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

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

1997-09-19

arxiv.org/abs/astro-ph/9709199v1

Astrophys.J.497:288-293,1998

Astronomy and Astrophysics

Astrophysics

Submitted to ApJ (15 pages, LaTeX)

Scientific paper

10.1086/305467

Radio observations of the afterglow of the gamma-ray burst GRB970508 provide unique new constraints on afterglow models. The quenching of diffractive scintillation at 4 week delay provides the first direct estimate of source size and expansion rate. It implies an apparent size $R\sim10^{17}$cm and expansion at a speed comparable to that of light at 4 weeks delay, 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) of order 10^{52}erg, consistent with the value inferred from observations at shorter delay. The observed radio behavior deviates from model predictions at delays larger than 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<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 $1/\gamma\sim0.5$. 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.

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