arXiv subjects

Wolfgang Tichy

Publications and source records attributed to Wolfgang Tichy.

At least 19 recordsLinked to original sources

arXiv2025

Gravitational scattering of two neutron stars

We present the first numerical relativity simulations of the gravitational scattering of two neutron stars. Constraint-satisfying initial data for two equal-mass nonspinning sequences are constructed at fixed energy and various initial angular momenta (impact parameter) and evolved with Einstein equations through the scattering process. The strong-field scattering dynamics are explored up to scattering angles of $220^\circ$ and the threshold of dynamical captures. The transition to bound orbits is aided by significant mass ejecta up to baryon mass ${\sim}0.1M_\odot$. A quantitative comparison with predictions of the scattering angle from state-of-the-art effective-one-body and post-Minkowskian calculations indicates quantitative agreement for large initial angular momenta although significant discrepancies in the tidal contribution emerge toward the capture threshold. Gravitational waveforms and radiated energy are in qualitative agreement with the analogous black hole problem and state-of-the-art effective-one-body predictions. Toward the capture threshold waveforms from scattering dynamics carry a strong imprint of matter effects, including the stars' $f$-mode excitations during the close encounter. Overall, our simulations open a new avenue to study tidal interactions in the relativistic two-body problem.

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arXiv2025

Neutron star evolution by combining discontinuous Galerkin and finite volume methods

We present here a new hybrid scheme that combines a discontinuous Galerkin (DG) method with compact finite volume (FV) and finite difference (FD) methods. The computational mesh is divided into smaller elements that touch but do not overlap. Like a pure DG method, our new hybrid scheme requires information exchange only at the surface of neighboring elements. This avoids the need for ghost zones that are usually many points deep in traditional FV implementations. Furthermore, unlike traditional FV implementations, that need information exchange between each element and its 26 surrounding neighbors on noncuboid meshes, our new hybrid method exchanges information only between each element and its six nearest neighbors. With this reduced communication, we aim to retain the high scalability of DG when using large supercomputers. In addition, the information exchange between adjacent elements is much simpler than in a traditional FV implementation, because we always have grid points at the interface, so that only surface interpolation is required. As a result it is much easier to implement adaptive mesh refinement. The goal is to use DG in elements with smooth matter fields and to fall back onto the more robust FV/FD method in elements that contain nonsmooth shocks or star surfaces. For this we devise trouble criteria to decide whether an element should be evolved with DG or FV/FD. We use the Nmesh program to implement and test the new scheme. We successfully evolve various single neutron star cases. These include the challenging cases of a neutron star initially in an unstable equilibrium migrating to a stable configuration and a boosted neutron star. These cases are simulated for the first time here in full 3D with general relativistic hydrodynamics using DG methods. We also describe additional numerical methods, such as the limiters and the atmosphere treatment we need for our simulations.

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arXiv2025

Numerical Relativity Simulations of Dark Matter Admixed Binary Neutron Stars

Binary neutron star mergers provide insight into strong-field gravity and the properties of ultra-dense nuclear matter. These events offer the potential to search for signatures of physics beyond the standard model, including dark matter. We present the first numerical-relativity simulations of binary neutron star mergers admixed with dark matter, based on constraint-solved initial data. Modeling dark matter as a non-interacting fermionic gas, we investigate the impact of varying dark matter fractions and particle masses on the merger dynamics, ejecta mass, post-merger remnant properties, and the emitted gravitational waves. Our simulations suggest that the dark matter morphology - a dense core or a diluted halo - may alter the merger outcome. Scenarios with a dark matter core tend to exhibit a higher probability of prompt collapse, while those with a dark matter halo develop a common envelope, embedding the whole binary. Furthermore, gravitational wave signals from mergers with dark matter halo configurations exhibit significant deviations from analytical models when the tidal deformability is calculated in a standard two-fluid framework. This highlights the need for refined models in calculating the tidal deformability when considering mergers with extended dark matter structures. These initial results provide a basis for further exploration of dark matter's role in binary neutron star mergers and their associated gravitational wave emission and can serve as a benchmark for future observations from advanced detectors and multi-messenger astrophysics.

astro-ph.HE↗

arXiv2025

The error budget of binary neutron star merger simulations for configurations with high spin

Numerical-relativity simulations offer a unique approach to investigating the dynamics of binary neutron star mergers and provide the most accurate predictions of waveforms in the late inspiral phase. However, the numerical predictions are prone to systematic biases originating from the construction of initial quasi-circular binary configurations, the numerical methods used to evolve them, and to extract gravitational signals. To assess uncertainties arising from these aspects, we analyze mergers of highly spinning neutron stars with dimensionless spin parameter $χ=0.5$. The initial data are prepared by two solvers, \textsc{FUKA} and \textsc{SGRID}, which are then evolved by two independent codes, \textsc{SACRA} and \textsc{BAM}. We assess the impact of numerical discretizations, finite extraction radii, and differences in numerical frameworks on the resulting gravitational waveforms. Our analysis reveals that the primary source of uncertainty in numerical waveforms is the evolution code, while the initial data solver has a smaller impact. We also compare our numerical-relativity waveforms with state-of-the-art analytical models, finding that the discrepancies between them exceed the estimated numerical uncertainties. Few suggestions are offered: (i) the analytic waveform becomes an inadequate approximation after the two neutron stars come into contact and the binary enters the essentially-one-body phase, (ii) the analytical models may not capture finite-size effects beyond quadrupole moment, and (iii) the inconsistent use of the binary black hole baseline in the analytical models may also be contributing to these discrepancies. The presented results benchmark the error budget for numerical waveforms of binary neutron star mergers, and provide information for the analytic models to explore further the high spin parameter space of binary neutron star mergers.

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arXiv2025

Eccentricity reduction of binary neutron star initial data with the entropy based flux limiting scheme

The construction of high-resolution shock-capturing schemes is vital in producing highly accurate gravitational waveforms from neutron star binaries. The entropy based flux limiting (EFL) scheme is able to perform fast converging binary neutron star merger simulations reaching up to fourth-order convergence in the gravitational waveform phase. In these results the EFL method was used only in the dynamical evolution of initial data constructed with the Lorene library. Here, we extend the use of the EFL method to the construction of eccentricity reduced initial data for neutron star binaries and present several new BNS simulations resulting from such initial data and show for the first time up to optimal fifth-order convergence in the gravitational waveform phase.

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arXiv2024

Entropy based flux limiting scheme for conservation laws

The entropy based flux-limiting (EFL) scheme is a novel approach designed to accurately resolve shocks and discontinuities in special and general relativistic hydrodynamics. By adaptively adjusting the numerical fluxes, the EFL method mitigates oscillations and preserves smooth transition across discontinuities in shock-dominated flows. Here, we extend the applicability of the EFL method beyond special/general relativistic hydrodynamics to scalar conservation laws and show how to treat systems without a thermodynamic entropy. This is an indication that the method has universal applicability to any system of partial differential equations that can be written in conservation form. We also present some further very challenging special/general relativistic hydrodynamics applications of the EFL method.

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arXiv2023

Quasi-equilibrium configurations of binary systems of dark matter admixed neutron stars

Using an adapted version of the SGRID code, we construct for the first time consistent quasi-equilibrium configurations for a binary system consisting of two neutron stars in which each is admixed with dark matter. The stars are modelled as a system of two non-interacting fluids minimally coupled to gravity. For the fluid representing baryonic matter the SLy equation of state is used, whereas the second fluid, which corresponds to dark matter, is described using the equation of state of a degenerate Fermi gas. We consider two different scenarios for the distribution of the dark matter. In the first scenario the dark matter is confined to the core of the star, whereas in the second scenario the dark matter extends beyond the surface of the baryonic matter, forming a halo around the baryonic star. The presence of dark matter alters the star's reaction to the companion's tidal forces, which we investigate in terms of the coordinate deformation and mass shedding parameters. The constructed quasi-equilibrium configurations mark the first step towards consistent numerical-relativity simulations of dark matter admixed neutron star binaries.

gr-qc↗

arXiv2023

Analytically improved and numerical-relativity informed effective-one-body model for coalescing binary neutron stars

Gravitational wave astronomy pipelines rely on template waveform models for searches and parameter estimation purposes. For coalescing binary neutron stars (BNS), such models need to accurately reproduce numerical relativity (NR) up to merger, in order to provide robust estimate of the stars' equation of state - dependent parameters. In this work we present an improved version of the Effective One Body (EOB) model $\tt TEOBResumS$ for gravitational waves from BNS systems. Building upon recent post-Newtonian calculations, we include subleading order tidal terms in the waveform multipoles and EOB metric potentials, as well as add up to 5.5PN terms in the gyro-gravitomagnetic functions entering the spin-orbit sector of the model. In order to further improve the EOB-NR agreement in the last few orbital cycles before merger, we introduce next-to-quasicircular corrections in the waveform -- informed by a large number of BNS NR simulations -- and introduce a new NR-informed parameter entering the tidal sector of our conservative dynamics. The performance of our model is then validated against 14 new eccentricity reduced simulations of unequal mass, spinning binaries with varying equation of state. A time-domain phasing analysis and mismatch computations demonstrate that the new model overall improves over the previous version of $\tt TEOBResumS$. Finally, we present a closed-form frequency domain representation of the (tidal) amplitude and phase of the new model. This representation accounts for mass-ratio, aligned spin and (resummed) spin-quadrupole effects in the tidal phase and -- within the calibration region -- it is faithful to the original model.

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arXiv2023

Second release of the CoRe database of binary neutron star merger waveforms

We present the second data release of gravitational waveforms from binary neutron star merger simulations performed by the Computational Relativity (CoRe) collaboration. The current database consists of 254 different binary neutron star configurations and a total of 590 individual numerical-relativity simulations using various grid resolutions. The released waveform data contain the strain and the Weyl curvature multipoles up to $\ell=m=4$. They span a significant portion of the mass, mass-ratio,spin and eccentricity parameter space and include targeted configurations to the events GW170817 and GW190425. CoRe simulations are performed with 18 different equations of state, seven of which are finite temperature models, and three of which account for non-hadronic degrees of freedom. About half of the released data are computed with high-order 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 ready-to-use fitting formulas for equation of state-insensitive relations at merger (e.g. merger frequency), luminosity peak, and post-merger spectrum.

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arXiv2022

The new discontinuous Galerkin methods based numerical relativity program Nmesh

Interpreting gravitational wave observations and understanding the physics of astrophysical compact objects such as black holes or neutron stars requires accurate theoretical models. Here, we present a new numerical relativity computer program, called Nmesh, that has the design goal to become a next generation program for the simulation of challenging relativistic astrophysics problems such as binary black hole or neutron star mergers. In order to efficiently run on large supercomputers, Nmesh uses a discontinuous Galerkin method together with a domain decomposition and mesh refinement that parallelizes and scales well. In this work, we discuss the various numerical methods we use. We also present results of test problems such as the evolution of scalar waves, single black holes and neutron stars, as well as shock tubes. In addition, we introduce a new positivity limiter that allows us to stably evolve single neutron stars without an additional artificial atmosphere, or other more traditional limiters.

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arXiv2022

Elliptica: a new pseudo-spectral code for the construction of initial data

Numerical studies of the dynamics of gravitational systems, e.g., black hole-neutron star systems, require physical and constraint-satisfying initial data. In this article, we present the newly developed pseudo-spectral code Elliptica, an infrastructure for construction of initial data for various binary and single gravitational systems of all kinds. The elliptic equations under consideration are solved on a single spatial hypersurface of the spacetime manifold. Using coordinate maps, the hypersurface is covered by patches whose boundaries can adapt to the surface of the compact objects. To solve elliptic equations with arbitrary boundary condition, Elliptica deploys a Schur complement domain decomposition method with a direct solver. In this version, we use cubed sphere coordinate maps and the fields are expanded using Chebyshev polynomials of the first kind. Here, we explain the building blocks of Elliptica and the initial data construction algorithm for a black hole-neutron star binary system. We perform convergence tests and evolve the data to validate our results. Within our framework, the neutron star can reach spin values close to breakup with arbitrary direction, while the black hole can have arbitrary spin with dimensionless spin magnitude $\sim 0.8$.

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arXiv2022

Conformally curved initial data for charged, spinning black hole binaries on arbitrary orbits

We present a method to construct conformally curved initial data for charged black hole binaries with spin on arbitrary orbits. We generalize the superposed Kerr-Schild, extended conformal thin sandwich construction from [Lovelace et al., Phys. Rev. D {78}, 084017 (2008)] to use Kerr-Newman metrics for the superposed black holes and to solve the electromagnetic constraint equations. We implement the construction in the pseudospectral code SGRID. The code thus provides a complementary and completely independent excision-based construction, compared to the existing charged black hole initial data constructed using the puncture method [Bozzola and Paschalidis, Phys. Rev. D {99}, 104044 (2019)]. It also provides an independent implementation (with some small changes) of the Lovelace et al. vacuum construction. We construct initial data for different configurations of orbiting binaries, e.g., with black holes that are highly charged or rapidly spinning (90 and 80 percent of the extremal values, respectively, for this initial test, though the code should be able to produce data with even higher values of these parameters using higher resolutions), as well as for generic spinning, charged black holes. We carry out exploratory evolutions with the finite difference, moving punctures codes BAM (in the vacuum case) and HAD (for head-on collisions including charge), filling inside the excision surfaces. In the charged case, evolutions of these initial data provide a proxy for binary black hole waveforms in modified theories of gravity. Moreover, the generalization of the construction to Einstein-Maxwell-dilaton theory should be straightforward.

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arXiv2022

High-accuracy high-mass ratio simulations for binary neutron stars and their comparison to existing waveform models

The subsequent observing runs of the advanced gravitational-wave detector network will likely provide us with various gravitational-wave observations of binary neutron star systems. For an accurate interpretation of these detections, we need reliable gravitational-wave models. To test and to point out how existing models could be improved, we perform a set of high-resolution numerical-relativity simulations for four different physical setups with mass ratios $q$ = $1.25$, $1.50$, $1.75$, $2.00$, and total gravitational mass $M = 2.7M_\odot$ . Each configuration is simulated with five different resolutions to allow a proper error assessment. Overall, we find approximately 2nd order converging results for the dominant $(2,2)$, but also subdominant $(2,1)$, $(3,3)$, $(4,4)$ modes, while, generally, the convergence order reduces slightly for an increasing mass ratio. Our simulations allow us to validate waveform models, where we find generally good agreement between state-of-the-art models and our data, and to prove that scaling relations for higher modes currently employed for binary black hole waveform modeling also apply for the tidal contribution. Finally, we also test if the current NRTidal model to describe tidal effects is a valid description for high-mass ratio systems. We hope that our simulation results can be used to further improve and test waveform models in preparation for the next observing runs.

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arXiv2021

Investigating GW190425 with numerical-relativity simulations

The third observing run of the LIGO-Virgo collaboration has resulted in about hundred gravitational-wave triggers including the binary neutron star merger GW190425. However, none of these events have been accompanied with an electromagnetic transient found during extensive follow-up searches. In this article, we perform new numerical-relativity simulations of binary neutron star and black hole - neutron star systems that have a chirp mass consistent with GW190425. Assuming that the GW190425's sky location was covered with sufficient accuracy during the electromagnetic follow-up searches, we investigate whether the non-detection of the kilonova is compatible with the source parameters estimated through the gravitational-wave analysis and how one can use this information to place constraints on the properties of the system. Our simulations suggest that GW190425 is incompatible with an unequal mass binary neutron star merger with a mass ratio $q<0.8$ when considering stiff or moderately stiff equations of state if the binary was face-on and covered by the observation. Our analysis shows that a detailed observational result for kilonovae will be useful to constrain the mass ratio of binary neutron stars in future events.

astro-ph.HE↗

arXiv2021

High-accuracy simulations of highly spinning binary neutron star systems

With an increasing number of expected gravitational-wave detections of binary neutron star mergers, it is essential that gravitational-wave models employed for the analysis of observational data are able to describe generic compact binary systems. This includes systems in which the individual neutron stars are millisecond pulsars for which spin effects become essential. In this work, we perform numerical-relativity simulations of binary neutron stars with aligned and anti-aligned spins within a range of dimensionless spins of $χ\sim [-0.28,0.58]$. The simulations are performed with multiple resolutions, show a clear convergence order and, consequently, can be used to test existing waveform approximants. We find that for very high spins gravitational-wave models that have been employed for the interpretation of GW170817 and GW190425 are not capable of describing our numerical-relativity dataset. We verify through a full parameter estimation study in which clear biases in the estimate of the tidal deformability and effective spin are present. We hope that in preparation of the next gravitational-wave observing run of the Advanced LIGO and Advanced Virgo detectors our new set of numerical-relativity data can be used to support future developments of new gravitational-wave models.

gr-qc↗

arXiv2020

Increasing the Accuracy of Binary Neutron Star Simulations with an improved Vacuum Treatment

Numerical relativity simulations are essential to study 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 multi-messenger observations with more accurate numerical relativity simulations.

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arXiv2020

Gravitational waves and mass ejecta from binary neutron star mergers: Effect of the spin orientation

We continue our study of the binary neutron star parameter space by investigating the effect of the spin orientation on the dynamics, gravitational wave emission, and mass ejection during the binary neutron star coalescence. We simulate seven different configurations using multiple resolutions to allow a reasonable error assessment. Due to the particular choice of the setups, five configurations show precession effects, from which two show a precession ("wobbling") of the orbital plane, while three show a "bobbing" motion, i.e., the orbital angular momentum does not precess, while the orbital plane moves along the orbital angular momentum axis. Considering the ejection of mass, we find that precessing systems can have an anisotropic mass ejection, which could lead to a final remnant kick of $\sim 40 \rm km/s$ for the studied systems. Furthermore, for the chosen configurations, antialigned spins lead to larger mass ejecta than aligned spins, so that brighter electromagnetic counterparts could be expected for these configurations. Finally, we compare our simulations with the precessing, tidal waveform approximant IMRPhenomPv2_NRTidalv2 and find good agreement between the approximant and our numerical relativity waveforms with phase differences below 1.2 rad accumulated over the last $\sim$ 16 gravitational wave cycles.

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arXiv2019

Constructing Binary Neutron Star Initial Data with High Spins, High Compactness, and High Mass-Ratios

The construction of accurate and consistent initial data for various binary parameters is a critical ingredient for numerical relativity simulations of the compact binary coalescence. In this article, we present an upgrade of the pseudospectral SGRID code, which enables us to access even larger regions of the binary neutron star parameter space. As a proof of principle, we present a selected set of first simulations based on initial configurations computed with the new code version. In particular, we simulate two millisecond pulsars close to their breakup spin, highly compact neutron stars with masses at about $98\%$ of the maximum supported mass of the employed equation of state, and an unequal mass systems with mass ratios even outside the range predicted by population synthesis models ($q = 2.03$). The discussed code extension will help us to simulate previously unexplored binary configurations. This is a necessary step to construct and test new gravitational wave approximants and to interpret upcoming binary neutron star merger observations. When we construct initial data, one has to specify various parameters, such as a rotation parameter for each star. Some of these parameters do not have direct physical meaning, which makes comparisons with other methods or models difficult. To facilitate this, we introduce simple estimates for the initial spin, momentum, mass, and center of mass of each individual star.

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