Implications of the Radio Afterglow from the Gamma‐Ray Burst of 1997 May 8

Abstract

Radio observations of the afterglow of the γ-ray burst GRB 970508 provide unique new constraints on afterglow models. The quenching of diffractive scintillation at ~4 weeks delay provides the first direct estimate of source size and expansion rate. It implies an apparent size R ~ 10 17 cm and an expansion at a speed comparable to that of light at t ~ 4 weeks, in agreement with the fireball model prediction, R = 10 17 ( t /week) 5/8 cm. The radio flux and its dependence on time and frequency at 1-5 weeks delay are in agreement with the model and imply a fireball energy (assuming spherical symmetry) ~10 52 ergs, consistent with the value inferred from observations at shorter delay. The observed radio behavior deviates from model predictions at delays greater than 5 weeks. This is expected, since at this delay the fireball is in transition from highly relativistic to subrelativistic expansion, with Lorentz factor γ ≤ 2. Deviation may result from a change in the physical processes associated with the shock wave as it becomes subrelativistic (e.g., a decrease in the fraction of energy carried by the magnetic field) or from the fireball being a cone of opening angle ~ 1/γ ~ . We predict the future behavior of the radio flux assuming that the latter interpretation is valid. These predictions may be tested by radio observations in the frequency range 0.1-10 GHz on a timescale of months.