Wigner Virgo Group

  • The main research areas of the Gravitational Physics Research Group are related to the study of gravitational phenomena. In addition to field theory research, other areas such as post-Newtonian general relativity, experimental gravitational-wave data analysis and their related algorithms and developing multi-core computer procedures. The group's research interest is motivated by gravitational-wave physics, since we are a member of the Virgo Collaboration operating the European Virgo gravitational-wave detector. In connection with the data evaluation tasks of the gravitational-wave observations restarted in the autumn of 2015, our group participates in using the data analysis packages of the LIGO–Virgo Collaboration.

    The members of the Gravitational Physics Research Group of the Wigner RCP have solid background in experimental and theoretical physics, in particular, general relativity. They also have experience in developing optimal numerical algorithms and coding these algorithms into efficient computer procedures that can run on grid and GPU clusters. One of the main motivation of our research interest originates in gravitational-wave physics as our group is a member of the Virgo Collaboration operating the Virgo interferometric gravitational-wave antenna, the European Gravitational Observatory.

  • List of publications in peer-reviewed journals (2012–2021)

    Authors: D. Barta
    Journal: Class. Quantum Grav. 38, 185002 (2021) , (arXiv:1908.02808)
    Impact Factor: 3.528

    Abstract

    This research paper complements our earlier qualitative study of the effect of viscosity and thermal conductivity on the radial oscillation and relaxation of non-rotating neutron stars. The fundamental and first two lowest-frequency excited modes of radial oscillation have been computed in the high nuclear density regime for a set of seven realistic equations of state (EoS) as functions of central energy density. Various types of zero-temperature EoS of cold nucleonic and hybrid nucleon–hyperon–quark matter models are used in the inner core to determine the internal structure in and around the hydrostatic equilibrium states and investigate the influence of each EoS on the dynamical behaviour of non-rotating neutron stars. We confirm the principal results of earlier, related studies that suggest an underlying correlation between the frequency spectrum of the fundamental oscillation mode and the variation of the adiabatic index over the high nuclear-density regime. We provide valuable information to impose further constraints on the plausible set of realistic EoS models, in addition to the practical applications for the rapidly evolving field of asteroseismology of compact objects.

    Authors: D. Barta, M. Vasúth
    Journal: Int. J. Mod. Phys. D 29, 2092001 (2020), (arXiv:2007.13415)
    Impact Factor: 3.071

    Abstract

    The study published in IJMPD 27(4):1850040, 2018 provided a numerical result for the frequency-shift of GWs due to dispersion in interstellar medium. In order to adjust the metric functions of the originally improperly matched `background' spacetime in sections 2.1, the authors have adopted Darmois--Israel junction conditions. In section 4.1 the code used in the original paper erroneously computed the magnitude of frequency-shift for the transient event GW150914 due to a missing conversion factor. In both cases where numerical errors and potential contradictions have been identified and eliminated, adjustments were undertaken in order to maintain consistency with closely-related earlier studies.

    Authors: D. Barta
    Journal: Class. Quantum Grav. 36, 215012 (2019) , (arXiv:1904.00907)
    Impact Factor: 3.487

    Abstract

    In this paper we present a generic formulation of the linearized dynamical equations governing small adiabatic radial oscillations of relativistic stars. The dynamical equations are derived by taking into consideration those effects of viscosity and thermal conductivity of neutron-star matter which directly determine the minimum period of observable pulsars. A variational principle is applied to determine a discrete set of eigenfunctions with complex eigenvalues. The real and imaginary parts of eigenvalues represent the squared natural frequencies and relaxation time of radial oscillations of non-rotating neutron stars, respectively. We provide a suitable framework which may be supplemented with various potential species of cold-nuclear-matter models to compute the spectra of the normalized eigenfrequencies with a certain numerical precision. In the last section, we provide a qualitative estimation of the rate at which viscosity and thermal conductivity drain the kinetic energy of radial oscillation mode in reasonably uniform neutron stars, without relying on explicit numerical computations.

    Authors: D. Barta, M. Vasúth
    Journal: Phys. Rev. D, 97, 124011 (2018), (arXiv:1803.00348)
    Impact Factor: 4.394

    Abstract

    A large number of theoretically predicted waveforms are required by matched-filtering searches for the gravitational-wave signals produced by compact binary coalescence. In order to substantially alleviate the computational burden in gravitational-wave searches and parameter estimation without degrading the signal detectability, we propose a novel reduced-order-model (ROM) approach with applications to adiabatic 3PN-accurate inspiral waveforms of sources that evolve on either highly or slightly eccentric orbits. We provide a singular-value decomposition-based reduced-basis method in the frequency domain to generate reduced-order approximations of any gravitational waves with acceptable accuracy and precision within the parameter range of the model. We construct efficient reduced bases comprised of a relatively small number of the most relevant waveforms over 3-dimensional parameter-space covered by the template bank (total mass 2.15M⊙≤M≤215M⊙, mass ratio 0.01≤q≤1, and initial orbital eccentricity 0≤e0≤0.95). The ROM is designed to predict signals in the frequency band from 10 Hz to 2 kHz for aLIGO and aVirgo design sensitivity. Beside moderating the data reduction, finer sampling of fiducial templates improves the accuracy of surrogates. Considerable increase in the speedup from several hundreds to thousands can be achieved by evaluating surrogates for low-mass systems especially when combined with high-eccentricity.

    Authors: D. Barta, M. Vasúth
    Journal: Int. J. Mod. Phys. D 27 1850040 (2018) , (arXiv:1708.05576)
    Impact Factor: 2.476

    Abstract

    We investigate the propagation of locally plane, small-amplitude, monochromatic gravitational waves through cold compressible interstellar gas in order to provide a more accurate picture of expected waveforms for direct detection. The quasi-isothermal gas is concentrated in a spherical symmetric cloud held together by self-gravitation. Gravitational waves can be treated as linearized perturbations on the background inner Schwarzschild spacetime. The perturbed quantities lead to the field equations governing the gas dynamics and describe the interaction of gravitational waves with matter. The resulted field equations decouple asymptotically for slowly varying short waves to a set of three PDEs of different orders of magnitude. A second-order WKB method provides transport equations for the wave amplitudes. The influence of background curvature already appears in the first-order amplitudes, which gives rise to diffraction. We have shown that the transport equation of these amplitudes provides numerical solutions for the frequency-alteration. The energy dissipating process is responsible for decreasing frequency. The decrease is significantly smaller than the magnitude of the original frequency and exhibits a power-law relationship between original and decreased frequencies. The frequency deviation was examined particularly for the transient signal GW150914. Considering AGNs as larger background structures and high-frequency signals emitted by BNS mergers, the frequency-deviation grows large enough to be relevant in future GW-observations with increased sensitivity.

    Authors: D. Barta
    Journal: Astron. Nachr. 334, 916 (2013), (arXiv:1803.00348)
    Impact Factor: 1.119

    Abstract

    We investigate insterstellar gas spheres by determining the metric functions, the material distribution, and the features of particle orbits in terms of stability and geodesics. An exact solution of the Einstein's equations for interstellar gas clouds is derived that is compatible with the results of recent astronomical measurements. The solution determines the distribution of pressure and density, and it is suitable to describe the energy, speed, trajectory, and further relevant physical features of the cloud's particles. We describe the spacetime inside the nebula and give the density profile and the geodesics of particles. We find that circular orbits are stable and the cloud rotates rigidly by an angular velocity that is inversely proportional to the radius.

    Authors: P. Csizmadia, G. Debreczeni, I. Rácz, M. Vasúth
    Journal: Class. Quantum Grav. 29, 245002 (2012) , (arXiv:1207.0001)
    Impact Factor: 3.647

    Abstract

    This paper is to introduce a new software called CBwaves which provides a fast and accurate computational tool to determine the gravitational waveforms yielded by generic spinning binaries of neutron stars and/or black holes on eccentric orbits. This is done within the post-Newtonian (PN) framework by integrating the equations of motion and the spin precession equations while the radiation field is determined by a simultaneous evaluation of the analytic waveforms. In applying CBwaves various physically interesting scenarios have been investigated. In particular, we have studied the appropriateness of the adiabatic approximation, and justified that the energy balance relation is indeed insensitive to the specific form of the applied radiation reaction term. By studying eccentric binary systems it is demonstrated that circular template banks are very ineffective in identifying binaries even if they possess tiny residual orbital eccentricity. In addition, by investigating the validity of the energy balance relation we show that, on contrary to the general expectations, the post-Newtonian approximation should not be applied once the post-Newtonian parameter gets beyond the critical value ∼0.08−0.1. Finally, by studying the early phase of the gravitational waves emitted by strongly eccentric binary systems –which could be formed e.g. in various many-body interactions in the galactic halo– we have found that they possess very specific characteristics which may be used to identify these type of binary systems.

Mátyás Vasúth, PhD

Mátyás Vasúth's profile pic

Head of group
Senior Research Fellow

Phone/ext.: +36 (1) 392-2222 / 2729
Room: 308
E-mail: vasuth.matyas "at" wigner.hu
ORCIDiD: 0000-0003-4573-8781

Dániel Barta, PhD

Balázs Kacskovics profile pic

Research Fellow

Phone/ext.: +36 (1) 392-2222 / 2729
Room: 308
E-mail: barta.daniel "at" wigner.hu
ORCIDiD: 0000-0001-6841-550X

Balázs Kacskovics

Balázs Kacskovics profile pic

PhD Student

Phone/ext.: +36 (1) 392-2222 / 2729
Room: 308
E-mail: kacskovics.balazs "at" wigner.hu
ORCIDiD: 0000-0001-9216-8713

~ Projects ~

Compact binary systems and gravitational waveforms

Weak gravitational fields and slow motion sources are described by the post-Newtonian (PN) approximation in a perturbative treatment where the dynamics of the binary and the emitted gravitational radiation is given to high accuracy. The PN approximation is proved to be accurately applied below the critical value ∼0.1 of the PN parameter. Both the radial motion and the parameterization of the orbit are known to high PN orders. As a first approximation, only the masses of the orbiting bodies are used to characterize the binary. The effects of other physical parameters (e.g. spins or the eccentricity of the orbit) and higher order general relativistic corrections have to be included for a more accurate description.
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Neutron stars as sources of gravitational radiation

Neutron stars are unique and fascinating astrophysical objects representing an extreme region of physics. These objects with high densities and strong magnetic fields produce large curvatures in spacetime. Together with black holes NSs are ideal sources for transient GW events. Black holes; however, represent simple objects compared to NS due to the small number of parameters needed for their characterization, e.g. their masses and spins. These characteristics for NSs are depending on the inner structure, their equation of state, representing much higher degrees of freedom. The observation of NS characteristics allows the detailed investigation of their inner structure and the behaviour of particles under extreme conditions. With the continuously increasing sensitivity of GW observatories these additional effects will play an important role in future studies.
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