2007ApJ...664..411P

We present high-sensitivity Very Long Baseline Interferometry (VLBI) observations 806 days after the γ-ray burst of 2003 March 29 (GRB 030329). The angular diameter of the radio afterglow is measured to be 0.347±0.09 mas, corresponding to 0.99±0.26 pc at the redshift of GRB 030329 (z=0.1685). The evolution of the image size favors a uniform external density over an R^–2 windlike density profile (at distances of R≳10¹⁸ cm from the source), although the latter cannot be ruled out yet. The current apparent expansion velocity of the image size is only mildly relativistic, suggesting a nonrelativistic transition time of t_NR∼1 yr. A rebrightening, or at least a significant flattening in the flux decay, is expected within the next several years as the counterjet becomes visible (this has not yet been observed). An upper limit of <1.9c is set on the proper motion of the flux centroid.