Gamma-Ray–Burst Afterglow: Supporting the Cosmological Fireball Model, Constraining Parameters, and Making Predictions

Cosmological fireball models of gamma-ray bursts (GRBs) predict delayed emission, ``after-glow,'' at longer wavelengths. We present several new results regarding the model predictions, and show that X-ray to optical observations of GRB970228 and GRB970402 are naturally explained by the model: The scaling of flux with time and frequency agrees with model predictions and requires a power law distribution of shock accelerated electrons, dlog N/dlog E=2.3+-0.1 (implying, and consistent with the observed, 1/t decline of flux observed at a given frequency); The absolute flux value agrees with that inferred through the model from observed gamma-ray fluence. The future after-glow emission of these bursts is predicted. The observations indicate that the ratio of magnetic field to equipartition value and the fraction F of dissipated kinetic energy carried by electrons are not much smaller than 1. More frequent observations at a fixed wavelength, or a wide spectrum at a fixed time, would put strong constraints on these parameters. We show that inverse-Compton scattering suppresses X-ray/ optical emission at delays t<T=10(F/0.3)^4 hr. Observations therefore imply F<0.3. The strong dependence of T on F implies that T may vary widely from burst to burst, and that frequent X-ray/optical observations at t~T would determine F. For F~0.2, inverse-Compton emission dominates during the first 2hr, producing >1GeV photons and providing a natural explanation to the delayed GeV emission observed in several strong bursts.