Wigner research group
Gergely Debreczeni, Dániel Barta#, Máté Ferenc Egri-Nagy, István Rácz, Mátyás Vasúth
The Gravitational Physics Research Group of Wigner RCP of the HAS conducts research on various fields including theoretical field theory, numerical and post-Newtonian general relativity calculations, experimental gravitational wave data analysis and fundamental research in algorithm optimization and many-core computer science. The progress and results of last year are summarized below.
Experimental gravitational wave data analysis. — During the year of 2014 the Virgo gravitational wave detector has gone through critical hardware upgrades to increase the sensitivity of the instrument. As such, no new data have been taken. The main activity of the collaboration and of our group was to prepare for the next data taking period, for the operation of the Advanced Virgo detector. As the result of the intense preparatory work, the analysis groups reached a quite high level of maturity in terms of operational and pipeline readiness. The main contribution from our group to this work was the coordination of the algorithmic and computational aspects of this effort. G. Debreczeni acted as the chair of the Virgo VDASC group co-chair of LVCComputing group and as the Computing Coordinator of the Virgo Collaboration. Scientific results and works can be summarized as follows:
1. The Wigner Virgo Group was working on the development of an algorithm (the 'GWorecast' pipeline – see Figure 1) which is, for the first time ever in history of gravitational wave research, able to predict the approximate time and sky location of a compact binary neutron star coalescence event by observing only the early inspiral part of the waves emitted. The applicability of this algorithms will be of utmost important in increasing the confidence level of gravitational wave events associated with gamma ray bursts.
2. By exploiting operational level concurrency of the algorithms involved, we managed to increase the sensitivity of the so-called Polynomial search pipeline which looks for the continuous gravitation wave signals emitted by compact binary sources. The outcome of this work was that we managed to extend the volume of the Universe that can be observed by gravitational wave detectors.
3. Significant progress has been achieved in the development of the search pipeline of continuously rotating, isolated neutron stars.
Reduced basis representations of gravitational waveform templates. — A large number of predicted waveform templates are used by data analysis of targeted search techniques for merging binary black hole sources based upon matched filtering. Waveforms for inspiralling binaries are parametrized by a set of intrinsic physical quantities that result in an eight-dimensional parameter space. The high eight-dimensionality makes gravitational wave searches,
# Ph.D. student
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parameter estimation, and modeling prohibitively expensive and computationally infeasible with most methods. To address these issues, the construction of high-accuracy reduced-basis representations that determines a relatively small set of the most relevant waveforms is essential.
Figure 1. Schematic flow diagram of the GWorecast algorithm. The goal of the pipeline is to predict the expected arrival time and sky location of the high-amplitude part of gravitational waves and associated gamma ray bursts. Having that information in time, it is possible to trigger collaborating electromagnetic telescopes, thus increase detection confidence by coincident observations.
Figure 2. Numerical integration of the evolution of the orbit and radiated gravitational waves.
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Our goal has been to develop interpolation techniques in the parameter space of waveforms: with a projection to a lower base, this allows one to significantly reduce the number of templates used and the computational demands for the search for signals. Thus the resulted reduction where the eccentricity is about to play the main role will prove to be just as significant as it has been shown in the case of spin. We would like to efficiently compress and accurately represent the space of waveforms for non-precessing binary black hole inspirals, which constitutes eight-dimensional parameter space. It is expected that the resultant reduction where the eccentricity plays the main role will prove to be just as significant as it has been shown in the case of spin.
We have reached a major stage by completing the following steps:
a.) generated a set of TaylorT4-expanded input waveforms that covers the multi-dimensional parameter space domain
b.) developed fast Fourier transforms (FFTs) of the time-domain via discrete sampling of the interpolating functions and by transformation of samples into the frequency domain
c.) defined frequency grids separately for amplitudes and phases over the multi-dimensional parameter space
d.) computed reduced bases for the amplitudes and phases with the SVD e.) interpolated over the parameter space
f.) assembled the frequency domain surrogate model
Figure 3. Basis waveforms of different stellar mass and eccentricity are stored in a waveform-databank. Graphical representation of waveforms in the time-domain.
Hyperbolic capture of compact binary systems. — The coalescence of compact binary systems with high orbital eccentricity is among the significant sources of gravitational
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waves. With completion of advanced gravitational wave detectors the detection of these sources is expected within the next few years.
The orbital evolution of inspiralling compact binaries can be conveniently described by the post-Newtonian (PN) approximation. In this weak field approach the gravitational potential and source velocities are considered as small parameters and the motion of the binary is approximated well by perturbed Keplerian orbits. The leading order contributions to the evolution of binaries on open orbits, moreover the energy flux, and the total energy emitted in GWs and the quadrupole contribution to the waveform during the hyperbolic interaction are well described in the literature. Moreover, there exists estimates on the expected rate of hyperbolic encounters in globular clusters and the Galactic Center. Waveforms of the multipole amplitudes for bound and unbound orbits, even the emitted energy and the energy spectrum are also presented.
In our work we have extended the description of binary systems up to the 1.5PN order, with the inclusion of the 1PN and spin-orbit (SO) contributions. Based on the radial motion of the system the conserved quantities characterizing the orbital evolution have been analyzed.
Moreover, a suitable parametrization valid for all three types of binary orbits (that is, elliptic, parabolic and hyperbolic) is also introduced. These results allow us to express the radiative change of the energy and angular momentum in terms of the conserved quantities.
Is the Bianchi identity always hyperbolic? — We consider n + 1 dimensional smooth Riemannian and Lorentzian spaces satisfying Einsteinʼs equa ons. The base manifold is assumed to be smoothly foliated by a one-parameter family of hypersurfaces. In both cases—likewise it is usually done in the Lorentzian case—Einsteinʼs equa ons may be split into ‘Hamiltonian’ and ‘momentum’ constraints and a ‘reduced’ set of field equations. It is shown that regardless of whether the primary space is Riemannian or Lorentzian, whenever the foliating hypersurfaces are Riemannian the ‘Hamiltonian’ and ‘momentum’ type expressions are subject to a subsidiary first order symmetric hyperbolic system. Since this subsidiary system is linear and homogeneous in the ‘Hamiltonian’ and ‘momentum’ type expressions, the hyperbolicity of the system implies that in both cases the solutions to the
‘reduced’ set of field equations are also solutions to the full set of equations provided that the constraints hold on one of the hypersurfaces foliating the base manifold.
Grants and international cooperation
COST-STSM-ECOST-STSM-MP1304-140414-042699: CompStar Short Term Scientific Mission
Publications
Articles
1. Rácz I: Stationary black holes as holographs II. CLASSICAL QUANT GRAV, 31:(3) Paper 035006. 35 p. (2014)
2. Rácz I: Is the Bianchi identity always hyperbolic? CLASSICAL QUANT GRAV, 31:(15) Paper 155004. 14 p. (2014)
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3. Rácz I: Gyenge gravitációs hullámok leírása az általános relativitáselméletben:
(Description of weak gravitational waves in general Relativity, in Hungarian).
Nagykanizsa: Czupi Kiadó, 2014.
Others
4. Rácz I: Magyar részvétel az európai gravitációshullám-kísérletekben - I. rész (Hungarian participation in the European gravity-wave experiments Part 1, in Hungarian). FIZIKAI SZEMLE, 64:(1) pp. 2-5. (2014)
5. Rácz I: Magyar részvétel az európai gravitációshullám-kísérletekben - II. Rész (Hungarian participation in the European gravity-wave experiments Part 2, in Hungarian). FIZIKAI SZEMLE, 64:(2) pp. 50-53. (2014)
LIGO and Virgo Collaborations
1. Aartsen MG, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [1186 authors]: Multimessenger search for sources of gravitational waves and high-energy neutrinos: Initial results for LIGO-Virgo and IceCube. PHYS REV D, 90:(10) Paper 102002. 22 p. (2014)
2. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [906 authors]:
First searches for optical counterparts to gravitational-wave candidate events.
ASTROPHYS J SUPPL S, 211:(1) Paper 7. 25 p. (2014)
3. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [878 authors]:
Application of a Hough search for continuous gravitational waves on data from the fifth LIGO science run. CLASSICAL QUANT GRAV, 31:(8) Paper 085014. 35 p. (2014)
4. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [877 authors]:
Constraints on cosmic strings from the ligo-virgo gravitational-wave detectors. PHYS REV LETT, 112:(13) Paper 131101. 10 p. (2014)
5. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [850 authors]:
First all-sky search for continuous gravitational waves from unknown sources in binary systems. PHYS REV D, 90: Paper 062010. 17 p. (2014)
6. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [888 authors]:
The NINJA-2 project: detecting and characterizing gravitational waveforms modelled using numerical binary black hole simulations. CLASSICAL QUANT GRAV, 31: Paper 115004. 45 p. (2014)
7. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [892 authors]:
Methods and results of a search for gravitational waves associated with gamma-ray bursts using the GEO600, LIGO, and Virgo detectors. PHYS REV D, 89: Paper 122004. 18 p. (2014)
8. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [848 authors]:
Search for gravitational radiation from intermediate mass black hole binaries in data
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from the second LIGO-Virgo joint science run. PHYS REV D, 89: Paper 122003. 15 p. (2014)
9. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [902 authors]:
Search for gravitational waves associated with γ-ray bursts detected by the InterPlanetary Network. PHYS REV LETT, 113:(1) Paper 011102. 14 p. (2014)
10. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [850 authors]:
Search for gravitational wave ringdowns from perturbed intermediate mass black holes in LIGO-Virgo data from 2005-2010. PHYS REV D, 89: Paper 102006. 18 p. (2014)
11. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [849 authors]:
Implementation of an F-statistic all-sky search for continuous gravitational waves in Virgo VSR1 data. CLASSICAL QUANT GRAV, 31:(16) Paper 165014. 27 p. (2014)
12. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [847 authors]:
Improved upper limits on the stochastic gravitational-wave background from 2009-2010 LIGO and Virgo data. PHYS REV LETT, 113: Paper 231101. 11 p. (2014)
13. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [898 authors]:
Gravitational waves from known pulsars: Results from the initial detector era.
ASTROPHYS J, 785:(2) Paper 119. 18 p. (2014)
14. Aasi J, et al. incl. Debreczeni G, Endroczi G, Nagy MF, Racz I, Vasuth M [195 authors]:
Reconstruction of the gravitational wave signal h(t) during the Virgo science runs and independent validation with a photon calibrator. CLASSICAL QUANT GRAV, 31:(16) Paper 165013. 30 p. (2014)
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