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.