Gamma-Ray-Burst Afterglow: Supporting the Cosmological Fireball Model, Constraining Parameters, and Making Predictions (original) (raw)
1538-4357/485/1/L5
Abstract
Cosmological fireball models of γ-ray bursts (GRBs) predict delayed emission, “afterglow,” at longer wavelengths. We present several new results regarding the model predictions and show that X-ray to optical observations of GRB 970228 and GRB 970402 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 d log N e/d log γ_e_ = -2.3 ± 0.1 (implying, and consistent with the observed, t_-1 decline of flux observed at a given frequency). The absolute flux value agrees with that inferred through the model from observed γ-ray fluence. The future afterglow emission of these bursts is predicted. The observations indicate that the ratio of magnetic field to equipartition value and the fraction ξ_e of dissipated kinetic energy carried by electrons are not much smaller than 1. More frequent observations at a single wavelength or a wide spectrum at a single time would put strong constraints on these parameters. We show that inverse Compton emission dominates at delays t < t_IC = 10(ξ_e/0.3)4 hr and may suppress X-ray/optical emission for several hours. The X-ray detection of GRB 970228 implies that ξ_e_ ≤ 0.6. Stronger constraints may be obtained from X-ray/optical observations at time delays of ~1 hr. For ξ_e_ ~ 0.2, inverse Compton emission dominates during the first 2 hr, producing photons of more than 1 GeV and providing a natural explanation to the delayed giga-electron-volt emission observed in several strong bursts.