VLBI observations of GRB afterglows

Once the afterglow from a GRB has been detected in the radio, Very Long Baseline Interferometry (VLBI) observations can provide a resolution of 1 milliarcsecond or better and absolute astrometry at a similar level. We will review the VLBI programs carried out to date which include observations of G970508, G980329, G980425, and G980703. All of these objects are found to be very compact (size 〈 1 mas) even months after the burst. This is consistent with the predictions of fireball models. Our ultimate goal is a direct measurement of the morphology and expansion of a GRB afterglow