We study equal-and unequal-mass neutron star mergers by means of new numerical relativity simulations in which the general relativistic hydrodynamics solver employs an algorithm that guarantees mass ...conservation across the refinement levels of the computational mesh. We focus on the post-merger dynamics and study the merger remnant, the dynamical ejecta, and the post-merger gravitational wave spectrum. Comparing results obtained with and without the conservative mesh refinement algorithm, we find that post-merger simulations can be affected by systematic errors if mass conservation is not enforced in the mesh refinement strategy. However, mass conservation also depends on grid details and on the artificial atmosphere setup; the latter are particularly significant in the computation of the dynamical ejecta.
In this work we study the dynamics of spinning binary black hole systems in the strong field regime. For this purpose we extract from numerical relativity simulations the binding energy, specific ...orbital angular momentum, and gauge-invariant frequency. The goal of our work is threefold: First, we extract the individual spin contributions to the binding energy, in particular the spin-orbit, spin-spin, and cubic-in-spin terms. Second, we compare our results with predictions from waveform models and find that while post-Newtonian approximants are not capable of representing the dynamics during the last few orbits before merger, there is good agreement between our data and effective-one-body approximants as well as the numerical relativity surrogate models. Finally, we present phenomenological representations for the binding energy for nonspinning systems with mass ratios up to q=10 and for the spin-orbit interaction for mass ratios up to q=8 obtaining accuracies of ≲0.1% and ≲6%, respectively.
Short gamma-ray bursts (sGRBs) show a large diversity in their properties. This suggests that the observed phenomenon can be caused by different 'central engines' or that the engine produces a ...variety of outcomes depending on its parameters, or possibly both. The most popular engine scenario, the merger of two neutron stars, has received support from the recent Fermi and INTEGRAL detection of a burst of gamma rays (GRB170817A) following the neutron star merger GW 170817, but at the moment, it is not clear how peculiar this event potentially was. Several sGRBs engine models involve the collapse of a supramassive neutron star that produces a black hole plus an accretion disc. We study this scenario for a variety of equations of states both via angular momentum considerations based on equilibrium models and via fully dynamical Numerical Relativity simulations. We obtain a broader range of disc forming configurations than earlier studies but we agree with the latter that none of these configurations is likely to produce a phenomenon that would be classified as an sGRB.
Numerical-relativity simulations are essential for studying the last stages of the binary neutron star coalescence. Unfortunately, for stable simulations there is the need to add an artificial ...low-density atmosphere. Here we discuss a new framework in which we can effectively set the density surrounding the neutron stars to zero to ensure a more accurate simulation. We test our method with a number of single star test cases and for an equal-mass binary neutron star simulation. While the bulk motion of the system is not influenced, and hence there is no improvement with respect to the emitted gravitational-wave signal, we find that the new approach is superior with respect to mass conservation and it allows a much better tracking of outward moving material. This will allow a more accurate simulation of the ejected material and supports the interpretation of present and future multimessenger observations with more accurate numerical-relativity simulations.
We present the computational relativity (CoRe) collaboration's public database of gravitational waveforms from binary neutron star mergers. The database currently contains 367 waveforms from ...numerical simulations that are consistent with general relativity and that employ constraint satisfying initial data in hydrodynamical equilibrium. It spans 164 physically distinct configuration with different binary parameters (total binary mass, mass-ratio, initial separation, eccentricity, and stars' spins) and simulated physics. Waveforms computed at multiple grid resolutions and extraction radii are provided for controlling numerical uncertainties. We also release an exemplary set of 18 hybrid waveforms constructed with a state-of-art effective-one-body model spanning the frequency band of advanced gravitational-wave detectors. We outline present and future applications of the database to gravitational-wave astronomy.
While the gravitational-wave (GW) signal GW170817 was accompanied by a variety of electromagnetic (EM) counterparts, sufficiently high-mass binary neutron star (BNS) mergers are expected to be unable ...to power bright EM counterparts. The putative high-mass binary BNS merger GW190425, for which no confirmed EM counterpart has been identified, may be an example of such a system. Since current and future GW detectors are expected to detect many more BNS mergers, it is important to understand how well we will be able to distinguish high-mass BNSs and low-mass binary black holes (BBHs) solely from their GW signals. To do this, we consider the imprint of the tidal deformability of the neutron stars on the GW signal for systems undergoing prompt black hole formation after merger. We model the BNS signals using hybrid numerical relativity–tidal effective-one-body waveforms. Specifically, we consider a set of five nonspinning equal-mass BNS signals with total masses of 2.7, 3.0, 3.2 M ⊙ and with three different equations of state, as well as the analogous BBH signals. We perform Bayesian parameter estimation on these signals at luminosity distances of 40 and 98 Mpc in an Advanced LIGO-Advanced Virgo network and an Advanced LIGO-Advanced Virgo-KAGRA network with sensitivities similar to the third and fourth observing runs (O3 and O4), respectively, and at luminosity distances of 369 and 835 Mpc in a network of two Cosmic Explorers and one Einstein Telescope, with a Cosmic Explorer sensitivity similar to Stage 2. Our analysis suggests that we cannot distinguish the signals from high-mass BNSs and BBHs at a 90% credible level with the O3-like network even at 40 Mpc. However, we can distinguish all but the most compact BNSs that we consider in our study from BBHs at 40 Mpc at a ≥ 95 % credible level using the O4-like network and can even distinguish them at a > 99.2 % (≥ 97%) credible level at 369 (835) Mpc using the 3G network. Additionally, we present a simple method to compute the leading effect of the Earth's rotation on the response of a gravitational wave detector in the frequency domain.
Interpreting high-energy, astrophysical phenomena, such as supernova explosions or neutron-star collisions, requires a robust understanding of matter at supranuclear densities. However, our knowledge ...about dense matter explored in the cores of neutron stars remains limited. Fortunately, dense matter is not probed only in astrophysical observations, but also in terrestrial heavy-ion collision experiments. Here we use Bayesian inference to combine data from astrophysical multi-messenger observations of neutron stars
and from heavy-ion collisions of gold nuclei at relativistic energies
with microscopic nuclear theory calculations
to improve our understanding of dense matter. We find that the inclusion of heavy-ion collision data indicates an increase in the pressure in dense matter relative to previous analyses, shifting neutron-star radii towards larger values, consistent with recent observations by the Neutron Star Interior Composition Explorer mission
,
. Our findings show that constraints from heavy-ion collision experiments show a remarkable consistency with multi-messenger observations and provide complementary information on nuclear matter at intermediate densities. This work combines nuclear theory, nuclear experiment and astrophysical observations, and shows how joint analyses can shed light on the properties of neutron-rich supranuclear matter over the density range probed in neutron stars.
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