the constraint violations are one or more orders of magnitude smaller than for the BSSNOK formulation. Furthermore, by comparing the CCZ4 and the BSSNOK formulations also for neutron star binaries with large initial constraint violations, we investigate their influence on the errors on physical quantities. We also give a new, simple and robust prescription for the damping parameter that removes the instabilities found when using the fully covariant version of CCZ4 in the evolution of black holes. Overall, we find that at essentially the same computational costs the CCZ4 formulation provides solutions that are stable and with a considerably smaller violation of the Hamiltonian constraint than the BSSNOK formulation. We also find that the performance of the CCZ4 formulation is very similar to another conformal and traceless, but noncovariant formulation of the Z4 system, i.e. the Z4c formulation.

Simulations of binary neutron stars and black hole-torus systems in general relativity

Simulations of binary neutron stars and black hole-torus systems in general relativity

Numerical relativity simulations of binary neutron stars

We present a new numerical relativity code designed for simulations of compact binaries involving matter. The code is an upgrade of the BAM code to include general relativistic hydrodynamics and implements state-of-the-art high-resolution-shock-capturing schemes on a hierarchy of mesh refined Cartesian grids with moving boxes. We test and validate the code in a series of standard experiments involving single neutron star spacetimes. We present test evolutions of quasiequilibrium equal-mass irrotational binary neutron star configurations in quasicircular orbits which describe the late inspiral to merger phases. Neutron star matter is modeled as a zero-temperature fluid; thermal effects can be included by means of a simple ideal gas prescription. We analyze the impact that the use of different values of damping parameter in the Gamma-driver shift condition has on the dynamics of the system. The use of different reconstruction schemes and their impact in the post-merger dynamics is investigated. We compute and characterize the gravitational radiation emitted by the system. Self-convergence of the waves is tested, and we consistently estimate error bars on the numerically generated waveforms in the inspiral phase.