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![Spectrograms of the gravitational wave signal for the simulations using the SFHo (left), LS220 (middle) and DD2 (right) equations of state, for an optimally oriented binary at 100 Mpc . The dashed horizontal black curves show the location of the peaks predicted in [42]. The time is calculated as the difference between the center of the window function used to compute the spectrogram (see text) and the peak of the gravitational wave amplitude. The dominant peak is clearly visible at (2 . 3 − 3) kHz . Secondary peaks in the (1 . 3 − 2 . 1) kHz range are poorly resolved, and quickly damped. The strong emission at frequencies f ∼ < 1 . 3 kHz is the merger signal itself.](profile/Francois-Foucart/publication/283117645/figure/fig6/AS:293928008994828@1447089306667/Spectrograms-of-the-gravitational-wave-signal-for-the-simulations-using-the-SFHo-left.png)
Spectrograms of the gravitational wave signal for the simulations using the SFHo (left), LS220 (middle) and DD2 (right) equations of state, for an optimally oriented binary at 100 Mpc . The dashed horizontal black curves show the location of the peaks predicted in [42]. The time is calculated as the difference between the center of the window function used to compute the spectrogram (see text) and the peak of the gravitational wave amplitude. The dominant peak is clearly visible at (2 . 3 − 3) kHz . Secondary peaks in the (1 . 3 − 2 . 1) kHz range are poorly resolved, and quickly damped. The strong emission at frequencies f ∼ < 1 . 3 kHz is the merger signal itself.
Source publication
Neutron star mergers are among the most promising sources of gravitational
waves for advanced ground-based detectors. These mergers are also expected to
power bright electromagnetic signals, in the form of short gamma-ray bursts,
infrared/optical transients, and radio emission. Simulations of these mergers
with fully general relativistic codes are...
Contexts in source publication
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... t < ∆ t , and W ( t ) = 0 otherwise. The choice of the window size ∆ t is a trade-off between high time resolution (small ∆ t ) and high spectral resolution (large ∆ t ). For Fig. 6, we use ∆ t = 6 ms , which causes a noticeable smoothing of the spectrogram in time but is necessary to start resolving the secondary peaks of emission. In the spectrograms, the fundamental peak is clearly visible, and strongest for the softest equation of state (SFHo). It is only mildly damped over the short duration of the simulations, but varies in frequency as the structure of the remnant evolves. This shift can be as large as 200 Hz in the case of the DD2 equation of state, and mostly occurs in the first ∼ 5 ms after merger. Fig. 6 also clearly shows that the gravitational wave emission at the secondary peaks is extremely short-lived, with a decay timescale of ∼ 1 ms − 3 ms . The emission at the secondary peak is both weaker and shorter-lived for the softest equation of state (SFHo). For low-mass systems, we thus see that only the fundamental mode remains significantly excited in the post-merger remnant. Gravitational wave emission at lower frequencies is largely coincident with the merger itself, and the secondary peaks are naturally broad in spectral space as the signal decays over only a few oscillation periods. For the low-mass systems studied here, there is thus a significant difference between the strong, long-lived peak corresponding to the fundamental mode, and the weak, broad and short-lived peaks at lower frequencies, which would naturally make the latter difficult to observe or disentangle from detector noise. The interpretation of the emission of gravitational waves at frequency f spiral as the result of rotating spiral structures within the core of the remnant is partially supported by visu- alizations of the rest mass density in the equatorial plane (see Fig. 8). Right after merger, when the emission at f spiral is the strongest, the LS220 and DD2 simulations show clear spiral arms, including in high-density regions. In the SFHo simulation, the spiral arms are significantly weaker. Later on, 3 ms after merger, lower density spiral structures remain visible in the LS220 and DD2 simulations, and are stronger in the latter simulation. Again, this is in agreement with the observed difference in the damping timescale of the emission at f spiral observed in the spectrograms (Fig. 6). Finally, 10 ms after merger, extended spiral structures remain visible, but they are confined to low-density regions and are unlikely to significantly contribute to gravitational wave emission. Our results thus appear consistent with the interpretation of f spiral proposed by Bauswein & Stergoulias [42]. An alternative universal relation, this time between the strongest peak of the post-merger gravitational wave signal and the tidal coupling constant κ , has been proposed ...
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... t < ∆ t , and W ( t ) = 0 otherwise. The choice of the window size ∆ t is a trade-off between high time resolution (small ∆ t ) and high spectral resolution (large ∆ t ). For Fig. 6, we use ∆ t = 6 ms , which causes a noticeable smoothing of the spectrogram in time but is necessary to start resolving the secondary peaks of emission. In the spectrograms, the fundamental peak is clearly visible, and strongest for the softest equation of state (SFHo). It is only mildly damped over the short duration of the simulations, but varies in frequency as the structure of the remnant evolves. This shift can be as large as 200 Hz in the case of the DD2 equation of state, and mostly occurs in the first ∼ 5 ms after merger. Fig. 6 also clearly shows that the gravitational wave emission at the secondary peaks is extremely short-lived, with a decay timescale of ∼ 1 ms − 3 ms . The emission at the secondary peak is both weaker and shorter-lived for the softest equation of state (SFHo). For low-mass systems, we thus see that only the fundamental mode remains significantly excited in the post-merger remnant. Gravitational wave emission at lower frequencies is largely coincident with the merger itself, and the secondary peaks are naturally broad in spectral space as the signal decays over only a few oscillation periods. For the low-mass systems studied here, there is thus a significant difference between the strong, long-lived peak corresponding to the fundamental mode, and the weak, broad and short-lived peaks at lower frequencies, which would naturally make the latter difficult to observe or disentangle from detector noise. The interpretation of the emission of gravitational waves at frequency f spiral as the result of rotating spiral structures within the core of the remnant is partially supported by visu- alizations of the rest mass density in the equatorial plane (see Fig. 8). Right after merger, when the emission at f spiral is the strongest, the LS220 and DD2 simulations show clear spiral arms, including in high-density regions. In the SFHo simulation, the spiral arms are significantly weaker. Later on, 3 ms after merger, lower density spiral structures remain visible in the LS220 and DD2 simulations, and are stronger in the latter simulation. Again, this is in agreement with the observed difference in the damping timescale of the emission at f spiral observed in the spectrograms (Fig. 6). Finally, 10 ms after merger, extended spiral structures remain visible, but they are confined to low-density regions and are unlikely to significantly contribute to gravitational wave emission. Our results thus appear consistent with the interpretation of f spiral proposed by Bauswein & Stergoulias [42]. An alternative universal relation, this time between the strongest peak of the post-merger gravitational wave signal and the tidal coupling constant κ , has been proposed ...
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... t < ∆ t , and W ( t ) = 0 otherwise. The choice of the window size ∆ t is a trade-off between high time resolution (small ∆ t ) and high spectral resolution (large ∆ t ). For Fig. 6, we use ∆ t = 6 ms , which causes a noticeable smoothing of the spectrogram in time but is necessary to start resolving the secondary peaks of emission. In the spectrograms, the fundamental peak is clearly visible, and strongest for the softest equation of state (SFHo). It is only mildly damped over the short duration of the simulations, but varies in frequency as the structure of the remnant evolves. This shift can be as large as 200 Hz in the case of the DD2 equation of state, and mostly occurs in the first ∼ 5 ms after merger. Fig. 6 also clearly shows that the gravitational wave emission at the secondary peaks is extremely short-lived, with a decay timescale of ∼ 1 ms − 3 ms . The emission at the secondary peak is both weaker and shorter-lived for the softest equation of state (SFHo). For low-mass systems, we thus see that only the fundamental mode remains significantly excited in the post-merger remnant. Gravitational wave emission at lower frequencies is largely coincident with the merger itself, and the secondary peaks are naturally broad in spectral space as the signal decays over only a few oscillation periods. For the low-mass systems studied here, there is thus a significant difference between the strong, long-lived peak corresponding to the fundamental mode, and the weak, broad and short-lived peaks at lower frequencies, which would naturally make the latter difficult to observe or disentangle from detector noise. The interpretation of the emission of gravitational waves at frequency f spiral as the result of rotating spiral structures within the core of the remnant is partially supported by visu- alizations of the rest mass density in the equatorial plane (see Fig. 8). Right after merger, when the emission at f spiral is the strongest, the LS220 and DD2 simulations show clear spiral arms, including in high-density regions. In the SFHo simulation, the spiral arms are significantly weaker. Later on, 3 ms after merger, lower density spiral structures remain visible in the LS220 and DD2 simulations, and are stronger in the latter simulation. Again, this is in agreement with the observed difference in the damping timescale of the emission at f spiral observed in the spectrograms (Fig. 6). Finally, 10 ms after merger, extended spiral structures remain visible, but they are confined to low-density regions and are unlikely to significantly contribute to gravitational wave emission. Our results thus appear consistent with the interpretation of f spiral proposed by Bauswein & Stergoulias [42]. An alternative universal relation, this time between the strongest peak of the post-merger gravitational wave signal and the tidal coupling constant κ , has been proposed ...
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... R 1 . 6 the radius of a neutron star of mass M NS = 1 . 6 M [in kilometers], f peak given in kHz, and we assume a total mass of 2 . 4 M . Here, we used the proportionality relation f peak ∝ M tot /R max 3 [53], with M tot the total mass of the binary at infinite separation, and R max the radius of a neutron star at the maximum mass M max allowed for this equation of state. We find disagreements of only ∼ (30 − 70) Hz between the numerical results and the fitting formula, which would translate to systematic errors of ∼ (100 − 200) m in the radius of a 1 . 6 M neutron star. We should also note that the merger of two 1 . 2 M neutron stars with the LS220 equation of state was studied with an approximate treatment of gravity in [53]. The dominant frequency of the post-merger signal was 2 . 55 kHz in that study. We find an extremely close value for that dominant frequency, 2 . 56 kHz . For the secondary peak, we compare our results to the linear fits to f spiral and f 2 − 0 provided in Fig. 4 of Bauswein & Stergoulias [42]. Bauswein & Stergoulias predict that the spiral mode should be the dominant secondary mode for mergers in which a stable or long-lived hypermassive neutron star is formed. This is in good agreement with our results as the first subdominant peak observed in our spectra agrees well with the frequency of f spiral . The SFHo equation of state waveform also shows a peak close to f 2 − 0 and the LS220 equation of state waveform has some extra power at f 2 − 0 . For the DD2 equation of state waveform, the predicted location of the f peak is too close to the merger frequency for any clear feature to be observed. We should note, however, that the predictions provided in [42] for f spiral and f 2 − 0 are not as powerful as the unique fitting formula provided for f peak . This is because a different linear relation between the frequency of the mode and the compactness of the star has to be determined for each choice of neutron star masses. A universal relation between the secondary peak of the post-merger waveform and the neutron star compactness has been proposed by Takami et al. [31, 32]. This universal relation would predict that the secondary peak is at ∼ 1 . 6 kHz for the LS220 and DD2 equations of state and at ∼ 1 . 7 kHz for the SFHo equation of state. The SFHo has its third strongest peak at that frequency, and that peak is not much weaker than the secondary peak. The other two equations of state show a difference of (100 − 200) Hz between the theoretical predictions and the location of the secondary peak, which would translate into (0 . 5 − 1 . 0) km errors in the determination of the neutron star radius. Using the universal formula from [31, 32] to infer the compactness of the neutron stars considered in this paper would thus lead to significant errors in the determination of neutron star radii. Simi- lar differences were observed by Bauswein & Stergoulias [42] for low-mass binaries. Like Takami et al. [31, 32] (but as opposed to Bauswein & Stergoulias [42]), our code is fully general relativistic. The observed differences are thus not due to the treatment of gravity. A more likely explanation is the use of a lower mass system combined with the use of nuclear- theory based equations of state. The choice of equation of state may also explain why Takami et al. [31] find significant differences between the frequency of the fundamental mode and f peak fit , while we do not. The largest differences in [31] were observed for the simple Γ = 2 polytrope, while more realistic equations of state performed better. The general relativistic simulations of Palenzuela et al. [27], performed for higher mass systems, found ∼ 10% disagreement between the numerical results and f peak fit – a larger difference than in our simulations, but one that would still allow the recovery of the neutron star radius with systematic errors 0 . 5 km . It is quite likely that the fitted frequency f peak fit is not universal, but nonetheless practically applicable to realistic neutron star equations of state. More insight can be gained in those post-merger features by considering a spectrogram of the gravitational wave signal, shown in Fig. 6. The quantity plotted there ...
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... compare f peak to our results in Table II. f peak is system- atically (0 . 2 − 0 . 3) kHz below the peak frequency measured in our simulations. The magnitude of the error is compara- ble to the scatter observed within the simulations presented in Bernuzzi et al. [86]. It is unclear whether the systematic underestimate of the peak frequency comes from our use of a hot nuclear theory based equation of state, the applica- tion of (13) to a low mass system, or the intrinsic scatter around (13). Bernuzzi et al. use piecewise polytropic equations of state with a Γ -law thermal component for the pressure, and f peak B is only fitted to mergers with total mass within the interval [2 . 45 M , 2 . 9 M ] , so that comparison with our results requires extrapolation of their fitting formula to lower mass systems. Comparing the standard and high resolution simulations for the LS220 equation of state does not reveal any significant resolution dependence in the spectrum of the gravitational wave signal. Even in the time domain, the low-resolution and high- resolution waveforms appear to only differ by a small time shift (see Fig. 7). We note that for both Fig. 6 and Fig. 7, we use waveforms extracted at finite radius ( r ∼ 130 M ) to have access to a longer post-merger signal. However, Fig. 6 shows that only a few milliseconds of post-merger signal are actu- ally necessary to study the secondary peaks in the post-merger spectrum, since the gravitational wave emission at those ...
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... compare f peak to our results in Table II. f peak is system- atically (0 . 2 − 0 . 3) kHz below the peak frequency measured in our simulations. The magnitude of the error is compara- ble to the scatter observed within the simulations presented in Bernuzzi et al. [86]. It is unclear whether the systematic underestimate of the peak frequency comes from our use of a hot nuclear theory based equation of state, the applica- tion of (13) to a low mass system, or the intrinsic scatter around (13). Bernuzzi et al. use piecewise polytropic equations of state with a Γ -law thermal component for the pressure, and f peak B is only fitted to mergers with total mass within the interval [2 . 45 M , 2 . 9 M ] , so that comparison with our results requires extrapolation of their fitting formula to lower mass systems. Comparing the standard and high resolution simulations for the LS220 equation of state does not reveal any significant resolution dependence in the spectrum of the gravitational wave signal. Even in the time domain, the low-resolution and high- resolution waveforms appear to only differ by a small time shift (see Fig. 7). We note that for both Fig. 6 and Fig. 7, we use waveforms extracted at finite radius ( r ∼ 130 M ) to have access to a longer post-merger signal. However, Fig. 6 shows that only a few milliseconds of post-merger signal are actu- ally necessary to study the secondary peaks in the post-merger spectrum, since the gravitational wave emission at those ...
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Citations
... Spectral-type methods have proven extremely useful in producing a large number of long and accurate gravitational waveforms from binary black hole merger simulations [11,12,13,14,15,16,17,18,19], as well as other applications in relativistic astrophysics [20,21,22,23,24,25]. The Spectral Einstein Code (SpEC) [12] performs binary neutron star merger simulations by solving the spacetime using pseudospectral methods and the magnetohydrodynamics (MHD) using finite difference methods [26,27,28,29,30,31,32,33]. However, these use completely separate grids requiring interpolation between them at every time/sub step. ...
... The transition layer and spherical shell are divided into 6 regions with 90 • opening angles (i.e. a cubed sphere geometry). We vary the resolution across the scenarios, but by convention, the cube consists of K 3 elements, where we may have K ∈ [8,16,32]. Each region of the inner spherical shells consists of (K/2) 3 elements, and each region the outer shells consists of (K/2) r × (K/4) 2 θ,ϕ elements. ...
... We use this case to provide an in-depth exploration of the effects of resolution on our results. As such, we test all K ∈ [8,16,32] and all DG basis functions P 5 through P 7 . In all cases, we run the simulation to t = 10 ms. ...
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.
... If the remnant object is less massive than the TOV limit, it survives as a neutron star. Efforts to numerically simulate the merger scenario by employing Einstein's theory of general relativity (GRHD) were started decades before detecting the first binary neutron star merger event, GW170817 [2,[5][6][7][8][9]. ...
We examine bulk viscosity, taking into account trapped neutrinos in baryonic matter, in the context of binary neutron star mergers. Following the merging event, the binary star can yield a remnant compact object with densities up to $5$ nuclear saturation density and temperature upto $50$ MeV resulting in the retention of neutrinos. We employ two relativistic mean field models, NL3 and DDME2, to describe the neutrino-trapped baryonic matter. The dissipation coefficient is determined by evaluating the Modified URCA interaction rate in the dense baryonic medium, and accounting for perturbations caused by density oscillations. We observe the resonant behavior of bulk viscosity as it varies with the temperature of the medium. The bulk viscosity peak remains within the temperature range of $\sim 13-50$ MeV, depending upon the underlying equation of states and lepton fractions. This temperature range corresponds to the relevant domain of binary neutron star mergers. We also note that in presence of neutrinos in the medium the bulk viscosity peak shifts towards higher temperature and the peak value of bulk viscosity also changes. The time scale of viscous dissipation is dictated by the beta-off-equilibrium susceptibilities derived from the nuclear equation of state. The resulting viscous decay time scale ranges from $32-100$ milliseconds, which aligns with the order of magnitude of the post-merger object's survival time in some specific scenarios.
... We denote the instantaneous frequency at the onset of merger at which the absolute amplitude |h| reaches its maximum as f merge , which is sometimes denoted as f peak or f 2,max in the literature. We also define f 2,peak as the frequency as the dominant peak in the Fourier spectrum of h eff in the post-merger phase, which is attributed to the l = m = 2 mode of the HMNS [15,36,[98][99][100][101]. The acceleration spectral density (ASD)h √ f (Hz −1/2 ) is plotted in Fig. 19 for this model assuming a source distance of 50 Mpc. ...
We investigate the properties of post-merger 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 long-lived $\phi-$mode in hypermassive neutron stars due to the presence of the massive scalar field which enhances the quasi-radial oscillation in the remnant. We investigate the descalarization condition in hypermassive neutron stars and discover a distinctive signature in post-merger gravitational waves.
... Fryer 1999;Chan et al. 2018;Burrows et al. 2023) or a massive remnant disk formed after disruption of a neutron star during compact binary merger events (e.g. Foucart 2012;Foucart et al. 2016;Radice et al. 2018;Bernuzzi 2020;Krüger & Foucart 2020;Camilletti et al. 2024). ...
In modeling a relativistic disk around a compact object, the self-gravity of the disk is often neglected while it needs to be incorporated for more accurate descriptions in several circumstances. Extending the Komatsu-Eriguchi-Hachisu self-consistent field method, we present numerical models of a rapidly rotating neutron star with a self-gravitating disk in stationary equilibrium. In particular, our approach allows us to obtain numerical solutions involving a massive disk with the rest mass $O(10^{-1})-O(10^0) M_\odot$ closely attached to a rotating neutron star. We also assess the impact of self-gravity on the internal structure of the disk and the neutron star. These axisymmetric, stationary solutions can be employed for simulations involving the neutron star-disk system in the context of high-energy transients and gravitational wave emissions.
... [29,31,35,[39][40][41][42][43][44][45] and includes information that may allow the inference of various neutron star properties, see, e.g. [8,39,[46][47][48][49][50]. This could be achieved, for instance, using empirical relations that relate f peak to radii of non-rotating neutron stars in the inspiral phase [20,29,31,43,[51][52][53][54][55]. ...
Gravitational waves in the postmerger phase of binary neutron star mergers may become detectable with planned upgrades of existing gravitational-wave detectors or with more sensitive next-generation detectors. The construction of template banks for the postmerger phase can facilitate signal detection and parameter estimation. Here, we investigate the performance of an artificial neural network in predicting simulation-based waveforms in the frequency domain (restricted to the magnitude of the frequency spectrum and to equal-mass models) that depend on three parameters that can be inferred through observations, neutron star mass, tidal deformability, and the gradient of radius versus mass. Compared to a baseline study using multiple linear regression, we find that the artificial neural network can predict waveforms with higher accuracy and more consistent performance in a cross-validation study. We also demonstrate, through a recalibration procedure, that future reduction of uncertainties in empirical relations that are used in our hierarchical scheme will result in more accurate predicted postmerger spectra.
... The remnant oscillates in various fluid (quasi) oscillation modes and emits GWs in the range of 2-4 kHz. The most prominent feature of the post-merger GW emission is associated with the fundamental quadrupolar oscillation mode (see [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62]). The frequency of this mode, denoted as f peak or f 2 , depends on the EOS. ...
We present a time-domain model for the gravitational waves emitted by equal-mass binary neutron star merger remnants for a fixed equation of state. We construct a large set of numerical relativity simulations for a single equation of state consistent with current constraints, totaling 157 equal-mass binary neutron star merger configurations. The gravitational-wave model is constructed using the supervised learning method of K-nearest neighbor regression. As a first step toward developing a general model with supervised learning methods that accounts for the dependencies on equation of state and the binary masses of the system, we explore the impact of the size of the dataset on the model. We assess the accuracy of the model for a varied dataset size and number density in total binary mass. Specifically, we consider five training sets of $\{ 20,40, 60, 80, 100\}$ simulations uniformly distributed in total binary mass. We evaluate the resulting models in terms of faithfulness using a test set of 30 additional simulations that are not used during training and which are equidistantly spaced in total binary mass. The models achieve faithfulness with maximum values in the range of $0.980$ to $0.995$. We assess our models simulating signals observed by the three-detector network of Advanced LIGO-Virgo. We find that all models with training sets of size equal to or larger than $40$ achieve an unbiased measurement of the main gravitational-wave frequency. We confirm that our results do not depend qualitatively on the choice of the (fixed) equation of state. We conclude that training sets, with a minimum size of $40$ simulations, or a number density of approximately $11$ simulations per $0.1\,M_\odot$ of total binary mass, suffice for the construction of faithful templates for the post-merger signal for a single equation of state and equal-mass binaries (abbreviated).
... The evolution of a long-lived super/hypermassive neutron star remnant formed from a binary neutron star (BNS) merger is primarily governed by turbulence, magnetohydrodynamics instabilities, and neutrino effects (Hotokezaka et al. 2013;Fujibayashi et al. 2017;Radice 2017;Kiuchi et al. 2018;Radice et al. 2018). Neutrinos play a critical role in altering the proton fraction of ejected material, contributing to the shedding of matter as a subrelativistic neutrino-driven wind, as well as facilitating cooling processes (Dessart et al. 2009;Hotokezaka et al. 2013;Metzger & Fernández 2014;Perego et al. 2014;Foucart et al. 2015Foucart et al. , 2016bFoucart et al. , 2016Radice et al. 2016;Fujibayashi et al. 2017). Also, neutrinos may play a role in the surrounding torus of the remnant (if any), and could potentially help power an ultrarelativistic jet as the source of short gamma-ray bursts (Ruffert et al. 1997;Rezzolla et al. 2011;Just et al. 2016). ...
... Examples include the calculations of neutrino opacities for β-processes and scattering with matter (Bruenn 1985;Burrows et al. 2006), the neutrino production rates of the e − e + pair annihilation and nucleon-nucleon Bremsstrahlung (Bruenn 1985;Hannestad & Raffelt 1998;Pons et al. 1998;Misiaszek et al. 2006). These calculations are applied as the collisional source terms in the Boltzmann equation for different simplified neutrino radiative transfer schemes utilized in the modeling of CCSNe (Bruenn 1985;Rampp & Janka 2002;Liebendörfer et al. 2005;Müller et al. 2010;Just et al. 2015;O'Connor 2015;Kuroda et al. 2016) and compact binary coalescence (Ruffert et al. 1997;Sekiguchi et al. 2015;Foucart et al. 2015Foucart et al. , 2016aFoucart et al. , 2016bFoucart et al. , 2016Radice et al. 2022;Musolino & Rezzolla 2024). ...
... Typical neutrino opacities utilized for cooling and nucleosynthesis in BNS postmerger systems are energy-averaged and primarily based on the calculations of Ruffert et al. (1996) and Ardevol-Pulpillo et al. (2019). Recent BNS simulations have been performed with these opacities (Foucart et al. 2015(Foucart et al. , 2016Radice et al. 2022;Musolino & Rezzolla 2024). Specifically, the energy-averaged absorption opacity is determined by averaging an energy-dependent neutrino distribution function (assuming in LTE) with the energy-dependent opacity, which is calculated under elastic approximation and without any corrections (see equation (B13) in Ardevol-Pulpillo et al. 2019). ...
We introduce Weakhub, a novel neutrino microphysics library that provides opacities and kernels beyond conventional interactions used in the literature. This library includes neutrino-matter, neutrino-neutrino interactions and plasma process, along with corresponding weak and strong corrections. A full kinematics approach is adopted for the calculations of β-processes, incorporating various weak corrections and medium modifications due to the nuclear equation of state. Calculations of plasma processes, electron neutrino-antineutrino annihilation, and nuclear de-excitation are also included. We also present the detailed derivations of weak interactions and the coupling to the two-moment based general-relativistic multigroup radiation transport in the general-relativistic multigrid numerical (Gmunu) code. We compare the neutrino opacity spectra for all interactions and estimate their contributions at hydrodynamical points in core-collapse supernovae and binary neutron star (BNS) postmerger remnants, and predict the effects of improved opacities in comparison to conventional ones for a BNS postmerger at a specific hydrodynamical point. We test the implementation of the conventional set of interactions by comparing it to an open-source neutrino library NuLib in a core-collapse supernova simulation. We demonstrate good agreement with discrepancies of less than ∼10% in luminosity for all neutrino species, while also highlighting the reasons contributing to the differences. To compare the advanced interactions to the conventional set in core-collapse supernova modeling, we perform simulations to analyze their impacts on neutrino signatures, hydrodynamical behaviors, and shock dynamics, showing significant deviations.
... The spacetime metric is given in standard 3 + 1 quantities: the lapse α, shift β i , and three-metric γ ij . This is a common procedure within general-relativistic truncated moment simulation codes (Shibata et al. 2011;Foucart et al. 2016b;Kuroda et al. 2016), but we make the procedure explicit here for completeness. Note that, in the main text, we revert to the more conventional symbols E and F to indicate tetrad quantities, but they refer to the lab-frame quantities in this appendix for consistency with Shibata et al. (2011). ...
Multi-messenger astrophysics has produced a wealth of data with much more to come in the future. This enormous data set will reveal new insights into the physics of core-collapse supernovae, neutron star mergers, and many other objects where it is actually possible, if not probable, that new physics is in operation. To tease out different possibilities, we will need to analyze signals from photons, neutrinos, gravitational waves, and chemical elements. This task is made all the more difficult when it is necessary to evolve the neutrino component of the radiation field and associated quantum-mechanical property of flavor in order to model the astrophysical system of interest—a numerical challenge that has not been addressed to this day. In this work, we take a step in this direction by adopting the technique of angular-integrated moments with a truncated tower of dynamical equations and a closure, convolving the flavor-transformation with spatial transport to evolve the neutrino radiation quantum field. We show that moments capture the dynamical features of fast flavor instabilities in a variety of systems, although our technique is by no means a universal blueprint for solving fast flavor transformation. To evaluate the effectiveness of our moment results, we compare to a more precise particle-in-cell method. Based on our results, we propose areas for improvement and application to complementary techniques in the future.
... The degrees of freedom considered in BNS merger simulations are nucleons, nuclei, electrons, and positrons. Aside from that, some include hyperons, quarks, and/or neutrinos [20,[108][109][110][111]. On the other hand, most state-of-the-art simulations of binary NS mergers do not include muons, even though the exotic temperature and density conditions in the BNS merger event favor their generation. ...
This work investigates hot quark matter under the thermodynamic conditions characteristic of a binary neutron star (BNS) merger remnants. We use the density-dependent quark mass model (DDQM) to access the microscopic nuclear equation of state (EoS). The strange quark matter (SQM) is studied at finite temperature and entropy in the presence of electrons and muons and their neutrinos to simulate the BNS merger conditions. We observe that as the entropy of the SQM increases, the merger remnant becomes more massive, and increases in size whereas the neutrino population also increases. In the fixed temperature case, on the other hand, we observe that the entropy spreads from the surface towards the center of the remnant. We determine the particle distribution in the remnants' core, the remnant's structure, the temperature profile, sound velocity, and the polytropic index and discuss their effects. The strange quark (SQ) remnants satisfy the 2 M⊙ mass constraint associated with neutron stars (NS).
... These schemes were shown to describe well the most relevant features of neutrino emission and reabsorption . The latter effect is crucial to predict the properties of the unbound matter, ultimately influencing the ejecta composition (e.g., Sekiguchi et al. 2015;Foucart et al. 2016;Perego et al. 2017;Radice et al. 2018). In all but one simulation, turbulent viscosity of magnetic origin was included via a large eddy scheme (Radice 2017(Radice , 2020. ...
The recent detection of the live isotopes ⁶⁰ Fe and ²⁴⁴ Pu in deep ocean sediments dating back to the past 3–4 Myr poses a serious challenge to the identification of their production site(s). While ⁶⁰ Fe is usually attributed to standard core-collapse supernovae, actinides are r -process nucleosynthesis yields, which are believed to be synthesized in rare events, such as special classes of supernovae or binary mergers involving at least one neutron star. Previous works concluded that a single binary neutron star merger cannot explain the observed isotopic ratio. In this work, we consider a set of numerical simulations of binary neutron star mergers producing long-lived massive remnants expelling both dynamical and spiral-wave wind ejecta. The latter, due to a stronger neutrino irradiation, also produce iron-group elements. Assuming that large-scale mixing is inefficient before the fading of the kilonova remnant and that the spiral-wave wind is sustained over a 100–200 ms timescale, the ejecta emitted at mid-high latitudes provide a ²⁴⁴ Pu over ⁶⁰ Fe ratio compatible with observations. The merger could have happened 80–150 pc away from the Earth and between 3.5 and 4.5 Myr ago. We also compute expected isotopic ratios for eight other live radioactive nuclides showing that the proposed binary neutron star merger scenario is distinguishable from other scenarios proposed in the literature.
