Radio observations of the afterglow of the \gamma -ray burst GRB970508 provide unique new constraints on afterglow models .
The quenching of diffractive scintillation at \sim 4 week delay provides the first direct estimate of source size and expansion rate .
It implies an apparent size R \sim 10 ^ { 17 } cm and expansion at a speed comparable to that of light at t \sim 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 week delay are in agreement with the model and imply a fireball energy ( assuming spherical symmetry ) \sim 10 ^ { 52 } erg , consistent with the value inferred from observations at shorter delay .
The observed radio behavior deviates from model predictions at delays > 5 weeks .
This is expected , since at this delay the fireball is in transition from highly-relativistic to sub-relativistic expansion , with Lorentz factor \gamma \leq 2 .
Deviation may be due to a change in the physical processes associated with the shock wave as it becomes sub-relativistic ( e.g .
a decrease in the fraction of energy carried by magnetic field ) , or to the fireball being a cone of opening angle \sim 1 / \gamma \sim 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 0.1–10GHz on time scale of months .