Implications of the radio afterglow from the gamma-ray burst of 1997 May 8

Radio observations of the afterglow of the gamma-ray burst GRB 970508 provide unique new constraints on afterglow models. The quenching of diffractive scintillation at similar to 4 weeks delay provides the first direct estimate of source size and expansion rate. It implies an apparent size R similar to 10(17) cm and an expansion at a speed comparable to that of light at t similar to 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) similar to 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 gamma less than or equal to 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 similar to 1/gamma similar to 1/2. 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.