  We calculate both analytically and numerically the evolution of a highly
relativistic fireball through the stages of free expansion and coasting, and
determine the dependence of the thermodynamic and radiation variables in the
comoving and laboratory frames. Magnetic fields may have been important in the
original impulsive event. We discuss their effect on the fireball dynamics, and
consider also their effects on the radiation emitted when the fireball runs
into an external medium and is decelerated. The inverse synchro-Compton
mechanism can then yield high radiative efficiency in the reverse shock,
producing a burst of non-thermal radiation mainly in the MeV to GeV range whose
total energy and duration agree with those of typical cosmic gamma ray bursts.
