Etude d'effets relativistes en champ gravitationnel fort

Abstract : The Galactic center is an ideal laboratory to put to the test general relativistic predictions. The supermassive black hole lying there has the biggest angular Schwarzschild radius (10 μas): its environment is thus a perfect candidate for the observation of strong relativistic phenomena. My first interest was to study the astrometric precision of the second generation VLTI instrument named GRAVITY. This beam combiner, which will take place in Paranal in 2014, was conceived in order to reach an astrometric precision of 10 μas. In order to check whether this goal is possible to achieve, I have used a programme simulating the instrument that allows to compute precisely the astrometric error that will affect the observed signal, taking into account the actual performance of GRAVITY. I came to the conclusion that the instrument is capable of reaching its goal, provided the observed point like source is bright enough. I was also able to show that GRAVITY can distinguish between a moving and a motionless source at a few Schwarzschild radii from the black hole. I then developed a new general relativistic ray-tracing code, GYOTO, allowing the computation of images and spectra of objects lying close to a black hole. A few astrophysical objects are implemented at the time being: a thin accretion disk, an accretion torus, a star orbiting around the black hole, for instance. GYOTO is also capable of integrating the radiative transfer equation inside optically thin objects. Let us highlight the fact that GYOTO will be made public in the near future. I was then able to use GYOTO in order to compute the silhouette and the spectrum of an accretion torus surrounding Sgr A*, emitting synchrotron radiation. The interest of such simulations is to constrain the black hole's parameters by fitting the theoretical model to observed data. I also computed the evolution of a blob of gas orbiting very close to the black hole's radius. This scenery can account for the radiation flares observed in the vicinity of the Galactic center. I performed realistic simulations of a GRAVITY observation of such a phenomenon, that allowed me to determine to what level GRAVITY could constrain this flare model, as well as the inclination of the black hole. I am also interested in the possibility to simulate observations of alternative compact objects at the Galactic center, in order to test the black hole paradigm. In this perspective, I developed a specific way of integrating geodesics with GYOTO, in the framework of the 3+1 formalism of general relativity. This formalism allows GYOTO to take into account non-standard metrics, computed numerically. This is a specific feature of GYOTO, that was not developed by other ray-tracing algorithm in the literature. The last part of my work was to give some perspectives of the use of GYOTO in numerically computed metrics. After having unsuccessfully tried to implement a gravastar metric inside the LORENE library, I developed the integration of null geodesics in the nonstationary spacetime of a collapsing neutron star. I computed images of such a neutron star, at different stages of its evolution. This kind of simulation could allow in the future to compute the electromagnetic counterpart of strong gravity phenomena, such as radiation emission in the vicinity of coalescing neutron stars.
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Submitted on : Monday, September 19, 2011 - 7:10:10 PM
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  • HAL Id : tel-00624816, version 1

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Frédéric Vincent. Etude d'effets relativistes en champ gravitationnel fort. Astrophysique [astro-ph]. Observatoire de Paris, 2011. Français. ⟨tel-00624816⟩

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