General Relativistic Hydrodynamic Simulations Research Articles

Emergence of Microphysical Bulk Viscosity in Binary Neutron Star Postmerger Dynamics

In nuclear matter in isolated neutron stars, the flavor content (e.g., proton fraction) is subject to weak interactions, establishing flavor (β-)equilibrium. However, there can be deviations from this equilibrium during the merger of two neutron stars. We study the resulting out-of-equilibrium dynamics during the collision by incorporating direct and modified Urca processes (in the neutrino-transparent regime) into general-relativistic hydrodynamics simulations with a simplified neutrino transport scheme. We demonstrate how weak-interaction-driven bulk viscosity in postmerger simulations can emerge and assess the bulk viscous dynamics of the resulting flow. We further place limits on the impact of the postmerger gravitational-wave strain. Our results show that weak-interaction-driven bulk viscosity can potentially lead to a phase shift of the postmerger gravitational-wave spectrum, although the effect is currently on the same level as the numerical errors of our simulation.

Secular Outflows from Long-Lived Neutron Star Merger Remnants

We study mass ejection from a binary neutron star merger producing a long-lived massive neutron star remnant with general-relativistic neutrino-radiation hydrodynamics simulations. In addition to outflows generated by shocks and tidal torques during and shortly after the merger, we observe the appearance of a wind driven by spiral density waves in the disk. This spiral-wave-driven outflow is predominantly located close to the disk orbital plane and have a broad distribution of electron fractions. At higher latitudes, a high electron-fraction wind is driven by neutrino radiation. The combined nucleosynthesis yields from all the ejecta components is in good agreement with Solar abundance measurements.

Probing nuclear physics with supernova gravitational waves and machine learning

ABSTRACT Core-collapse supernovae (CCSNe) are sources of powerful gravitational waves (GWs). We assess the possibility of extracting information about the equation of state (EOS) of high density matter from the GW signal. We use the bounce and early post-bounce signals of rapidly rotating supernovae. A large set of GW signals is generated using general relativistic hydrodynamics simulations for various EOS models. The uncertainty in the electron capture rate is parametrized by generating signals for six different models. To classify EOSs based on the GW data, we train a convolutional neural network (CNN) model. Even with the uncertainty in the electron capture rates, we find that the CNN models can classify the EOSs with an average accuracy of about 87 per cent for a set of four distinct EOS models.

General relativistic moving-mesh hydrodynamic simulations with arepo and applications to neutron star mergers

ABSTRACT We implement general relativistic hydrodynamics in the moving-mesh code arepo. We also couple a solver for the Einstein field equations employing the conformal flatness approximation. The implementation is validated by evolving isolated static neutron stars using a fixed metric or a dynamical space–time. In both tests, the frequencies of the radial oscillation mode match those of independent calculations. We run the first moving-mesh simulation of a neutron star merger. The simulation includes a scheme to adaptively refine or derefine cells and thereby adjusting the local resolution dynamically. The general dynamics are in agreement with independent smoothed particle hydrodynamics and static-mesh simulations of neutron star mergers. Coarsely comparing, we find that dynamical features like the post-merger double-core structure or the quasi-radial oscillation mode persist on longer time scales, possibly reflecting a low numerical diffusivity of our method. Similarly, the post-merger gravitational wave emission shows the same features as observed in simulations with other codes. In particular, the main frequency of the post-merger phase is found to be in good agreement with independent results for the same binary system, while, in comparison, the amplitude of the post-merger gravitational wave signal falls off slower, i.e. the post-merger oscillations are less damped. The successful implementation of general relativistic hydrodynamics in the moving-mesh arepo code, including a dynamical space–time evolution, provides a fundamentally new tool to simulate general relativistic problems in astrophysics.

Ab-initio General-relativistic Neutrino-radiation Hydrodynamics Simulations of Long-lived Neutron Star Merger Remnants to Neutrino Cooling Timescales

We perform the first 3D ab-initio general-relativistic neutrino-radiation hydrodynamics of a long-lived neutron star merger remnant spanning a fraction of its cooling timescale. We find that neutrino cooling becomes the dominant energy loss mechanism after the gravitational-wave dominated phase (∼20 ms postmerger). Electron flavor antineutrino luminosity dominates over electron flavor neutrino luminosity at early times, resulting in a secular increase of the electron fraction in the outer layers of the remnant. However, the two luminosities become comparable ∼20–40 ms postmerger. A dense gas of electron antineutrinos is formed in the outer core of the remnant at densities ∼1014.5 g cm−3, corresponding to temperature hot spots. The neutrinos account for ∼10% of the lepton number in this region. Despite the negative radial temperature gradient, the radial entropy gradient remains positive, and the remnant is stably stratified according to the Ledoux criterion for convection. A massive accretion disk is formed from the material squeezed out of the collisional interface between the stars. The disk carries a large fraction of the angular momentum of the system, allowing the remnant massive neutron star to settle to a quasi-steady equilibrium within the region of possible, stable, rigidly rotating configurations. The remnant is differentially rotating, but it is stable against the magnetorotational instability. Other MHD mechanisms operating on longer timescales are likely responsible for the removal of the differential rotation. Our results indicate the remnant massive neutron star is thus qualitatively different from a protoneutron stars formed in core-collapse supernovae.

General relativistic simulations of high-mass binary neutron star mergers: rapid formation of low-mass stellar black holes

ABSTRACT Almost a hundred compact binary mergers have been detected via gravitational waves by the LIGO-Virgo-KAGRA collaboration in the past few years providing us with a significant amount of new information on black holes and neutron stars. In addition to observations, numerical simulations using newly developed modern codes in the field of gravitational wave physics will guide us to understand the nature of single and binary degenerate systems and highly energetic astrophysical processes. We here presented a set of new fully general relativistic hydrodynamic simulations of high-mass binary neutron star systems using the open-source EINSTEIN TOOLKIT and LORENE codes. We considered systems with total baryonic masses ranging from 2.8$\, \mathrm{M}_\odot$ to 4.0$\, \mathrm{M}_\odot$ and used the SLy equation of state. We analysed the gravitational wave signal for all models and report potential indicators of systems undergoing rapid collapse into a black hole that could be observed by future detectors like the Einstein Telescope and the Cosmic Explorer. The properties of the post-merger black hole, the disc and ejecta masses, and their dependence on the binary parameters were also extracted. We also compared our numerical results with recent analytical fits presented in the literature and provided parameter-dependent semi-analytical relations between the total mass and mass ratio of the systems and the resulting black hole masses and spins, merger frequency, BH formation time, ejected mass, disc mass, and radiated gravitational wave energy.

Impact of anisotropic ejecta on jet dynamics and afterglow emission in binary neutron-star mergers

ABSTRACT Binary neutron-stars mergers widely accepted as potential progenitors of short gamma-ray bursts. After the remnant of the merger has collapsed to a black hole, a jet is powered and may breakout from the the matter expelled during the collision and the subsequent wind emission. The interaction of the jet with the ejecta may affect its dynamics and the resulting electromagnetic counterparts. We here examine how an inhomogeneous and anisotropic distribution of ejecta affects such dynamics, dictating the properties of the jet-ejecta cocoon and of the afterglow radiated by the jet upon deceleration. More specifically, we carry out general-relativistic hydrodynamical simulations of relativistic jets launched within a variety of geometrically inhomogeneous and anisotropic distributions of ejected matter. We find that different anisotropies impact the variance of the afterglow light curves as a function of the jet luminosity and ejected mass. A considerable amount of the jet energy is deposited in the cocoon through the jet-ejecta interaction with a small but important dependence on the properties of the ejecta. Furthermore, all configurations show a two-component behaviour for the polar structure of the jet, with a narrow core at large energies and Lorentz factors and a shallow segment at high latitudes from the jet axis. Hence, afterglows measured on off-axis lines of sight could be used to deduce the properties of the ejected matter, but also that the latter need to be properly accounted for when modelling the afterglow signal and the jet-launching mechanisms.

General relativistic hydrodynamic simulations of perturbed transonic accretion

Contact. Comparing horizon-scale observations of Sgr A* and M 87* with numerical simulations has provided considerable insight into their interpretation. Most of these simulations are variations of the same physical scenario consisting of a rotation-supported torus seeded with poloidal magnetic fields. However, this approach has several well-known limitations such as secular decreasing trends in mass-accretion rates that render long-term variability studies difficult; a lack of connection with the large-scale accretion flow, which is replaced by an artificial medium emulating vacuum; and significant differences with respect to the predictions of models of accretion onto Sgr A* fed by stellar winds. Aims. We aim to study the flow patterns that arise on horizon scales in more general accretion scenarios that have a clearer connection with the large-scale flow, and are at the same time controlled by a reduced set of parameters. Methods. As a first step in this direction, we performed three-dimensional general relativistic hydrodynamic simulations of rotating transonic flows with velocity perturbations injected from a spherical boundary located far away from the central object (1000 gravitational radii). We studied the general properties of these flows with varying perturbation amplitudes and angular momentum. We analyzed time series of mass and angular-momentum radial fluxes, angle- and time-averaged profiles, and synthetic bremsstrahlung light curves, as well as the three-dimensional structure of the flow, and quantified shock and sonic transitions in the solutions. Results. We observe a rich phenomenology in accretion patterns, which includes smooth Bondi-like flows, turbulent torus-like structures, shocks, filaments, and complex sonic structures. For sufficiently large perturbations and angular momentum, radial profiles deviate from the constant entropy and constant angular-momentum profiles used for initialization and resemble those of advection-dominated accretion flows, showing evidence of entropy generation and angular-momentum redistribution not mediated by magnetic fields. Time series do not show the secular decreasing trend and are suitable for long-term variability studies. We see that the fluctuations are amplified and extend further in frequency than the injected spectrum, producing a red noise spectrum both for the mass-accretion rate and the synthetic light curves. Conclusions. We present a simulation setup that can produce a wide variety of flow patterns at horizon scales and incorporate information from large scale accretion models. The future inclusion of magnetic fields and radiative cooling could make this type of simulation a viable alternative for the numerical modeling of general low-luminosity active galactic nuclei (AGNs).

Evolution and explosions of metal-enriched supermassive stars: proton rich general relativistic instability supernovae

ABSTRACT The assembly of supermassive black holes poses a challenge primarily because of observed quasars at high redshift, but additionally because of the current lack of observations of intermediate mass black holes. One plausible scenario for creating supermassive black holes is direct collapse triggered by the merger of two gas-rich galaxies. This scenario allows the creation of supermassive stars with solar metallicity. We investigate the behaviour of metal enriched supermassive stars which collapse due to the general relativistic radial instability during hydrogen burning. These stars contain both hydrogen and metals and thus may explode due to the CNO cycle (carbon–nitrogen–oxygen) and the rp process (rapid proton capture). We perform a suite of stellar evolution simulations for a range of masses and metallicities, with and without mass-loss. We evaluate the stability of these supermassive stars by solving the pulsation equation in general relativity. When the stars becomes unstable, we perform 1D general relativistic hydrodynamical simulations coupled to a 153 isotope nuclear network with cooling from neutrino reactions, in order to determine if the stars explode. If the stars do explode, we post process the nucleosynthesis using a 514 isotope network which includes additional proton rich isotopes. These explosions are characterized by enhanced nitrogen and intermediate mass elements ($16\ge \rm {A}\ge 25$), and suppressed light elements ($8\ge \rm {A}\ge 14$), and we comment on recent observations of super-solar nitrogen in GN-z11.

Simulating bulk viscosity in neutron stars. I. Formalism

The faithful inclusion of the effects of bulk viscosity induced by the presence of chemical reactions is an important issue for simulations of core-collapse supernovae, binary neutron star mergers, and neutron star oscillations, where particle abundances are locally pushed out of chemical equilibrium by rarefaction and compression of the fluid elements. In this work, we discuss three different approaches that can be used to implement bulk viscosity in general relativistic hydrodynamic simulations of neutron stars: the exact multi-component reacting fluid, and two M\"uller-Israel-Stewart theories, namely the second order Hiscock-Lindblom model and its linear limit, the Maxwell-Cattaneo model. After discussing the theory behind the three approaches, we specialize their dynamics equations to spherical symmetry in the radial gauge-polar slicing (i.e., Schwarzschild) coordinates. We also discuss a particular choice for the equation of state of the fluid and the associated neutrino emission rates, which are used in a companion paper for the numerical comparison of the three frameworks, and we obtain the effective sound speed for the Hiscock-Lindblom theory in the non-linear regime.

Extremely Relativistic Tidal Disruption Events

Extreme tidal disruption events (eTDEs), which occur when a star passes very close to a supermassive black hole, may provide a way to observe a long-sought general relativistic effect: orbits that wind several times around a black hole and then leave. Through general relativistic hydrodynamics simulations, we show that such eTDEs are easily distinguished from most tidal disruptions, in which stars come close, but not so close, to the black hole. Following the stellar orbit, the debris is initially distributed in a crescent, it then turns into a set of tight spirals circling the black hole, which merge into a shell expanding radially outwards. Some mass later falls back toward the black hole, while the remainder is ejected. Internal shocks within the infalling debris power the observed emission. The resulting lightcurve rises rapidly to roughly the Eddington luminosity, maintains this level for between a few weeks and a year (depending on both the stellar mass and the black hole mass), and then drops. Most of its power is in thermal X-rays at a temperature ∼(1–2) × 106 K (∼100–200 eV). The debris evolution and observational features of eTDEs are qualitatively different from ordinary TDEs, making eTDEs a new type of TDE. Although eTDEs are relatively rare for lower-mass black holes, most tidal disruptions around higher-mass black holes are extreme. Their detection offers a view of an exotic relativistic phenomenon previously inaccessible.

Gravitational waves or x‐ray counterpart? No need to choose

AbstractBinary black holes emit gravitational waves (GWs) as they inspiral toward coalescence. Searches for electromagnetic counterparts to these GWs rely on looking for common sources producing both signals. In this article, we take a different approach: we investigate the impact of radiation zone effects, including retardation effects and gravitational wave propagation onto the circumbinary disk around stellar‐mass, spinning black holes, using general relativistic hydrodynamical simulations. Then we used a general relativistic ray‐tracing code to extract its x‐ray spectrum and lightcurve. This allowed us to show that radiation zone effects leave an imprint onto the disk, leading to quasi‐periodic patterns in the x‐ray lightcurve. The amplitude of the modulation is weak () but increases with time and is strongly dependent on the inclination angle.

On the origin of the lump in circumbinary discs

ABSTRACT Accreting binary black holes (BBHs) are multimessenger sources, emitting copious electromagnetic (EM) and gravitational waves. One of their most promising EM signatures is the light-curve modulation caused by a strong unique and extended azimuthal overdensity structure orbiting at the inner edge of the circumbinary disc (CBD), dubbed ‘lump’. In this paper, we investigate the origin of this structure using 2D general-relativistic (GR) hydrodynamical simulations of a CBD in an approximate BBH space–time. First, we use the symmetric mass-ratio case to study the transition from the natural m = 2 mode to m = 1. The asymmetry with respect to m = 2 grows exponentially, pointing to an instability origin. We indeed find that the CBD edge is prone to a (magneto)hydrodynamical instability owing to the disc edge density sharpness: the Rossby Wave Instability (RWI). The RWI criterion is naturally fulfilled at the CBD edge and we report the presence of vortices, which are typical structures of the RWI. The RWI is also at work in the asymmetric mass-ratio cases (from 0.1 to 0.5). However, the CBD edge sharpness decreases with a decreasing mass ratio, and so the lump. By proposing a scenario for this lump formation, our work further supports its existence in astrophysical CBDs and potential source for an EM signature of BBHs. Finally, because the RWI is not caused by GR effects, it is also a robust candidate for the lump origin in CBDs around non-compact objects, e.g. binary protostars.

Comprehensive Study of Mass Ejection and Nucleosynthesis in Binary Neutron Star Mergers Leaving Short-lived Massive Neutron Stars

By performing general relativistic hydrodynamics simulations with an approximate neutrino radiation transfer, the properties of ejecta in the dynamical and post-merger phases are investigated in the cases in which the remnant massive neutron star collapses into a black hole in ≲20 ms after the onset of the merger. The dynamical mass ejection is investigated in three-dimensional simulations. The post-merger mass ejection is investigated in two-dimensional axisymmetric simulations with viscosity using the three-dimensional post-merger systems as the initial conditions. We show that the typical neutron richness of the dynamical ejecta is higher for the merger of more asymmetric binaries; hence, heavier r-process nuclei are dominantly synthesized. The post-merger ejecta are shown to have only mild neutron richness, which results in the production of lighter r-process nuclei, irrespective of the binary mass ratios. Because of the larger disk mass, the post-merger ejecta mass is larger for more asymmetric binary mergers. Thus, the post-merger ejecta can compensate for the underproduced lighter r-process nuclei for asymmetric merger cases. As a result, by summing up both ejecta components, the solar residual r-process pattern is reproduced within the average deviation of a factor of three, irrespective of the binary mass ratio. Our result also indicates that the (about a factor of a few) light-to-heavy abundance scatter observed in r-process-enhanced stars can be attributed to variation in the binary mass ratio and total mass. Implications of our results associated with the mass distribution of compact neutron star binaries and the magnetar scenario of short gamma-ray bursts are discussed.

Evolution of equal mass binary bare quark stars in full general relativity: Could a supramassive merger remnant experience prompt collapse?

We have evolved mergers of equal-mass binary quark stars, the total mass of which is close to the mass shedding limit of uniformly rotating configurations, in fully general relativistic hydrodynamic simulations, aimed at investigating the post-merger outcomes. In particular, we have identified the threshold mass for prompt black hole formation after the merger, by tracing the minimum lapse function as well as the amount of ejected material during the merger simulation. A semi-analytical investigation based on the angular momentum contained in the merger remnant is also performed to verify the results. For the equation of state considered in this work, the maximum mass of TOV solutions for which is 2.10 $M_\odot$, the threshold mass is found between 3.05 and 3.10 $M_\odot$. This result is consistent (with a quantitative error smaller than 1%) with the universal relation derived from the numerical results of symmetric binary neutron star mergers. Contrary to the neutron star case, the threshold mass is close to the mass shedding limit of uniformly rotating quark star. Consequently, we have found that binary quark stars with total mass corresponding to the long-lived supramassive remnant for neutron star case, could experience collapse to black hole within several times dynamical timescale, making quark stars as exceptions of the commonly accepted post-merger scenarios for binary neutron star mergers. We have suggested explanation for both the similarity and the difference, between quark stars and neutron stars.

Stability analysis of supermassive primordial stars: a new mass range for general relativistic instability supernovae

ABSTRACT Observed supermassive black holes in the early Universe have several proposed formation channels in part because most of these channels are difficult to probe. One of the more promising channels, the direct collapse of a supermassive star has several possible probes including the explosion of a helium-core supermassive star triggered by a general relativistic instability. We develop a straightforward method for evaluating the general relativistic radial instability without simplifying assumptions and apply it to population III supermassive stars taken from a post Newtonian stellar evolution code. This method is more accurate than previous determinations and it finds that the instability occurs earlier in the evolutionary life of the star. Using the results of the stability analysis, we perform 1D general relativistic hydrodynamical simulations, and we find two general relativistic instability supernovae fuelled by alpha capture reactions as well as several lower mass pulsations, analogous to the puslational pair instability process. The mass range for the events ($2.6-3.0\times 10^4{\rm M}_\odot$) is lower than that had been suggested by previous works ($5.5\times 10^4{\rm M}_\odot$), because the instability occurs earlier in the star’s evolution. The explosion may be visible to among others JWST, while the discovery of the pulsations opens up additional possibilities for observation.

Three-dimensional General-relativistic Simulations of Neutrino-driven Winds from Rotating Proto-neutron Stars

We explore the effects of rapid rotation on the properties of neutrino-heated winds from proto-neutron stars (PNS) formed in core-collapse supernovae or neutron-star mergers by means of three-dimensional general-relativistic hydrodynamical simulations with M0 neutrino transport. We focus on conditions characteristic of a few seconds into the PNS cooling evolution when the neutrino luminosities obey erg s−1, and over which most of the wind mass loss will occur. After an initial transient phase, all of our models reach approximately steady-state outflow solutions with positive energies and sonic surfaces captured on the computational grid. Our nonrotating and slower rotating models (angular velocity relative to Keplerian Ω/ΩK ≲ 0.4; spin period P ≳ 2 ms) generate approximately spherically symmetric outflows with properties in good agreement with previous PNS wind studies. By contrast, our most rapidly spinning PNS solutions (Ω/ΩK ≳ 0.75; P ≈ 1 ms) generate outflows focused in the rotational equatorial plane with much higher mass-loss rates (by over an order of magnitude), lower velocities, lower entropy, and lower asymptotic electron fractions, than otherwise similar nonrotating wind solutions. Although such rapidly spinning PNS are likely rare in nature, their atypical nucleosynthetic composition and outsized mass yields could render them important contributors of light neutron-rich nuclei compared to more common slowly rotating PNS birth. Our calculations pave the way to including the combined effects of rotation and a dynamically important large-scale magnetic field on the wind properties within a three-dimensional GRMHD framework.

Exploring the Phase Diagram of V-QCD with Neutron Star Merger Simulations

Determining the phase structure of Quantum Chromodynamics (QCD) and its Equation of State (EOS) at densities and temperatures realized inside neutron stars and their mergers is a long-standing open problem. The holographic V-QCD framework provides a model for the EOS of dense and hot QCD, which describes the deconfinement phase transition between a dense baryonic and a quark matter phase. We use this model in fully general relativistic hydrodynamic (GRHD) simulations to study the formation of quark matter and the emitted gravitational wave signal of binary systems that are similar to the first ever observed neutron star merger event GW170817.

Accretion onto black holes inside neutron stars with piecewise-polytropic equations of state: Analytic and numerical treatments

We consider spherically symmetric accretion onto a small, possibly primordial, black hole residing at the center of a neutron star governed by a cold nuclear equation of state (EOS). We generalize the relativistic Bondi solution for such EOSs, approximated by piecewise polytropes, and thereby obtain analytical expressions for the steady-state matter profiles and accretion rates. We compare these rates with those found by time-dependent, general relativistic hydrodynamical simulations upon relaxation and find excellent agreement. We consider several different candidate EOSs, neutron star masses and central densities and find that the accretion rates vary only little, resulting in an accretion rate that depends primarily on the black hole mass, and only weakly on the properties of the neutron star.

Dynamical ejecta of neutron star mergers with nucleonic weak processes – II: kilonova emission

ABSTRACT The majority of existing results for the kilonova (or macronova) emission from material ejected during a neutron-star (NS) merger is based on (quasi-) one-zone models or manually constructed toy-model ejecta configurations. In this study, we present a kilonova analysis of the material ejected during the first $\sim 10\,$ ms of a NS merger, called dynamical ejecta, using directly the outflow trajectories from general relativistic smoothed-particle hydrodynamics simulations, including a sophisticated neutrino treatment and the corresponding nucleosynthesis results, which have been presented in Part I of this study. We employ a multidimensional two-moment radiation transport scheme with approximate M1 closure to evolve the photon field and use a heuristic prescription for the opacities found by calibration with atomic-physics-based reference results. We find that the photosphere is generically ellipsoidal but augmented with small-scale structure and produces emission that is about 1.5–3 times stronger towards the pole than the equator. The kilonova typically peaks after $0.7\!-\!1.5\,$ d in the near-infrared frequency regime with luminosities between $3\!-\!7\times 10^{40}\,$ erg s−1 and at photospheric temperatures of $2.2\!-\!2.8\times 10^3\,$ K. A softer equation of state or higher binary-mass asymmetry leads to a longer and brighter signal. Significant variations of the light curve are also obtained for models with artificially modified electron fractions, emphasizing the importance of a reliable neutrino-transport modelling. None of the models investigated here, which only consider dynamical ejecta, produces a transient as bright as AT2017gfo. The near-infrared peak of our models is incompatible with the early blue component of AT2017gfo.