γ-Ray Burst Afterglow: 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 areproduced by relativistic expanding blastwaves, ``fireballs,'' at cosmologicaldistances. The detection of absorption lines in the optical afterglow of theGRB of 8 May 1997 confirms that the sources lie at cosmological distance. Weshow here that the new features detected in GRB970508 afterglow, radio emissionone week following the burst and a 2 day increase in optical flux, areconsistent with the blastwave model. The fireball optical depth at radiofrequencies is much smaller than previously estimated, which accounts for theobserved radio emission. The initial suppression of optical flux is consistentwith that predicted due to electron cooling. The combined radio and opticaldata imply that the fireball energy is $\sim10^{52}$erg, and that the densityof the medium into which the blastwave expands is $\sim 1cm^{-3}$, a valuetypical for gas within galaxies. We predict the time dependence of the radioflux and the absorption frequency, which constitute tests of the fireball modelas described in this paper.Comment: More general discussion of radio flux and self-absorption; Ap. J. Lett. in press (v. 489