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ABSTRACT We present a systematic numerical relativity study of the impact of different physics input and grid resolution in binary neutron star mergers. We compare simulations employing a neutrino leakage scheme, leakage plus M0 scheme, the M1 transport scheme, and pure hydrodynamics. Additionally, we examine the effect of a subgrid scheme for turbulent viscosity. We find that the overall dynamics and thermodynamics of the remnant core are robust, implying that the maximum remnant density could be inferred from gravitational wave observations. Black hole collapse instead depends significantly on viscosity and grid resolution. Differently from recent work, we identify possible signatures of neutrino effects in the gravitational waves only at the highest resolutions considered; new highresolution simulations will be thus required to build accurate gravitational wave templates to observe these effects. Different neutrino transport schemes impact significantly mass, geometry, and composition of the remnant’s disc and ejecta; M1 simulations show systematically larger proton fractions, reaching maximum values larger than 0.4. rprocess nucleosynthesis yields reflect the different ejecta compositions; they are in agreement and reproduce residual solar abundances only if M0 or M1 neutrino transport schemes are adopted. We compute kilonova light curves using sphericallysymmetric radiationhydrodynamics evolutions up to 15 d postmerger, finding that they are mostly sensitive to the ejecta mass and electron fraction; accounting for multiple ejecta components appears necessary for reliable light curve predictions. We conclude that advanced neutrino schemes and resolutions higher than current standards are essential for robust longterm evolutions and detailed astrophysical predictions.more » « lessFree, publiclyaccessible full text available January 28, 2024

ABSTRACT We investigate rprocess nucleosynthesis and kilonova emission resulting from binary neutron star (BNS) mergers based on a threedimensional (3D) generalrelativistic magnetohydrodynamic (GRMHD) simulation of a hypermassive neutron star (HMNS) remnant. The simulation includes a microphysical finitetemperature equation of state (EOS) and neutrino emission and absorption effects via a leakage scheme. We track the thermodynamic properties of the ejecta using Lagrangian tracer particles and determine its composition using the nuclear reaction network SkyNet. We investigate the impact of neutrinos on the nucleosynthetic yields by varying the neutrino luminosities during postprocessing. The ejecta show a broad distribution with respect to their electron fraction Ye, peaking between ∼0.25–0.4 depending on the neutrino luminosity employed. We find that the resulting rprocess abundance patterns differ from solar, with no significant production of material beyond the second rprocess peak when using luminosities recorded by the tracer particles. We also map the HMNS outflows to the radiation hydrodynamics code SNEC and predict the evolution of the bolometric luminosity as well as broadband light curves of the kilonova. The bolometric light curve peaks on the timescale of a day and the brightest emission is seen in the infrared bands. This is the first direct calculation of the rprocess yields and kilonova signal expected from HMNS winds based on 3D GRMHD simulations. For longerlived remnants, these winds may be the dominant ejecta component producing the kilonova emission.

Abstract We present the second data release of gravitational waveforms from binary neutron star (BNS) merger simulations performed by the Computational Relativity (
CoRe ) collaboration. The current database consists of 254 different BNS configurations and a total of 590 individual numericalrelativity simulations using various grid resolutions. The released waveform data contain the strain and the Weyl curvature multipoles up to . They span a significant portion of the mass, massratio, spin and eccentricity parameter space and include targeted configurations to the events GW170817 and GW190425. $\ell =m=4$CoRe simulations are performed with 18 different equations of state, seven of which are finite temperature models, and three of which account for nonhadronic degrees of freedom. About half of the released data are computed with highorder hydrodynamics schemes for tens of orbits to merger; the other half is computed with advanced microphysics. We showcase a standard waveform error analysis and discuss the accuracy of the database in terms of faithfulness. We present readytouse fitting formulas for equation of stateinsensitive relations at merger (e.g. merger frequency), luminosity peak, and postmerger spectrum. 
Abstract Neutrinos are copiously emitted by neutron star mergers, due to the high temperatures reached by dense matter during the merger and its aftermath. Neutrinos influence the merger dynamics and shape the properties of the ejecta, including the resulting
r process nucleosynthesis and kilonova emission. In this work, we analyse neutrino emission from a large sample of binary neutron star merger simulations in Numerical Relativity, covering a broad range of initial masses, nuclear equation of state and viscosity treatments. We extract neutrino luminosities and mean energies, and compute quantities of interest such as the peak values, peak broadnesses, time averages and decrease time scales. We provide a systematic description of such quantities, including their dependence on the initial parameters of the system. We find that for equalmass systems the total neutrino luminosity (several ) decreases as the reduced tidal deformability increases, as a consequence of the less violent merger dynamics. Similarly, tidal disruption in asymmetric mergers leads to systematically smaller luminosities. Peak luminosities can be twice as large as the average ones. Electron antineutrino luminosities dominate (initially by a factor of 23) over electron neutrino ones, while electron neutrinos and heavy flavour neutrinos have similar luminosities. Mean energies are nearly constant in time and independent on the binary parameters. Their values reflect the different decoupling temperature inside the merger remnant. Despite present uncertainties in neutrino modelling, our results provide a broad and physically grounded characterisation of neutrino emission, and they can serve as a reference point to develop more sophisticated neutrino transport schemes.$$10^{53}{\hbox {erg}~{\hbox {s}}^{1}}$$ ${10}^{53}\text{erg}\phantom{\rule{0ex}{0ex}}{\text{s}}^{1}$ 
ABSTRACT GW190425 was the second gravitational wave (GW) signal compatible with a binary neutron star (BNS) merger detected by the Advanced LIGO and Advanced Virgo detectors. Since no electromagnetic counterpart was identified, whether the associated kilonova was too dim or the localization area too broad is still an open question. We simulate 28 BNS mergers with the chirp mass of GW190425 and mass ratio 1 ≤ q ≤ 1.67, using numericalrelativity simulations with finitetemperature, composition dependent equations of state (EOS) and neutrino radiation. The energy emitted in GWs is $\lesssim 0.083\mathrm{\, M_\odot }c^2$ with peak luminosity of 1.1–$2.4\times ~10^{58}/(1+q)^2\, {\rm {erg \, s^{1}}}$. Dynamical ejecta and disc mass range between 5 × 10−6–10−3 and 10−5–$0.1 \mathrm{\, M_\odot }$, respectively. Asymmetric mergers, especially with stiff EOSs, unbind more matter and form heavier discs compared to equal mass binaries. The angular momentum of the disc is 8–$10\mathrm{\, M_\odot }~GM_{\rm {disc}}/c$ over three orders of magnitude in Mdisc. While the nucleosynthesis shows no peculiarity, the simulated kilonovae are relatively dim compared with GW170817. For distances compatible with GW190425, AB magnitudes are always dimmer than ∼20 mag for the B, r, and K bands, with brighter kilonovae associated to more asymmetric binaries and stiffer EOSs. We suggest that, even assuming a good coverage of GW190425’s sky location, the kilonova could hardly have been detected by present widefield surveys and no firm constraints on the binary parameters or EOS can be argued from the lack of the detection.

ABSTRACT We present a new momentbased energyintegrated neutrino transport code for neutron star merger simulations in general relativity. In the merger context, ours is the first code to include Doppler effects at all orders in υ/c, retaining all nonlinear neutrino–matter coupling terms. The code is validated with a stringent series of tests. We show that the inclusion of full neutrino–matter coupling terms is necessary to correctly capture the trapping of neutrinos in relativistically moving media, such as in differentially rotating merger remnants. We perform preliminary simulations proving the robustness of the scheme in simulating abinitio mergers to black hole collapse and longterm neutron star remnants up to ${\sim }70\,$ ms. The latter is the longest dynamical spacetime, 3D, general relativistic simulations with full neutrino transport to date. We compare results obtained at different resolutions and using two different closures for the moment scheme. We do not find evidences of significant outofthermodynamic equilibrium effects, such as bulk viscosity, on the postmerger dynamics or gravitational wave emission. Neutrino luminosities and average energies are in good agreement with theory expectations and previous simulations by other groups using similar schemes. We compare dynamical and early wind ejecta properties obtained with M1 and with our older neutrino treatment. We find that the M1 results have systematically larger proton fractions. However, the differences in the nucleosynthesis yields are modest. This work sets the basis for future detailed studies spanning a wider set of neutrino reactions, binaries, and equations of state.

ABSTRACT We develop a method to compute synthetic kilonova light curves that combine numerical relativity simulations of neutron star mergers and the SNEC radiation–hydrodynamics code. We describe our implementation of initial and boundary conditions, rprocess heating, and opacities for kilonova simulations. We validate our approach by carefully checking that energy conservation is satisfied and by comparing the SNEC results with those of two semianalytic lightcurve models. We apply our code to the calculation of colour light curves for three binaries having different mass ratios (equal and unequal mass) and different merger outcome (shortlived and longlived remnants). We study the sensitivity of our results to hydrodynamic effects, nuclear physics uncertainties in the heating rates, and duration of the merger simulations. We find that hydrodynamics effects are typically negligible and that homologous expansion is a good approximation in most cases. However, pressure forces can amplify the impact of uncertainties in the radioactive heating rates. We also study the impact of shocks possibly launched into the outflows by a relativistic jet. None of our models match AT2017gfo, the kilonova in GW170817. This points to possible deficiencies in our merger simulations and kilonova models that neglect nonLTE effects and possible additional energy injection from the merger remnant and to the need to go beyond the assumption of spherical symmetry adopted in this work.