Synchrotron Self-Absorption in Gamma-Ray Burst Afterglow

Gamma-ray burst (GRB) afterglow is reasonably described by synchrotron emission from relativistic blast waves at cosmological distances. We perform detailed calculations taking into account the effect of synchrotron self-absorption. We consider emission from the whole region behind the shock front, and use the Blandford-McKee self-similar solution to describe the fluid behind the shock. We calculate the spectra and the observed image of a GRB afterglow near the self-absorption frequency, νa, and derive an accurate expression for νa. We show that the image is rather homogeneous for ν < νa, as opposed to the bright ring at the outer edge and the dim center, which appear at higher frequencies. We compare the spectra we obtain to radio observations of GRB 970508. We combine the calculations of the spectra near the self-absorption frequency with other parts of the spectra and obtain revised estimates for the physical parameters of the burst: E52 = 0.53, e = 0.59, B = 0.014, n1 = 3.0. These estimates different by up to 2 orders of magnitude from the estimates based on an approximate spectrum.