Paires de quasars et mirages gravitationnels : Etudes observationnelles et implications astrophysiques

The present work deals with the gravitational lensing phenomenon through the study of three pairs of quasars with different redshifts separated by a few arcseconds on the sky as well as through the study of a new gravitationnally lensed system that we had the chance to serendipitously discover during our thesis.
In the first part, we investigate the possibility of measuring the mass of the foreground quasar using the latter as a potential cosmic lens that deviates the light from the background quasar. For this purpose, we have observed with the Hubble Space Telescope (HST) the known close quasar pairs with discrepant redshifts in order to unveil secondary images of the background quasar in the vicinity of the foreground one. Among the three known quasar pairs, Q1009-0252 possesses one of its two components lensed by a foreground galaxy. Because of the lack of a spectrum of the lensing galaxy, we have combined various data sets obtained for this system in order to determine its type and its redshift as well as the causes of the observed chromatic differences in the flux ratios between the two lensed images. Finally, motivated by the results of Burbidge et al. (1997) ,we have re-examined the expected number of quasar pairs (with different redshifts) that should have been discovered.
The second part is dedicated to the study of the complex gravitationally lensed system RXS J113155.4-123155. First, we expose the results related to the discovery of this system and discuss its properties (i.e. number of images, their relative positions, their brightnesses, redshifts of the source and of the lensing galaxy). Next, we present the results obtained from the multi-colour and multi-epoch follow-up imaging of this sytem. These results include the level of differential extinction affecting the lensed images, the confirmation of an important level of intrinsic variability of the source, the evidence of a microlensing event affecting one of the lensed image as well as a modeling of the lensing galaxy. This analysis enabled us to understand the possible phenomena responsible for the observed flux ratios.