Stirring the Embers: High-Sensitivity VLBI Observations of GRB 030329

We present high-sensitivity Very Long Baseline Interferometry (VLBI)observations 806 days after the γ-ray burst of 2003 March 29 (GRB030329). The angular diameter of the radio afterglow is measured to be0.347+/-0.09 mas, corresponding to 0.99+/-0.26 pc at the redshift of GRB030329 (z=0.1685). The evolution of the image size favors a uniformexternal density over an R^-2 windlike density profile (atdistances of R>~10¹⁸ cm from the source), although thelatter cannot be ruled out yet. The current apparent expansion velocityof the image size is only mildly relativistic, suggesting anonrelativistic transition time of t_NR~1 yr. A rebrightening,or at least a significant flattening in the flux decay, is expectedwithin the next several years as the counterjet becomes visible (thishas not yet been observed). An upper limit of 1.9c is set on theproper motion of the flux centroid.