The 1.4 GHz light curve of GRB 970508
  We report on Westerbork 1.4 GHz radio observations of the radio counterpart to $\gamma$-ray burst GRB~970508, between 0.80 and 138 days after this event. The 1.4 GHz light curve shows a transition from optically thick to thin emission between 39 and 54 days after the event. We derive the slope $p$ of the spectrum of injected electrons ($dN/d\gamma_{e}\propto\gamma_{e}^{-p}$) in two independent ways which yield values very close to $p=2.2$. This is in agreement with a relativistic dynamically near-adiabatic blast wave model whose emission is dominated by synchrotron radiation and in which a significant fraction of the electrons cool fast. 
T.J. Galama (1), R.A.M.J. Wijers (2), M. Bremer (3), P.J. Groot (1),   R.G. Strom (1,4), A.G. de Bruyn (4,5), C. Kouveliotou...