The Radio–to–X-Ray Spectrum of GRB 970508 on 1997 May 21.0 UT

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Abstract
We have reconstructed the spectrum of the afterglow of GRB 970508 on 1997 May 21.0 UT (12.1 days after the gamma-ray burst) on the basis of observations spanning the X-ray-to-radio range. The low-frequency power-law index of the spectrum, α=0.44 ± 0.07 (Fν ∝ να), is in agreement with the expected value α=1/3 for optically thin synchrotron radiation. The 1.4 GHz emission is self-absorbed. We infer constraints on the break frequencies νc and νm on 1997 May 21.0 UT from a spectral transition from Fν ~ν−0.6 to Fν ~ν−1.1 in the optical passband around 1.4 days. A model of an adiabatically expanding blast wave emitting synchrotron radiation, in which a significant fraction of the electrons cool rapidly, provides a successful and consistent description of the afterglow observations over nine decades in frequency, ranging in time from trigger until several months later.
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