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E. Gourgoulhon

Publications and source records attributed to E. Gourgoulhon.

At least 55 records · Page 3Linked to original sources

Rapidly rotating strange stars

We study effects of the strange quark mass and of the QCD coupling constant in the MIT bag model of quark matter with lowest order quark-gluon interactions on the rapid rotation of strange stars. The influence of rotation on global parameters of strange stars is greater than in the case of the neutron star models. We show that independently of the strange quark mass and the coupling constans the ratio of the rotational kinetic energy to the absolute value of the gravitational potential energy for rotating strange stars is significantly higher than for ordinary neutron stars. This might indicate that rapidly rotating strange stars could be important sources of gravitational waves.

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Rotating compact strange stars

We compute numerical models of uniformly rotating strange stars (SS) in general relativity for the recently proposed QCD-based equation of state (EOS) of strange quark matter (Dey et al. 1998). Static models based on this EOS are characterised by a larger surface redshift than strange stars within the MIT bag model. The frequencies of the fastest rotating configurations described by Dey model are much higher than these for neutron stars (NS) and for the simplest SS MIT bag model. We determine a number of physical parameters for such stars and compare them with those obtained for NS. We construct constant baryon mass equilibrium sequences both normal and supramassive. Similarly to the NS a supramassive SS, prior to collapse to a black hole, spins up as it loses angular momentum. We find the upper limits on maximal masses and maximal frequencies of the rotating configurations. We show that the maximal rotating frequency for each of considered evolutionary sequences is never the Keplerian one. A normal and low mass supramassive strange stars gaining angular momentum always slows down just before reaching the Keplerian limit. For a high mass supramassive SS sequence the Keplerian configuration is the one with the lowest rotational frequency in the sequence. The value of $T/W$ for rapidly rotating SS of any mass is significantly higher than those for ordinary NS. For Keplerian configurations it increases as mass decreases. The results are robust for all linear self-bound equations of state.

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Innermost stable circular orbits around strange stars and kHz QPOs in low-mass X-ray binaries

Exact calculations of innermost stable circular orbit (ISCO) around rotating strange stars are performed within the framework of general relativity. Equations of state (EOS) of strange quark matter based on the MIT Bag Model with massive strange quarks and lowest order QCD interactions, are used. The presence of a solid crust of normal matter on rotating, mass accreting strange stars in LMXBs is taken into account. It is found that, contrary to neutron stars, above some minimum mass a gap always separates the ISCO and stellar surface, independently of the strange star rotation rate. For a given baryon mass of strange star, we calculate the ISCO frequency as function of stellar rotation frequency, from static to Keplerian configuration. For masses close to the maximum mass of static configurations the ISCO frequencies for static and Keplerian configurations are similar. However, for masses significantly lower than the maximum mass of static configurations, the minimum value of the ISCO frequency is reached in the Keplerian limit. Presence of a solid crust increases the ISCO frequency for the Keplerian configuration by about ten percent compared to that for a bare strange star of the same mass.

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Darwin-Riemann problems in general relativity

A review is given of recent results about the computation of irrotational Darwin-Riemann configurations in general relativity. Such configurations are expected to represent fairly well the late stages of inspiralling binary neutron stars.

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Fast rotation of strange stars

Exact models of uniformly rotating strange stars, built of self bound quark matter, are calculated within the framework of general relativity. This is made possible thanks to a new numerical technique capable to handle the strong density discontinuity at the surface of these stars. Numerical calculations are done for a simple MIT bag model equation of state of strange quark matter. Evolutionary sequences of models of rotating strange stars at constant baryon mass are calculated. Maximally rotating configurations of strange stars are determined, assuming that the rotation frequency is limited by the mass shedding and the secular instability with respect to axisymmetric perturbations. Exact formulae which give the dependence of the maximum rotation frequency, and of the maximum mass and corresponding radius of rotating configurations, on the value of the bag constant, are obtained. The values of T/W for rapidly rotating massive strange stars are significantly higher than those for ordinary neutron stars. This might indicate particular susceptibility of rapidly rotating strange stars to triaxial instabilities.

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Evolutionary sequences of irrotational binary neutron stars

We present results of numerical computations of quasiequilibrium sequences of binary neutron stars with zero vorticity, in the general relativistic framework. The Einstein equations are solved under the assumption of a conformally flat spatial 3-metric (Wilson-Mathews approximation). The evolution of the central density of each star is monitored as the orbit shrinks in response to gravitational wave emission. For a compactification ratio M/R=0.14, the central density remains rather constant (with a slight increase, below 0.1%) before decreasing. For a higher compactification ratio M/R=0.17 (i.e. stars closer to the maximum mass configuration), a very small density increase (at most 0.3%) is observed before the decrease. This effect remains within the error induced by the conformally flat approximation. It can be thus concluded that no substantial compression of the stars is found, which would have indicated a tendency to individually collapse to black hole prior to merger. Moreover, no turning point has been found in the binding energy or angular momentum along evolutionary sequences, which may indicate that these systems do not have any innermost stable circular orbit (ISCO).

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Numerical models of irrotational binary neutron stars in general relativity

We report on general relativistic calculations of quasiequilibrium configurations of binary neutron stars in circular orbits with zero vorticity. These configurations are expected to represent realistic situations as opposed to corotating configurations. The Einstein equations are solved under the assumption of a conformally flat spatial 3-metric (Wilson-Mathews approximation). The velocity field inside the stars is computed by solving an elliptical equation for the velocity scalar potential. Results are presented for sequences of constant baryon number (evolutionary sequences). Although the central density decreases much less with the binary separation than in the corotating case, it still decreases. Thus, no tendency is found for the stars to individually collapse to black hole prior to merger.

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Spectral methods in general relativistic astrophysics

We present spectral methods developed in our group to solve three-dimensional partial differential equations. The emphasis is put on equations arising from astrophysical problems in the framework of general relativity.

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Numerical approach for high precision 3-D relativistic star models

A multi-domain spectral method for computing very high precision 3-D stellar models is presented. The boundary of each domain is chosen in order to coincide with a physical discontinuity (e.g. the star's surface). In addition, a regularization procedure is introduced to deal with the infinite derivatives on the boundary that may appear in the density field when stiff equations of state are used. Consequently all the physical fields are smooth functions on each domain and the spectral method is absolutely free of any Gibbs phenomenon, which yields to a very high precision. The power of this method is demonstrated by direct comparison with analytical solutions such as MacLaurin spheroids and Roche ellipsoids. The relative numerical error reveals to be of the order of $10^{-10}$. This approach has been developed for the study of relativistic inspiralling binaries. It may be applied to a wider class of astrophysical problems such as the study of relativistic rotating stars too.

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Strategy to detect the gravitational radiation counterpart of gamma-ray bursts

Both observational and theoretical rates of binary neutron star coalescence give low prospects for detection of a single event by the initial LIGO/VIRGO interferometers. However, by utilizing at the best all the a priori information on the expected signal, a positive detection can be achieved. This relies on the hypothesis that $γ$-ray bursts are the electromagnetic signature of neutron star coalescences. The information about the direction of the source can then be used to add in phase the signals from different detectors in order (i) to increase the signal-to-noise ratio and (ii) to make the noise more Gaussian. Besides, the information about the time of arrival can be used to drastically decrease the observation time and thereby the false alarm rate. Moreover the fluence of the $γ$-ray emission gives some information about the amplitude of the gravitational signal. One can then add the signals from $\sim 10^4$ observation boxes ($\sim$ number of $γ$-ray bursts during 10 years) to yield a positive detection. Such a detection, based on the Maximum a Posteriori Probability Criterium, is a minimal one, in the sense that no information on the position and time of the events, nor on any parameter of the model, is collected. The advantage is that this detection requires an improvement of the detector sensitivity by a factor of only $\sim 1.5$ with respect to the initial LIGO/VIRGO interferometers, and that, if positive, it will confirm the $γ$-ray burst model.

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Spontaneous symmetry breaking of rapidly rotating stars in general relativity: influence of the 3D-shift vector

An analytical scheme and a numerical method in order to study the effects of general relativity on the viscosity driven secular bar mode instability of rapidly rotating stars are presented. The approach consists in perturbing an axisymmetric and stationary configuration and studying its evolution by constructing a series of triaxial quasi-equilibrium configurations. These are obtained by solution of an approximate set of field equations where only the dominant non-axisymmetric terms are taken into account. The progress with respect to our former investigation consists in a higher relativistic order of the non-axisymmetric terms included into the computation, namely the fully three-dimensional treatment of the vector part of the space-time metric tensor as opposed to the scalar part, solely, in the former case. The scheme is applied to rotating stars built on a polytropic equation of state and compared to our previous results. The 3D-vector part turns out to inhibit the symmetry breaking efficiently. Nevertheless, the bar mode instability is still possible for an astrophysically relevant mass of M_ns=1.4 M_sun when a stiff polytropic equation of state with an adiabatic index of gamma=2.5 is employed. Triaxial neutron stars may be efficient emitters of gravitational waves and are thus potentially interesting sources for the forthcoming laser interferometric gravitational wave detectors such as LIGO, VIRGO and GEO600. From a numerical point of view, the solution of the three-dimensional minimal-distortion shift vector equation in spherical coordinates is an important achievement of our code.

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A relativistic formalism to compute quasi-equilibrium configurations of non-synchronized neutron star binaries

A general relativistic version of the Euler equation for perfect fluid hydrodynamics is applied to a system of two neutron stars orbiting each other. In the quasi-equilibrium phase of the evolution of this system, a first integral of motion can be derived for certain velocity fields of the neutron star fluid including the (academic) case of co-rotation with respect to the orbital motion (synchronized binaries) and the realistic case of counter-rotation with respect to the orbital motion. The velocity field leading to this latter configuration can be computed by solving three-dimensional vector and scalar Poisson equations.

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On gravitational waves emitted by an ensemble of rotating neutron stars

We study the possibility to detect the gravitational wave background generated by all the neutron stars in the Galaxy with only one gravitational wave interferometric detector. The proposed strategy consists in squaring the detector's output and searching for a sidereal modulation. The shape of the squared signal is computed for a disk and a halo distribution of neutron stars. The required noise stability of the interferometric detector is discussed. We argue that a possible population of old neutron stars, originating from a high stellar formation rate at the birth of the Galaxy and not emitting as radio pulsars, could be detected by the proposed technique in the low frequency range of interferometric experiments.

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Triaxial neutron stars -- a possible source of gravitational radiation

Triaxial neutron stars may be important sources of gravitational radiation for the forthcoming generation of interferometric gravitational wave detectors such as LIGO, VIRGO, and GEO600. We investigate the viscosity triggered bar mode secular instability of rapidly rotating neutron stars by means of a perturbation analysis of numerically constructed ``exact'' general relativistic axisymmetric star models. In the theoretical approach, only the dominant parts of the nonaxisymmetric terms of the 3D-Einstein equations are taken into account. A comparison of our results with previous studies of Newtonian polytropic stars confirms James' classical result gamma_crit=2.238 for the critical polytropic index. Beyond the Newtonian regime, gamma_crit reveals a slight increase toward highly relativistic configurations. Six out of twelve employed realistic dense matter equations of state admit the spontaneous symmetry breaking for masses above 1.6 M_sol.

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Gravitational waves from accreting neutron stars

We show that accreting neutron stars in binary systems or in Landau-Thorne-Zytkow objects are good candidates for continuous gravitational wave emission. Their gravitational radiation is strong enough to be detected by the next generation of detectors having a typical noise of 10^{-23} Hz^{-1/2}.

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Gravitational waves from neutron stars

Contents: 1. Introduction 2. Spontaneous symmetry breaking 2.1 Review of classical results about Maclaurin/Jacobi ellipsoids 2.2 Spontaneous breaking of symmetry: a general phenomenom 2.3 Previous results for compressible Newtonian stars 2.4 Generation of gravitational waves 2.5 Finding the equilibrium configurations of a rotating star in the Newtonian regime 2.6 Extension to general relativity 2.7 First integral of fluid motion in general relativity 2.8 Gravitational field equations 2.9 Numerical results 3. CW emission from pulsars 4. Discussion and conclusion Appendix: Relativistic hydrodynamics in an accelerated frame

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Gravitational waves from isolated neutron stars

Continuous wave gravitational radiation from isolated rotating neutron stars is discussed. The general waveform and orders of magnitude for the amplitude are presented for various known pulsars. The specific case of gravitational radiation resulting from the distortion induced by the stellar magnetic field is presented. Finally some preliminary results about the signal from the whole population of neutron stars in the Galaxy are discussed.

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Spectral methods in general relativity -- toward the simulation of 3D-gravitational collapse of neutron stars

Several applications of spectral methods to problems related to the relativistic astrophysics of compact objects are presented. Based on a proper definition of the analytical properties of regular tensorial functions we have developed a spectral method in a general sphericallike coordinate system. The applications include the investigation of spherically symmetric neutron star collapse as well as the solution of the coupled 2D-Einstein-Maxwell equations for magnetized, rapidly rotating neutron stars. In both cases the resulting codes are efficient and give results typically several orders of magnitude more accurate than equivalent codes based on finite difference schemes. We further report the current status of a 3D-code aiming at the simulation of non-axisymmetric neutron star collapse where we have chosen a tensor based numerical scheme.

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