Accurate Calorimetry of GRB030329

We report late-time observations of the radio afterglow of GRB 030329. The light curves show a clear achromatic flattening at 50 days after the explosion. We interpret this flattening as resulting from the blast wave becoming transrelativistic. Modeling of this transition enables us to make estimates of the energy content of the burst, regardless of the initial jet structure or the distribution of initial Lorentz factors of the ejecta. We find, in accordance with other events, that GRB030329 is well-described by an explosion with total energy of a few times 10 erg expanding into a circumburst medium with a density of order unity. Subject headings: gamma-ray bursts: specific (GRB030329) – radio continuum: general – cosmology: observations