Infrared interferometric imaging and prospectives for the interferometric observation of the Galactic Center: the GRAVITY project.

The Galactic Center hosts a supermassive black hole called Sgr A*. Thanks to the GRAVITY instrument, the high angular resolution that the VLTI delivers will allow the direct observation of the immediate vicinity of such a black hole. To reach this astrophysical goal, it is mandatory to get very accurate interferometric observables to sucessfully apply imaging reconstruction methods. In that framework and as the first part of my thesis, I could use different infrared interferometric imaging techniques applied to the data obtained on the red supergiant Alpha Orionis (Betelgeuse) with the three telescope interferometer IOTA. These works have unveiled the presence of bright asymmetric structures on the stellar surface whose origin is likely to be convective.<br /><br />The quality of image reconstruction depends on the accuracy of the interferometric observables. In a second part of my thesis, I could study the simulated interferometric performances of GRAVITY to estimate the accuracy on phases and visibilities and checked that they meet the requirements. Finally, in order to optimize the GRAVITY future observations, it is important to get an idea of the spatial and temporal behaviour of its primary scientific goal: Sgr A*. To that aim, I could participate to a large multi-wavelength observation campaign of Sgr A*. I could use the BURST mode of the VISIR spectro-imager to get a high resolution on images and a high sensitivity to Sgr A* radiations. This allowed me to derive an unprecedented upper limit on Sgr A* flux at 8.6 microns. These observations also showed a flare in L' band exhibiting a quasi-periodicity that is twice longer than the ones previously observed. Even if the radiating process is still not fully understood, these observations confirm that the flares are due to the orbital motion of matter at a few Schwarzschild radii from Sgr A*. <br /><br />Thanks to an astrometric accuracy of 10 mas, which represents 1 Schwarzschild radius at the distance of the Galactic Center, GRAVITY will be able to resolve the orbital motion of these hot spots and to understand the nature of these radiations. Moreover, it will also allow to directly measure the space-time metric and test general relativity in strong field regime due the presence of a supermassive black hole located at the center of a galaxy.