Recently, many gravitational wave events from compact binary mergers have been detected by LIGO. Determining the final mass and spin of the remnant black holes (RBHs) is a fundamental issue and is also important in astrophysics. In this paper, unified models for predicting the final mass and spin of the RBHs from compact binary mergers is proposed. The models achieve a good accuracy within the parameter range of interest. In addition, the rotational energy of the RBHs is also studied which is relevant to the electromagnetic counterparts of the mergers. It is found the distribution of the rotational energy of the RBHs from different types of mergers of compact binary has its own characteristics, which might help identify the electromagnetic counterparts associated with the mergers.
Neutron star (NS) mergers provide us with information rich in physics using multi-messenger astrophysical observations. One of the probable remnants of such a merger is a differentially rotating hot hypermassive neutron star. The stability of the merger remnant depends crucially on the underlying Equation of State (EoS) and thus provides a method to probe the nature of dense matter in NSs. In this work, we search for possible signatures of strangeness containing matter in the NS interior on the secular stability of the merger remnant. We also use recently proposed methods to make a rough estim…
We study high-energy emission from the mergers of neutron star binaries as electromagnetic counterparts to gravitational waves aside from short gamma-ray bursts. The mergers entail significant mass ejection, which interacts with the surrounding medium to produce similar but brighter remnants than supernova remnants in a few years. We show that electrons accelerated in the remnants can produce synchrotron radiation in X-rays detectable at $\sim 100$ Mpc by current generation telescopes and inverse Compton emission in gamma rays detectable by the \emph{Fermi} Large Area Telescopes and the Cheren…
Both the core collapse of rotating massive stars, and the coalescence of neutron star (NS) binaries, result in the formation of a hot, differentially rotating NS remnant. The timescales over which differential rotation is removed by internal angular-momentum transport processes (`viscosity') has key implications for the remnant's long-term stability and the NS equation-of-state (EOS). Guided by a non-rotating model of a cooling proto-NS, we estimate the dominant sources of viscosity using an externally imposed angular velocity profile $Ω(r)$. Although the magnetorotational instability provides…
The coalescence of a binary neutron star (NS) system may in some cases produce a massive NS remnant that is long-lived and, potentially, indefinitely stable to gravitational collapse. Such a remnant has been proposed as an explanation for the late X-ray emission observed following some short duration gamma-ray bursts (GRBs) and as possible electromagnetic counterparts to the gravitational wave chirp. A stable NS merger remnant necessarily possesses a large rotational energy > 1e52 erg, the majority of which is ultimately deposited into the surrounding circumburst medium (CBM) at mildly relativ…
The inspiral and merger of two black holes produces a remnant black hole with mass and spin determined by the properties of its parent black holes. Using the inferred population properties of component black holes from the first two and a half observing runs of Advanced LIGO and Virgo, we calculate the population properties of the leftover remnant black holes. By integrating their rate of formation over the age of the universe, we estimate the number density of remnant black holes today. Using simple prescriptions for the cosmic star formation rate and black hole inspiral delay times, we deter…
Spinning down neutron-star merger remnants with the Tayler-Spruit dynamo: Global simulations reveal the formation of massive disks and neutron-rich ejecta
California Institute of Technology（加州理工学院）；Walter Burke Institute for Theoretical Physics, California Institute of Technology（加州理工学院沃尔特·伯克理论物理研究所）
Limited by the sensitivities of the current gravitational wave (GW) detectors, the central remnant of the binary neutron star (NS) merger associated with GW170817 remains an open question. Considering the relatively large total mass, it is generally proposed that the merger of GW170817 would lead to a shortly lived hypermassive NS or directly produce a black hole (BH). There is no clear evidence to support or rule out a long-lived NS as the merger remnant. Here we utilize the GW and electromagnetic (EM) signals to comprehensively investigate the parameter space that allows a long-lived NS to s…
We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some…
Massive (hypermassive and supramassive) neutron stars are likely to be often formed after the merger of binary neutron stars. We explore the evolution process of the remnant massive neutron stars and gravitational waves emitted by them, based on numerical-relativity simulations for binary neutron star mergers employing a variety of equations of state and choosing a plausible range of the neutron-star mass of binaries. We show that the lifetime of remnant hypermassive neutron stars depends strongly on the total binary mass and also on the equations of state. Gravitational waves emitted by the r…
The first neutron star binary merger detected in gravitational waves, GW170817 and the subsequent detection of its emission across the electromagnetic spectrum showed that these systems are viable progenitors of short $γ$-ray bursts (sGRB). The afterglow signal of GW170817 has been found to be consistent with a structured GRB jet seen off-axis, requiring significant amounts of relativistic material at large angles. This trait can be attributed to the interaction of the relativistic jet with the external wind medium. Here we perform numerical simulations of relativistic jets interacting with re…
We performed general relativistic, long-term, axisymmetric neutrino radiation hydrodynamics simulations for the remnant formed after the binary neutron star merger, which consist of a massive neutron star and a torus surrounding it. As an initial condition, we employ the result derived in a three-dimensional, numerical relativity simulation for the binary neutron star merger. We investigate the properties of neutrino-driven ejecta. Due to the pair-annihilation heating, the dynamics of the neutrino-driven ejecta is significantly modified. The kinetic energy of the ejecta is about two times larg…
We study properties of neutrino transfer in a remnant of neutron star merger, consisting of a massive neutron star and a surrounding torus. We perform numerical simulations of the neutrino transfer by solving the Boltzmann equation with momentum-space angles and energies of neutrinos for snapshots of the merger remnant having elongated shapes. The evaluation of the neutrino distributions in the multi-dimensions enable us to provide the detailed information of angle and energy spectra and neutrino reaction rates. We demonstrate features of asymmetric neutrino fluxes from the deformed remnant an…
We present a novel universal relation for binary neutron star mergers with long-lived neutron star remnants: inspired by recent work based on numerical relativity simulations, we propose a novel approach using perturbative calculations that allow us to relate the pre-merger neutron star binary tidal deformability to the effective compactness of the post-merger remnant. Our results allow for the prediction of the stellar parameters of a long-lived remnant neutron star from the study of gravitational waves emitted during the pre-merger phase.
Massive, rapidly-spinning magnetar remnants produced as a result of binary neutron star (BNS) mergers may deposit a fraction of their energy into the surrounding kilonova ejecta, powering a synchrotron radio signal from the interaction of the ejecta with the circumburst medium. We present 6.0 GHz Very Large Array (VLA) observations of nine, low-redshift short gamma-ray bursts (SGRBs; $z<0.5$) on rest-frame timescales of $\approx2.4-13.9$ yr following the bursts. We place $3σ$ limits on radio continuum emission of $F_ν\lesssim6-20\,μ$Jy at the burst positions, or $L_ν\lesssim(0.6-8.3)\times10^{…
Outflows launched from a remnant disc of compact binary merger may have essential contribution to the kilonova emission. Numerical calculations are conducted in this work to study the structure of accretion flows and outflows. By the incorporation of limited-energy advection in the hyper-accretion discs, outflows occur naturally from accretion flows due to imbalance between the viscous heating and the sum of the advective and radiative cooling. Following this spirit, we revisit the properties of the merger outflow ejecta. Our results show that around $10^{-3} \sim 10^{-1} M_\odot$ of the disc …
It is known that a binary neutron star merger produces a hypermassive neutron star. The lifetime of this compact remnant depends on the total mass and the equation of state. The collapse of this compact remnant to a black- hole-torus system is expected to give rise to a powerful jet and a short gamma-ray burst. Nevertheless, if the collapse is delayed half a second or so, the surrounding matter would be already accreted and/or expelled and hence no torus will be formed. However, the collapse itself will give rise to a quasi-isotropic magnetized fireball. This magnetic bomb will dissipate much …
Long-term neutrino-radiation resistive-magnetohydrodynamics simulations in full general relativity are performed for a system composed of a massive neutron star and a torus formed as a remnant of binary neutron star mergers. The simulation is performed in axial symmetry incorporating a mean-field dynamo term for a hypothetical amplification of the magnetic-field strength. We first calibrate the mean-field dynamo parameters by comparing the results for the evolution of black hole-disk systems with viscous hydrodynamics results. We then perform simulations for the system of a remnant massive neu…
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…
Neutron star mergers produce a substantial amount of fast-moving ejecta, expanding outwardly for years after the merger. The interaction of these ejecta with the surrounding medium may produce a weak isotropic radio remnant, detectable in relatively nearby events. We use late-time radio observations of short duration gamma-ray bursts (sGRBs) to constrain this model. Two samples of events were studied: four sGRBs that are possibly in the local (<200 Mpc) universe were selected to constrain the remnant non-thermal emission from the sub-relativistic ejecta, whereas 17 sGRBs at cosmological distan…
Gravitational-wave (GW) and electromagnetic (EM) signals from the merger of a Neutron Star (NS) and a Black Hole (BH) are a highly anticipated discovery in extreme gravity, nuclear-, and astrophysics. We develop a simple formula that distinguishes between merger outcomes and predicts the post-merger remnant mass, validated with 75 simulations. Our formula improves on existing results by describing critical unexplored regimes: comparable masses and higher BH spins. These are important to differentiate NSNS from NSBH mergers, and to infer source physics from EM signals.
The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multi-dimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an $α$-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al. (2011), which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towar…
Measuring the collapse time of a binary neutron star merger remnant can inform the physics of extreme matter and improve modelling of short gamma-ray bursts and associated kilonova. The lifetime of the post-merger remnant directly impacts the mechanisms available for the jet launch of short gamma-ray bursts. We develop and test a method to measure the collapse time of post-merger remnants. We show that for a GW170817-like event at $\sim\!40\,$Mpc, a network of Einstein Telescope with Cosmic Explorer is required to detect collapse times of $\sim\!10\,$ms. For a two-detector network at A+ design…
Multi-messenger observations of GW170817 have not conclusively established whether the merger remnant is a black hole (BH) or a neutron star (NS). We show that a long-lived magnetized NS with a poloidal field $B\approx 10^{12}$G is fully consistent with the electromagnetic dataset, when spin down losses are dominated by gravitational wave (GW) emission. The required ellipticity $ε\gtrsim 10^{-5}$ can result from a toroidal magnetic field component much stronger than the poloidal component, a configuration expected from a NS newly formed from a merger. Abrupt magnetic dissipation of the toroida…
Although multidimensional simulations have investigated the processes of double WD mergers, post-merger evolution only focused on the carbon-oxygen (CO) WD or helium (He) WD merger remnants. In this work, we investigate for the first time the evolution of the remnants stemmed from the merger of oxygen-neon (ONe) WDs with CO WDs. Our simulation results indicate that the merger remnants can evolve to hydrogen- and helium-deficient giants with maximum radius of about 300Rsun. Our models show evidence that merger remnants more massive than 1.95Msun can ignite Ne before significant mass-loss ensues…
We investigate non-thermal electron and nuclei energy losses within the binary neutron star merger remnant produced by the event GW170817. The lack of a cooling feature within the detected synchrotron emission from the source is used to constrain the magnetic field at the mG level, assuming that this emission is electron synchrotron in origin, and that the accelerated spectrum in the electrons follows the form $dN/dE_e \propto E_e^{-2}$. The level of subsequent gamma-ray emission from the source is demonstrated to provide a further constraint on the source magnetic field strength. We also put …
Detection and parameter estimation of binary neutron star merger remnants can shed light on the physics of hot matter at supranuclear densities. Here we develop a fast, simple model that can generate gravitational waveforms, and show it can be used for both detection and parameter estimation of post-merger remnants. The model consists of three exponentially-damped sinusoids with a linear frequency-drift term. The median fitting factors between the model waveforms and numerical-relativity simulations exceed 0.90. We detect remnants at a post-merger signal-to-noise ratio of $\ge 7$ using a Bayes…
Close binaries of double white dwarfs (DWDs) inspiral and merge by emitting gravitational wave (GW). Orbital motion of some of these binaries are expected to be observed at low frequency band by space-borne laser interferometric detectors of GW. The merger remnant may suffer thermonuclear runaway and explode as type Ia supernova if they are massive enough. As GW sources the remnants have so far been scarcely studied. Here we propose a new mechanism of GW emission from DWD merger remnants which may be observed by planned GW detectors in decihertz frequency band. A remnant is temporarily expecte…
We present stellar evolution calculations of the remnant of the merger of two carbon-oxygen white dwarfs (CO WDs). We focus on cases that have a total mass in excess of the Chandrasekhar mass. After the merger, the remnant manifests as an $L \sim 3 \times 10^4 L_\odot$ source for $\sim 10^4$ yr. A dusty wind may develop, leading these sources to be self-obscured and to appear similar to extreme AGB stars. Roughly $\sim 10$ such objects should exist in the Milky Way and M31 at any time. As found in previous work, off-center carbon fusion is ignited within the merger remnant and propagates inwar…
We construct evolutionary models of the remnant of the merger of two carbon-oxygen (CO) core white dwarfs (WDs). With total masses in the range $1-2 {\rm M_\odot}$, these remnants may either leave behind a single massive WD or undergo a merger-induced collapse to a neutron star (NS). On the way to their final fate, these objects generally experience a $\sim 10$ kyr luminous giant phase, which may be extended if sufficient helium remains to set up a stable shell-burning configuration. The uncertain, but likely significant, mass loss rate during this phase influences the final remnant mass and f…
Merging neutron stars are expected to produce hot, metastable remnants in rapid differential rotation, which subsequently cool and evolve into rigidly rotating neutron stars or collapse to black holes. Studying this metastable phase and its further evolution is essential for the prediction and interpretation of the electromagnetic, neutrino, and gravitational signals from such a merger. In this work, we model binary neutron star merger remnants and propose new rotation and thermal laws that describe post-merger remnants. Our framework is capable to reproduce quasi-equilibrium configurations fo…
We study the neutrino flavor evolution in the neutrino-driven wind from a binary neutron star merger remnant consisting of a massive neutron star surrounded by an accretion disk. With the neutrino emission characteristics and the hydrodynamical profile of the remnant consistently extracted from a three-dimensional simulation, we compute the flavor evolution by taking into account neutrino coherent forward scattering off ordinary matter and neutrinos themselves. We employ a "single-trajectory" approach to investigate the dependence of the flavor evolution on the neutrino emission location and a…
We explore the electromagnetic counterparts that will associate with binary neutron star mergers for the case that remnant massive neutron stars survive for $\gtrsim 0.5\,$s after the merger. For this study, we employ the outflow profiles obtained by long-term general-relativistic neutrino-radiation magneto-hydrodynamics simulations with a mean field dynamo effect. We show that a synchrotron afterglow with high luminosity can be associated with the merger event if the magnetic fields of the remnant neutron stars are significantly amplified by the dynamo effect. We also perform a radiative tran…
A supra-massive neutron star (NS) spinning extremely rapidly could survive from a merger of NS-NS binary. The spin-down of this remnant NS that is highly magnetized would power the isotropic merger ejecta to produce a bright mergernova emission in ultraviolet/optical bands. Before the mergernova, the early interaction between the NS wind and the ejecta can drive a forward shock propagating outwards into the ejecta. As a result, a remarkable amount of heat can be accumulated behind the shock front and the final escaping of this heat can produce a shock breakout emission. We describe the dynamic…
Gravitational waves have been detected from the inspiral of a binary neutron-star, GW170817, which allowed constraints to be placed on the neutron star equation of state. The equation of state can be further constrained if gravitational waves from a post-merger remnant are detected. Post-merger waveforms are currently generated by numerical-relativity simulations, which are computationally expensive. Here we introduce a hierarchical model trained on numerical-relativity simulations, which can generate reliable post-merger spectra in a fraction of a second. Our spectra have mean fitting factors…
The question "what is the observable electromagnetic (EM) signature of a compact binary merger?" is an intriguing one with crucial consequences to the quest for gravitational waves (GW). Compact binary mergers are prime sources of GW, targeted by current and next generation detectors. Numerical simulations have demonstrated that these mergers eject energetic sub-relativistic (or even relativistic) outflows. This is certainly the case if the mergers produce short GRBs, but even if not, significant outflows are expected. The interaction of such outflows with the surround matter inevitably leads …
When binary black holes merge in dense star clusters, their remnants can pair up with other black holes in the cluster, forming heavier and heavier black holes in a process called hierarchical merger. The most important condition for hierarchical merger to occur is that remnants formed by mergers are retained by the host star cluster. Using the publicly available gravitational-wave event database, we infer the magnitudes of kick velocities imparted to the remnant black holes due to anisotropic emission of gravitational waves and use that to quantify the retention probability of each event as a…
Two neutron stars merge somewhere in the Universe approximately every 10 seconds, creating violent explosions observable in gravitational waves and across the electromagnetic spectrum. The transformative coincident gravitational-wave and electromagnetic observations of the binary neutron star merger GW170817 gave invaluable insights into these cataclysmic collisions, probing bulk nuclear matter at supranuclear densities, the jet structure of gamma-ray bursts, the speed of gravity, and the cosmological evolution of the local Universe, among other things. Despite the wealth of information, it is…
The binary neutron star merger GW170817/GRB170817A confirmed that at least some neutron star mergers are the progenitors of short gamma-ray bursts. Many short gamma-ray bursts have long-term x-ray afterglows that have been interpreted in terms of post-merger millisecond magnetars---rapidly rotating, highly magnetised, massive neutron stars. We review our current understanding of millisecond magnetars born in short gamma-ray bursts, focusing particularly three main topics. First, whether millisecond magnetars really do provide the most plausible explain for the x-ray plateau. Second, determinin…
Multiple approaches are required to study the evolution of black-hole binaries. While the post-Newtonian approximation is sufficient to describe the early inspiral (even from infinitely large orbital separation), only numerical relativity can capture the full complexity of the dynamics near merger. We combine multi-timescale post-Newtonian integrations with numerical-relativity surrogate models, thus mapping the entire history of the binary from its asymptotic configuration at past-time infinity to the post-merger remnant. This approach naturally allows us to assess the impact of the precessio…
The coalescence of a neutron-star binary is likely to result in the formation of a neutron-star merger remnant for a large range of binary mass configurations. The massive merger remnant shows strong oscillations, which are excited by the merging process, and emits gravitational waves. Here we discuss possibilities and prospects of inferring unknown stellar properties of neutron stars by the detection of postmerger gravitational-wave emission, which thus leads to constraints of the equation of state of high-density matter. In particular, the dominant oscillation frequency of the postmerger rem…
Black hole thermodynamics has brought strong hints of a profound and fundamental connection between gravity, thermodynamics, and quantum theory. If the black hole does behave like a natural thermodynamic system, it should be thermodynamically stable in a clean environment. In this paper, using the observational data of binary black hole (BBH) mergers observed by LIGO, Virgo, and KAGRA detectors, we check whether the black hole remnants produced from BBH mergers in the LIGO-Virgo-KAGRA catalog GWTC-3 are thermodynamically stable. The criterion for the thermodynamic stability is quite simple and…
The Matter-Neutrino Resonance (MNR) phenomenon has the potential to significantly alter the flavor content of neutrinos emitted from compact object mergers. We present the first calculations of MNR transitions using neutrino self interaction potentials and matter potentials generated selfconsistently from a dynamical model of a three-dimensional neutron star merger. In the context of the single angle approximation, we find that Symmetric and Standard MNR transitions occur in both normal and inverted hierarchy scenarios. We examine the spatial regions above the merger remnant where propagating …
Super-Chandrasekhar remnants of double white dwarf mergers could sometimes collapse into a rapidly rotating neutron star (NS), accompanying with a mass ejection of a few times $0.01M_{\odot}$. Bright optical transient emission can be produced by the ejecta due to heating by radioactivities and particularly by energy injection from the NS. Since the merger remnants before collapse resemble a star evolving from the asymptotic giant branch phase to the planetary nebula phase, an intense dusty wind is considered to be driven about several thousand years ago before the collapse and surround the rem…
In this work, we study the merger of two neutron stars with a gravitational mass of 1.4 M_sol each, employing the Shen-Horowitz-Teige equation of state. This equation of state is a corner case, allowing the formation of a stable neutron star with the given total baryonic mass of 3.03 M_sol. We investigate in unprecedented detail the structure of the remnant, in particular the mass distribution, the thermal structure, and the rotation profile. We also compute fluid trajectories both inside the remnant and those relevant for the formation of the disk. We find a peanut-shaped fluid flow inside th…
The connection between short gamma-ray bursts (SGRBs) and binary neutron star (BNS) mergers was recently confirmed by the association of GRB 170817A with the merger event GW170817. However, no conclusive indications were obtained on whether the merger remnant that powered the SGRB jet was an accreting black hole (BH) or a long-lived massive neutron star (NS). Here, we explore the latter case via BNS merger simulations covering up to 250 ms after merger. We report, for the first time in a full merger simulation, the formation of a magnetically-driven collimated outflow along the spin axis of th…
Fast pairwise conversions of neutrinos are predicted to be ubiquitous in neutron star merger remnants with potentially major implications on the nucleosynthesis of the elements heavier than iron. We present the first sophisticated numerical solution of the neutrino flavor evolution above the remnant disk within a (2+1+1) dimensional setup: two spatial coordinates, one angular variable, and time. We look for a steady-state flavor configuration above the remnant disk. Albeit the linear stability analysis predicts flavor instabilities at any location above the remnant disk, our simulations in the…
We analyze the properties of the gravitational wave signal emitted after the merger of a binary neutron star system when the remnant survives for more than a 80 ms (and up to 140ms). We employ four different piecewise polytropic equations of state supplemented by an ideal fluid thermal component. We find that the post-merger phase can be subdivided into three phases: an early post-merger phase (where the quadrupole mode and a few subdominant features are active), the intermediate post-merger phase (where only the quadrupole mode is active) and the late post-merger phase (where convective insta…
