4 Discussion

We have calculated the light curve and spectra due to synchrotron emission from an ultra-relativistic adiabatic blast wave, which is described by the Blandford-McKee (1976) self similar solution. We obtained a flat peak for the light curve, which rules out the interpretation of the sharp optical peak of GRB 970508 as the expected peak of the light curve. The observed image of an afterglow has been calculated over a wide range of frequencies. The image at $\nu<\nu_{\rm a}$is quite homogeneous, while at higher frequencies there is a bright ring at the outer edge and the center is dim.

We have calculated the physical parameters of GRB 970508 based on detailed calculations of the spectra near $\nu_{\rm a}$ and $\nu_{\rm m}$. The values we have obtained are different by up to two orders of magnitude from the values obtained using a simple broken power law theoretical spectrum (Wijers & Galama 1998). This stresses the sensitivity of this method to the exact details of the theoretical model.

Acknowledgements

This research was supported by the US-Israel BSF grant 95-328 and by a grant from the Israeli Space Agency. Re'em Sari thanks the Sherman Fairchild Foundation for support.