GAMMA-RAY BURSTS - POSTBURST EVOLUTION OF FIREBALLS

The post-burst evolution of fireballs that produce gamma-ray bursts (G RBs) is studied, assuming the expansion of fireballs to be adiabatic a nd relativistic. Numerical results as well as an approximate analytic solution for the evolution are presented. Owing to the adoption of a n ew relation between t, R and gamma, our results differ markedly from p revious studies. Synchrotron radiation from the shocked interstellar m edium is carefully calculated, using a conventional set of equations, The observed X-ray flux of GRB afterglows can be reproduced easily. Al though the optical afterglows seem much more complicated, our results can still present a rather satisfactory agreement with observations. W e also find that the expansion will no longer be highly relativistic a bout 4 d after the main GRB. We thus suggest that the marginally relat ivistic phase of the expansion should be investigated so as to check t he afterglows observed a week or more later.