GASDYNAMICS OF RELATIVISTICALLY EXPANDING GAMMA-RAY BURST SOURCES - KINEMATICS, ENERGETICS, MAGNETIC-FIELDS, AND EFFICIENCY

We calculate both analytically and numerically the evolution of highly relativistic fireballs through the stages of free expansion and coasting, and determine the dependence of the thermodynamic and radiation variables in the comoving and laboratory frames. The dynamics and the comoving geometry change at the (lab) expansion factors r/r0 > eta and r/r0 > eta2, respectively, where eta = E0/M0c2 is the initial Lorentz factor. In the lab, the gas appears concentrated in a thin shell of width r0 until r/r0 less than or similar to eta2, and increases linearly after that. Magnetic fields may have been important in the original impulsive event. We discuss their effect on the fireball dynamics and also consider 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 (and through turbulent instabilities and mixing also in the forward blast wave), producing a burst of nonthermal radiation mainly in the MeV to GeV range. The energy and duration depend on eta, the magnetic field strength, and the external density, and can match the range of properties observed in cosmic gamma-ray bursts.