gamma-ray burst after-glow: Confirming the cosmological fireball model

The recent detection of delayed X-ray and optical emission, ``afterglow,'' associated with gamma-ray bursts (GRBs) supports models, where the bursts are produced by relativistic expanding blastwaves, ``fireballs,'' at cosmological distances. The detection of absorption lines in the optical afterglow of the GRB of 8 May 1997 confirms that the sources lie at cosmological distance. We show here that the new features detected in GRB970508 afterglow, radio emission one week following the burst and a 2 day increase in optical flux, are consistent with the blastwave model. The fireball optical depth at radio frequencies is much smaller than previously estimated, which accounts for the observed radio emission. The initial suppression of optical flux is consistent with that predicted due to electron cooling. The combined radio and optical data imply that the fireball energy is $\sim10^{52} erg$, and that the density of the medium into which the blastwave expands is $\sim 1 cm^{-3}$, a value typical for gas within galaxies. We predict the time dependence of the radio flux and the absorption frequency, which constitute tests of the fireball model as described in this paper.