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

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 powerlaw index of the spectrum, ( ), is in agreement with the expected value for a a 0.44 0.07 F n a 1/3 n optically thin synchrotron radiation. The 1.4 GHz emission is self-absorbed. We infer constraints on the break frequencies n c and n m on 1997 May 21.0 UT from a spectral transition from to in the optical 0.6 1.1