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 d iffractive scintillation at similar to 4 weeks delay provides the firs t direct estimate of source size and expansion rate. It implies an app arent size R similar to 10(17) cm and an expansion at a speed comparab le to that of light at t similar to 4 weeks, in agreement with the fir eball 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 agreem ent with the model and imply a fireball energy (assuming spherical sym metry) similar to 10(52) ergs, consistent with the value inferred from observations at shorter delay. The observed radio behavior deviates f rom model predictions at delays greater than 5 weeks. This is expected , since at this delay the fireball is in transition from highly relati vistic to subrelativistic expansion, with Lorentz factor gamma less th an or equal to 2. Deviation may result from a change in the physical p rocesses associated with the shock wave as it becomes subrelativistic (e.g., a decrease in the fraction of energy carried by the magnetic fi eld) or from the fireball being a cone of opening angle similar to 1/g amma similar to 1/2. We predict the future behavior of the radio flux assuming that the latter interpretation is valid. These predictions ma y be tested by radio observations in the frequency range 0.1-10 GHz on a timescale of months.