GRB AFTERGLOWS - FROM ULTRA-RELATIVISTIC TO NONRELATIVISTIC PHASE

Postburst evolution of adiabatic fireballs that produce gamma-ray burs ts is studied. Emphasis has been put on the transition from the highly relativistic phase to the non-relativistic phase, which, according to our calculation, should happen much earlier than previously expected. The theoretical light curves decline a little sharper at the non-rela tivistic stage than at early times, but still can fit the observations well. However, disagreements are obvious when t greater than or equal to 100 d, implying a large initial energy Eo, a low interstellar dens ity n, or the possible existence of a persistent energy source at the center of the fireball.