Binary Neutron Star Merger Research Articles

Binary neutron star mergers using a discontinuous Galerkin-finite difference hybrid method

Abstract We present a discontinuous Galerkin-finite difference hybrid scheme that allows high-order shock capturing with the discontinuous Galerkin method for general relativistic magnetohydrodynamics in dynamical spacetimes. We present several optimizations and stability improvements to our algorithm that allow the hybrid method to successfully simulate single, rotating, and binary neutron stars. The hybrid method achieves the efficiency of discontinuous Galerkin methods throughout almost the entire spacetime during the inspiral phase, while being able to robustly capture shocks and resolve the stellar surfaces. We also use Cauchy-characteristic evolution to compute the first gravitational waveforms at future null infinity from binary neutron star mergers. The simulations presented here are the first successful binary neutron star inspiral and merger simulations using discontinuous Galerkin methods.

Thermal Conductivity and Thermal Hall Effect in Dense Electron-Ion Plasma

In this study, we examine thermal conductivity and the thermal Hall effect in electron-ion plasmas relevant to hot neutron stars, white dwarfs, and binary neutron star mergers, focusing on densities found in the outer crusts of neutron stars and the interiors of white dwarfs. We consider plasma consisting of single species of ions, which could be either iron [56]Fe or carbon [12]C nuclei. The temperature range explored is from the melting temperature of the solid T∼109 K up to 1011 K. This covers both degenerate and non-degenerate electron regimes. We find that thermal conductivity increases with density and temperature for which we provide analytical scaling relations valid in different regimes. The impact of magnetic fields on thermal conductivity is also analyzed, showing anisotropy in low-density regions and the presence of the thermal Hall effect characterized by the Righi–Leduc coefficient. The transition froma degenerate to non-degenerate regime is characterized by a minimum ratio of thermal conductivity to temperature, which is analogous to the minimum observed already in the case of electrical conductivity. We provide also formulas fit to our numerical results, which can be used in dissipative magneto-hydrodynamics simulations of warm compact stars.

Binary neutron star mergers in massive scalar-tensor theory: Properties of postmerger remnants

We investigate the properties of postmerger remnants of binary neutron star mergers in the framework of Damour–Esposito-Farese-type scalar-tensor theory of gravity with a massive scalar field by numerical relativity simulation. It is found that the threshold mass for prompt collapse is raised in the presence of the excited scalar field. Our simulation results also suggest the existence of a long-lived ϕ mode in hypermassive neutron stars due to the presence of the massive scalar field that enhances the quasiradial oscillation in the remnant. We investigate the descalarization condition in hypermassive neutron stars and discover a distinctive signature in postmerger gravitational waves. Published by the American Physical Society 2024

Incidence of afterglow plateaus in gamma-ray bursts associated with binary neutron star mergers

One of the most surprising gamma-ray burst (GRB) features discovered with the Swift X-ray telescope (XRT) is a plateau phase in the early X-ray afterglow light curves. These plateaus are observed in the majority of long GRBs, while their incidence in short GRBs (SGRBs) is still uncertain due to their fainter X-ray afterglow luminosity with respect to long GRBs. An accurate estimate of the fraction of SGRBs with plateaus is of utmost relevance given the implications that the plateau may have for our understanding of the jet structure and possibly of the nature of the binary neutron star (BNS) merger remnant. This work presents the results of an extensive data analysis of the largest and most up-to-date sample of SGRBs observed with the XRT, and for which the redshift has been measured. We find a plateau incidence of 18-37<!PCT!> in SGRBs, which is a significantly lower fraction than that measured in long GRBs (>50<!PCT!>). Although still debated, the plateau phase could be explained as energy injection from the spin-down power of a newly born magnetized neutron star (NS; magnetar). We show that this scenario can nicely reproduce the observed short GRB (SGRBs) plateaus, while at the same time providing a natural explanation for the different plateau fractions between short and long GRBs. In particular, our findings may imply that only a minority of BNS mergers generating SGRBs leave behind a sufficiently stable or long-lived NS to form a plateau. From the probability distribution of the BNS remnant mass, a fraction 18-37<!PCT!> of short GRB plateaus implies a maximum NS mass in the range $ odot

Stable case BB/BC mass transfer to form GW190425-like massive binary neutron star mergers

Context. On April 25, 2019, the LIGO-Virgo Collaboration discovered a gravitational-wave (GW) signal from a binary neutron star (BNS) merger, that is, GW190425. Due to the inferred large total mass, the origin of GW190425 remains unclear. Aims. Assuming GW190425 originated from the standard isolated binary evolution channel, its immediate progenitor is considered to be a close binary system, consisting of a He-rich star and a NS just after the common envelope phase. We aim to study the formation of GW190425 in a solar-like environment by using the detailed binary evolution code MESA. Methods. We perform detailed stellar structure and binary evolution calculations that take into account mass loss, internal differential rotation, and tidal interactions between a He-rich star and a NS companion. We explore the parameter space of the initial binary properties, including initial NS and He-rich masses and initial orbital period. Results. We find that the immediate post-common-envelope progenitor system, consisting of a primary ∼2.0 M⊙ (∼1.7 M⊙) NS and a secondary He-rich star with an initial mass of ∼3.0 − 5.5 M⊙ (∼5.5 − 6.0 M⊙) in a close binary with an initial period of ∼0.08 − 0.5 days (∼0.08 − 0.4 days), that experiences stable Case BB/BC mass transfer (MT) during binary evolution, can reproduce the formation of GW190425-like BNS events. Our studies reveal that the secondary He-rich star of the GW190425’s progenitor before its core collapse can be efficiently spun up through tidal interaction, finally remaining as a NS with rotational energy even reaching ∼1052 erg, which is always much higher than the neutrino-driven energy of the supernova (SN) explosion. If the newborn secondary NS is a magnetar, we expect that GW190425 can be the remnant of a magnetar-driven SN, namely a magnetar-driven ultra-stripped SN, a superluminous SN, or a broad-line Type Ic SN. Conclusions. Our results show that GW190425 could be formed through the isolated binary evolution, which involves a stable Case BB/BC MT just after the common envelope phase. On top of that, we show the He-rich star can be tidally spun up, potentially forming a spinning magnetized NS (magnetar) during the second SN explosion.

The Binary Black Hole Merger Rate Deviates from the Cosmic Star Formation Rate: A Tug of War between Metallicity and Delay Times

Gravitational-wave detectors are now making it possible to investigate how the merger rate of binary black holes (BBHs) evolves with redshift. In this study, we examine whether the BBH merger rate of isolated binaries deviates from a scaled star formation rate density (SFRD)—a frequently used model in state-of-the-art research. To address this question, we conduct population synthesis simulations using COMPAS with a grid of stellar evolution models, calculate their cosmological merger rates, and compare them to a scaled SFRD. We find that our simulated rates deviate by factors up to 3.5 at z ∼ 0 and 5 at z ∼ 9 due to two main phenomena: (i) the formation efficiency of BBHs is an order of magnitude higher at low metallicities than at solar metallicity, and (ii) BBHs experience a wide range of delays (from a few megayears to many gigayears) between formation and merger. The deviations are similar when comparing to a delayed SFRD, and even larger (up to ∼10×) when comparing to SFRD-based models scaled to the local merger rate. Interestingly, our simulations find that the BBH delay time distribution is redshift dependent, increasing the complexity of the redshift distribution of mergers. We find similar results for simulated merger rates of black hole–neutron stars (BHNSs) and binary neutron stars (BNSs). We conclude that the rate of BBH, BHNS, and BNS mergers from the isolated channel can significantly deviate from a scaled SFRD, and that future measurements of the merger rate will provide insights into the formation pathways of gravitational-wave sources.

Simulating Short Gamma-Ray Burst Jets in Realistic Late Binary Neutron Star Merger Environments

The electromagnetic emission and the afterglow observations of the binary neutron star merger event GW170817A confirmed the association of the merger with a short gamma-ray burst (GRB) harboring a narrow (5°–10°) and powerful (1049–1050 erg) jet. Using the 1 s long neutrino-radiation general relativistic MHD simulation of coalescing neutron stars of K. Kiuchi et al., and following the semi-analytical estimates of M. Pais et al., we inject a narrow, powerful, unmagnetized jet into the post-merger phase. We explore different opening angles, luminosities, central engine durations, and times after the merger. We explore early (0.1 s following the merger) and late (1 s) jet launches; the latter is consistent with the time delay of ≈1.74 s observed between GW170817 and GRB 170817A. We demonstrate that the semi-analytical estimates correctly predict the jets’ breakout and collimation conditions. When comparing our synthetic afterglow light curves to the observed radio data of GW170807, we find a good agreement for a 3 × 1049 erg jet launched late with an opening angle in the range ≃5°–7°.

Rates and Beaming Angles of Gamma-Ray Bursts Associated with Compact Binary Coalescences

Some, if not all, binary neutron star (BNS) coalescences, and a fraction of neutron star–black hole (NSBH) mergers, are thought to produce sufficient mass ejection to power gamma-ray bursts (GRBs). However, this fraction, as well as the distribution of beaming angles of BNS-associated GRBs, is poorly constrained from observation. Recent work applied machine learning tools to analyze GRB light curves observed by Fermi/Gamma-Ray Burst Monitor (GBM) and Swift/Burst Alert Telescope (BAT). GRBs were segregated into multiple distinct clusters, with the tantalizing possibility that one of them (BNS cluster) could be associated with BNSs and another (NSBH cluster) with NSBHs. As a proof of principle, assuming that all GRBs detected by Fermi/GBM and Swift/BAT associated with BNSs (NSBHs) lie in the BNS (NSBH) cluster, we estimate their rates (Gpc−3 yr−1). We compare these rates with corresponding BNS and NSBH rates estimated by the LIGO–Virgo–KAGRA (LVK) collaboration from the first three observing runs (O1, O2, O3). We find that the BNS rates are consistent with LVK’s rate estimates, assuming a uniform distribution of beaming fractions (f b ∈ [0.01, 0.1]). Conversely, using the LVK’s BNS rate estimates, assuming all BNS mergers produce GRBs, we are able to constrain the beaming angle distribution to θ j ∈ [0.°8, 33.°5] at 90% confidence. We similarly place limits on the fraction of GRB-bright NSBHs as f B ∈ [1.3%, 63%] (f B ∈ [0.4%, 15%]) with Fermi/GBM (Swift/BAT) data.

Magnetic dissipation in short gamma-ray-burst jets

Aims. Short gamma-ray bursts originate when relativistic jets emerge from the remnants of binary neutron star (BNS) mergers, as observed in the first multi-messenger event GW170817–GRB 170817A, which coincided with a gravitational wave signal. Both the jet and the remnant are believed to be magnetized, and the presence of magnetic fields is known to influence the jet propagation across the surrounding post-merger environment. In the magnetic interplay between the jet and the environment itself, effects due to a finite plasma conductivity may be important, especially in the first phases of jet propagation. We aim to investigate these effects, from jet launching to its final breakout from the post-merger environment. Methods. We used the PLUTO numerical code to perform 2D axisymmetric and full 3D resistive relativistic magnetohydrodynamic (MHD) simulations, employing spherical coordinates with spatial radial stretching. We considered and compared different models for physical resistivity, which must be small but still dominating over the intrinsic numerical dissipation (which yields unwanted smearing of structures in any ideal MHD code). Stiff terms in the current density are treated with IMplicit-EXplicit Runge Kutta algorithms for time-stepping. A Synge-like gas (Taub equation of state) is also considered. All simulations were performed using an axisymmetric analytical model for both the jet propagation environment and the jet injection; we leave the case of jet propagation in a realistic environment (i.e., imported from an actual BNS merger simulation) to a future study. Results. As expected, no qualitative differences are detected due to the effect of a finite conductivity, but significant quantitative differences in the jet structure and induced turbulence are clearly seen in 2D axisymmetric simulations, and we also compare different resistivity models. We see the formation of regions with a resistive electric field parallel to the magnetic field, and nonthermal particle acceleration may be enhanced there. The level of dissipated Ohmic power is also dependent on the various recipes for resistivity. Most of the differences arise before the breakout from the inner environment, whereas once the jet enters the external weakly magnetized environment region, these differences are preserved during further propagation despite the lower grid refinement. Finally, we show and discuss the 3D evolution of the jet within the same environment in order to highlight the emergence of nonaxisymmetric features.

Binary neutron star merger offsets from their host galaxies. GW 170817 as a case study

The locations of binary neutron star (BNS) mergers within their host galaxies encode the systemic kicks that these systems received in the supernova aftermath. We investigate how the galactic potential and the systemic kicks shape the offset distribution of BNS mergers with a case study of GW 170817 and its host NGC 4993. We derived dynamical constraints on the host potential from integral field spectroscopy with Jeans anisotropic modelling. We evolved the trajectories of synthetic BNSs from the BPASS code in the galactic potential, using two different kick prescriptions to investigate how the observed offsets might differentiate between these two possibilities. The simulation was repeated after swapping the host potential with that of a dwarf galaxy, to test the effect of the potential on the offsets. The location of GW 170817 is entirely consistent with our predictions, regardless of large or small kicks, because the strong potential of NGC 4993 is only diagnostic of very large kicks. In galaxies of similar or greater mass, large offsets can constrain large kicks, while small offsets do not provide much information. In an old dwarf galaxy, on the other hand, small offsets can constrain small kicks, while large offsets would prevent host association.

Off-Axis Color Characteristics of Binary Neutron Star Merger Events: Applications for Space Multi-Band Variable Object Monitor and James Webb Space Telescope

With advancements in gravitational wave detection technology, an increasing number of binary neutron star (BNS) merger events are expected to be detected. Due to the narrow opening angle of jet cores, many BNS merger events occur off-axis, resulting in numerous gamma-ray bursts (GRBs) going undetected. Models suggest that kilonovae, which can be observed off-axis, offer more opportunities to be detected in the optical/near-infrared band as electromagnetic counterparts of BNS merger events. In this study, we calculate kilonova emission using a three-dimensional semi-analytical code and model the GRB afterglow emission with the open-source Python package afterglowpy at various inclination angles. Our results show that it is possible to identify the kilonova signal from the observed color evolution of BNS merger events. We also deduce the optimal observing window for SVOM/VT and JWST/NIRCam, which depends on the viewing angle, jet opening angle, and circumburst density. These parameters can be cross-checked with the multi-band afterglow fitting. We suggest that kilonovae are more likely to be identified at larger inclination angles, which can also help determine whether the observed signals without accompanying GRBs originate from BNS mergers.

Prospect of Detecting Magnetic Fields from Strong-magnetized Binary Neutron Stars

Binary neutron star mergers are unique sources of gravitational waves in multi-messenger astronomy. The inspiral phase of binary neutron stars can emit gravitational waves as chirp signals. The present waveform models of gravitational waves only considered the gravitational interaction. In this paper, we derive the waveform of the gravitational wave signal taking into account the presence of magnetic fields. We found that the electromagnetic interaction and radiation can introduce different frequency-dependent power laws for both the amplitude and frequency of the gravitational wave. We show from the results of the Fisher information matrix that the third-generation observation may detect magnetic dipole moments if the magnetic field is ∼1017 G.

Probing kaon meson condensations through gravitational waves during neutron star inspiral phases

Gravitational waves from binary neutron star merger events have shown unparalleled advantages in deciphering the stellar internal structure, yet it remains underexplored to seek clues about kaon meson condensations inside massive neutron stars through these events. To address this gap, this study examines the binary star inspiral processes with kaon meson condensations, employing various kaon meson potential wells to probe unique imprints in the inspiral gravitational waves. Our analysis reveals that the inspiral process of kaon meson condensation binary stars exhibits lower gravitational wave frequencies and shorter retarded times compared to traditional neutron stars. Furthermore, a deeper kaon meson potential well in these systems further shortens the inspiral duration time, resulting in distinctive gravitational waveforms and notable orbital phase deviations. These findings underscore the pivotal role of kaon meson condensations in modulating the inspiral gravitational waves, suggesting the inspiral gravitational waves could serve as a powerful tool for probing potential signals of kaon mesons within neutron stars.

Measuring neutron star radius with second and third generation gravitational wave detector networks

Abstract The next generation of ground-based interferometric gravitational wave detectors will observe mergers of black holes and neutron stars throughout cosmic time. A large number of the binary neutron star merger events will be observed with extreme high fidelity, and will provide stringent constraints on the equation of state of nuclear matter. In this paper, we investigate the systematic improvement in the measurability of the equation of state with increase in detector sensitivity by combining constraints obtained on the radius of a $1.4 \, \mathrm{M}_{\odot}$ neutron star from a simulated source population. Since the measurability of the equation of state depends on its stiffness, we consider a range of realistic equations of state that span the current observational constraints. We show that a single 40km Cosmic Explorer detector can pin down the neutron star radius for a soft, medium and stiff equation of state to an accuracy of 10m within a decade, whereas the current generation of ground-based detectors like the Advanced LIGO-Virgo network would take $\mathcal{O}(10^5)$ years to do so for a soft equation of state. 

A simulation study on the sub-threshold joint gravitational wave-electromagnetic wave observation on binary neutron star mergers

Abstract The coalescence of binary neutron stars (BNS) is a prolific source of gravitational waves (GWs) and electromagnetic (EM) radiation, offering a dual observational window into the Universe. Lowering the signal-to-noise ratio (S/N) threshold is a simple and cost-effective way to enhance the detection probability of GWs from BNS mergers. In this study, we introduce a metric of the purity of joint GW and EM detections Pjoint, which is in analogue to Pastro in GW only observations. By simulating BNS merger GWs jointly detected by the HLV network and EM counterparts (kilonovae and short Gamma-ray bursts, sGRBs) with an assumed merger rate density of BNS, we generate catalogs of GW events and EM counterparts. Through this simulation, we analyze joint detection pairs, both correct and misidentified. We find the following: 1. For kilonovae, requiring Pjoint > 95% instead of $P_{\rm astro}>95~{{\%}}$ reduces the S/N from 9.2 to 8.5-8.8, allowing 5-13 additional joint detections per year and increasing the GW detection volume by 9-17%; 2. For sGRBs, requiring Pjoint > 95% instead of Pastro reduces the S/N from 9.2 to 8.1-8.5; 3. Increasing kilonova or sGRB detection capability does not improve Pjoint due to a higher rate of misidentifications. We also show that sub-threshold GW and kilonova detections can reduce the uncertainty in measuring the Hubble constant to 89-92% of its original value, and sub-threshold GW and sGRB observations can enhance the precision of constraining the speed of GWs to 88% of previously established values.

Black hole—neutron star binary mergers: the impact of stellar compactness

Abstract Recent gravitational wave (GW) observations include possible detections of black hole—neutron star binary mergers. As with binary black hole mergers, numerical simulations help characterize the sources. For binary systems with neutron star components, the simulations help to predict the imprint of tidal deformations and disruptions on the GW signals. In a previous study, we investigated how the mass of the black hole has an impact on the disruption of the neutron star and, as a consequence, on the shape of the GWs emitted. We extend these results to study the effects of varying the compactness of the neutron star. We consider neutron star compactness in the 0.113–0.2 range for binaries with mass ratios of 3 and 5. As the compactness and the mass ratio increase, the binary system behaves during the late inspiral and merger more like a black hole binary. For the cases with the least compact neutron star, the GWs emitted, in terms of mismatches, are the most distinguishable from those by a binary black hole. The disruption of the star significantly suppresses the kicks on the final black hole. The disruption also affects, although not dramatically, the spin of the final black hole. Lastly, for neutron stars with low compactness, the quasi-normal ringing of the black hole after the merger does not show a clean quasi-normal ringing because of the late accretion of debris from the neutron star.

Delayed jet launching in binary neutron star mergers with realistic initial magnetic fields

Delayed jet launching in binary neutron star mergers with realistic initial magnetic fields

Disk mass after a binary neutron star merger as a constraining parameter for short gamma-ray bursts

Context. The coincident detection of GW170817 and gamma-ray burst GRB170817A marked a milestone for the connection between binary neutron star (BNS) mergers and short gamma-ray bursts (sGRBs). These mergers can lead to the formation of a black hole that is surrounded by a disk and to the generation of a powerful jet. It spends energy to break free from the merger ejecta, and then a portion of it is dissipated to produce observable emissions. Aims. Our primary goal is to enhance our comprehension of BNS mergers by constraining the disk mass for a selection of sGRBs. To do this, we used the isotropic gamma-ray luminosity and corresponding emission times as key indicators. Methods. We leveraged data from GW170817 to estimate the disk mass surrounding the BNS merger remnant, and we subsequently inferred the efficiency of the accretion onto the jet. We then statistically examined other sGRB observations to estimate whether they might have been induced by BNS mergers Results. Our findings suggest that when similar physical parameters are employed as in the only observed BNS-powered GRB event, GRB170817A, a substantial fraction of sGRBs would need an unrealistically massive disk remnant. Conclusions. This observation raises the possibility that either a different mechanism powered those events or that the post-collapse disk efficiency varies significantly in different BNS merger scenarios.

Toward Accelerated Nuclear-physics Parameter Estimation from Binary Neutron Star Mergers: Emulators for the Tolman–Oppenheimer–Volkoff Equations

Gravitational-wave observations of binary neutron-star (BNS) mergers have the potential to revolutionize our understanding of the nuclear equation of state (EOS) and the fundamental interactions that determine its properties. However, Bayesian parameter estimation frameworks do not typically sample over microscopic nuclear-physics parameters that determine the EOS. One of the major hurdles in doing so is the computational cost involved in solving the neutron-star structure equations, known as the Tolman–Oppenheimer–Volkoff (TOV) equations. In this paper, we explore approaches to emulating solutions for the TOV equations: multilayer perceptrons (MLPs), Gaussian processes, and a data-driven variant of the reduced basis method (RBM). We implement these emulators for three different parameterizations of the nuclear EOS, each with a different degree of complexity represented by the number of model parameters. We find that our MLP-based emulators are generally more accurate than the other two algorithms, whereas the RBM results in the largest speedup with respect to the full high-fidelity TOV solver. We employ these emulators for a simple parameter inference using a potentially loud BNS observation and show that the posteriors predicted by our emulators are in excellent agreement with those obtained from the full TOV solver.

Diversity in Fermi/GBM Gamma-Ray Bursts: New Insights from Machine Learning

Classification of gamma-ray bursts (GRBs) has been a long-standing puzzle in high-energy astrophysics. Recent observations challenge the traditional short versus long viewpoint, where long GRBs are thought to originate from the collapse of massive stars and short GRBs from compact binary mergers. Machine learning (ML) algorithms have been instrumental in addressing this problem, revealing five distinct GRB groups within the Swift Burst Alert Telescope (BAT) light-curve data, two of which are associated with kilonovae (KNe). In this work, we extend our analysis to the Fermi Gamma-ray Burst Monitor catalog and identify five clusters using unsupervised ML techniques, consistent with the Swift/BAT results. These five clusters are well separated in the fluence-duration plane, hinting at a potential link between fluence, duration, and complexities (or structures) in the light curves of GRBs. Further, we confirm two distinct classes of KN-associated GRBs. The presence of GRB 170817A in one of the two KN-associated clusters lends evidence to the hypothesis that this class of GRBs could potentially be produced by binary neutron star mergers. The second KN-associated GRB cluster could potentially originate from neutron star–black hole mergers. Future multimessenger observations of compact binaries in gravitational waves and electromagnetic waves can be paramount in understanding these clusters better.