Résumé :
Using Very Long Baseline Interferometry (VLBI) the radio afterglows of
Gamma-Ray Bursts can be imaged at sub-milliarcsecond resolution. Absolute
astrometry to localize the burst within the host galaxy, or to constrain
proper motions has also been achieved at similar levels. Several GRBs have
been detected with this technique in recent years. The nearby (z=0.1685)
gamma-ray burst of 29 March 2003 has presented us with a unique
opportunity for a detailed examination of a GRB. This burst reached flux
density levels at centimeter wavelengths more than 50 times brighter than
any previously studied event. I will present the results of VLBI
observations at multiple epochs designed to measure source diameters and
proper motions of GRB 030329 at sub-milliarcsecond levels. These
observations can constrain theories describing the dynamical evolution of
the fireball.
