The angular size and proper motion of the afterglow of GRB 030329

The bright, nearby (z = 0.1685) gamma-ray burst (GRB) of 2003 March 29 has presented us with the first opportunity to directly image the expansion of a GRB. This burst reached flux density levels at centimeter wavelengths more than 50 times brighter than any previously studied event. Here we present the results of a VLBI campaign using the Very Long Baseline Array, Very Large Array, Green Bank, Effelsberg, Arecibo, and Westerbork telescopes that resolves the radio afterglow of GRB 030329 and constrains its rate of expansion. The size of the afterglow is found to be similar to 0.07 mas (0.2 pc) 25 days after the burst and 0.17 mas (0.5 pc) 83 days after the burst, indicating an average velocity of 3c - 5c. This expansion is consistent with expectations of the standard fireball model. We measure the projected proper motion of GRB 030329 in the sky to less than 0.3 mas in the 80 days following the burst. In observations taken 52 days after the burst, we detect an additional compact component at a distance from the main component of 0.28 +/- 0.05 mas (0.80 pc). The presence of this component is not expected from the standard model.