198 research outputs found
Last orbits of binary black holes
Binary black hole systems in the pre-coalescence stage are numerically
constructed by demanding that the associated spacetime admits a helical Killing
vector. Comparison with third order post-Newtonian calculations indicates a
rather good agreement until the innermost stable circular orbit.Comment: 4 pages, 2 figures, invited talk at Journees Relativistes 2001, to
appear in International Journal of Modern Physics
New code for equilibriums and quasiequilibrium initial data of compact objects. II. Convergence tests and comparisons of binary black hole initial data
COCAL is a code for computing equilibriums or quasiequilibrium initial data
of single or binary compact objects based on finite difference methods. We
present the results of supplementary convergence tests of COCAL code using time
symmetric binary black hole data (Brill-Lindquist solution). Then, we compare
the initial data of binary black holes on the conformally flat spatial slice
obtained from COCAL and KADATH, where KADATH is a library for solving a wide
class of problems in theoretical physics including relativistic compact objects
with spectral methods. Data calculated from the two codes converge nicely
towards each other, for close as well as largely separated circular orbits of
binary black holes. Finally, as an example, a sequence of equal mass binary
black hole initial data with corotating spins is calculated and compared with
data in the literature.Comment: 9 pages, PRD in pres
Models of rotating boson stars and geodesics around them: new type of orbits
We have developed a highly accurate numerical code capable of solving the
coupled Einstein-Klein-Gordon system, in order to construct rotating boson
stars in general relativity. Free fields and self-interacting fields, with
quartic and sextic potentials, are considered. In particular, we present the
first numerical solutions of rotating boson stars with rotational quantum
number and , as well as the first determination of the maximum mass
of free-field boson stars with . We have also investigated timelike
geodesics in the spacetime generated by a rotating boson star for ,
and . A numerical integration of the geodesic equation has enabled us to
identify a peculiar type of orbits: the zero-angular-momentum ones. These
orbits pass very close to the center and are qualitatively different from
orbits around a Kerr black hole. Should such orbits be observed, they would put
stringent constraints on astrophysical compact objects like the Galactic
center
Pôles de compétitivité : les systèmes productifs recomposés par les réseaux d’innovation
International audienceThis paper focuses on the French competitiveness cluster policy. It questions the tension between the spatial anchorage of firms and their implication in science-industry networks of “competitiveness clusters”. It shows that the recent spatial and industrial transformations of the regional production system are a major key to understanding the way “competitiveness clusters” were created and work. Far from being only innovation networks, “competitiveness clusters” are a major actor of the spatial organization of the production system.Cet article interroge la tension sensible dans la politique des pôles de compétitivité entre logiques territoriales d’ancrage des entreprises et logiques réticulaires d’insertion dans des réseaux de recherche science-industrie. En s’appuyant sur l’exemple de la région Provence-Alpes-Côte d’Azur (PACA), il montre que les mutations spatiales et industrielles récentes des systèmes productifs régionaux sont une clé de lecture essentielle de la construction et du fonctionnement des pôles de compétitivité. Loin de n’être que des réseaux d’innovation, les pôles de compétitivité sont aussi un des acteurs de l’organisation des systèmes productifs
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.Comment: 20 pages, 11 PostScript figures, uses PTPTeX, to appear in the
Proceedings of Yukawa International Seminar 99 "Black Holes and Gravitational
Waves", edited by T. Nakamura & H. Kodama, Prog. Theor. Phys. Supp
Spectral Methods for Numerical Relativity
Version published online by Living Reviews in Relativity.International audienceEquations arising in General Relativity are usually too complicated to be solved analytically and one has to rely on numerical methods to solve sets of coupled partial differential equations. Among the possible choices, this paper focuses on a class called spectral methods where, typically, the various functions are expanded onto sets of orthogonal polynomials or functions. A theoretical introduction on spectral expansion is first given and a particular emphasis is put on the fast convergence of the spectral approximation. We present then different approaches to solve partial differential equations, first limiting ourselves to the one-dimensional case, with one or several domains. Generalization to more dimensions is then discussed. In particular, the case of time evolutions is carefully studied and the stability of such evolutions investigated. One then turns to results obtained by various groups in the field of General Relativity by means of spectral methods. First, works which do not involve explicit time-evolutions are discussed, going from rapidly rotating strange stars to the computation of binary black holes initial data. Finally, the evolutions of various systems of astrophysical interest are presented, from supernovae core collapse to binary black hole mergers
Imaging a boson star at the Galactic center
Millimeter very long baseline interferometry will soon produce accurate
images of the closest surroundings of the supermassive compact object at the
center of the Galaxy, Sgr A*. These images may reveal the existence of a
central faint region, the so-called shadow, which is often interpreted as the
observable consequence of the event horizon of a black hole. In this paper, we
compute images of an accretion torus around Sgr A* assuming this compact object
is a boson star, i.e. an alternative to black holes within general relativity,
with no event horizon and no hard surface. We show that very relativistic
rotating boson stars produce images extremely similar to Kerr black holes,
showing in particular shadow-like and photon-ring-like structures. This result
highlights the extreme difficulty of unambiguously telling the existence of an
event horizon from strong-field images.Comment: 21 pages, 9 figures, accepted in CQG; main difference wrt previous
version is the last paragraph of the conclusio
Hyperboloidal evolution of test fields in three spatial dimensions
We present the numerical implementation of a clean solution to the outer
boundary and radiation extraction problems within the 3+1 formalism for
hyperbolic partial differential equations on a given background. Our approach
is based on compactification at null infinity in hyperboloidal scri fixing
coordinates. We report numerical tests for the particular example of a scalar
wave equation on Minkowski and Schwarzschild backgrounds. We address issues
related to the implementation of the hyperboloidal approach for the Einstein
equations, such as nonlinear source functions, matching, and evaluation of
formally singular terms at null infinity.Comment: 10 pages, 8 figure
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