IMPACT OF RELATIVISTIC FIREBALLS ON EXTERNAL MATTER - NUMERICAL-MODELS OF COSMOLOGICAL GAMMA-RAY BURSTS

We numerically model the interaction between an expanding fireball and a stationary external medium whose density is either homogeneous or v aries with distance as a power law. The evolution is followed until mo st of the fireball kinetic energy is converted into internal energy. T he density, pressure, and flow Lorentz factor profiles are shown at di fferent stages, including shock and rarefaction wave reflections, for a fireball of initial bulk Lorentz factor Gamma = 100 in both the adia batic and the nonadiabatic (radiative) regimes. For cooling times shor ter than the dynamic time, bolometric light curves are computed for va lues of Gamma = 50, 100, and 200. We compare the numerical light curve s with analytic results and find that for a homogeneous external mediu m there is a simple scaling relationship between light curves obtained for different parameters. The light curves for power-law external den sities are similar in shape to those in the homogeneous case. We discu ss the implications of a comparison of the results with observed gamma -ray burst time histories.