6. FIREBALLS

Before turning to the question of how is the kinetic energy of the relativistic flow converted to radiation we ask is it possible to produce the needed flows? More specifically, is it possible to accelerate particles to relativistic velocities? It is remarkable that a relativistic particle flow is almost the unavoidable outcome of a "fireball" - a large concentration of energy (radiation) in a small region of space in which there are relatively few baryons. The relativistic fireball model was proposed by Goodman [217] and by Paczynski [53]. They have shown that the sudden release of a large quantity of gamma ray photons into a compact region can lead to an opaque photon-lepton "fireball" through the production of electron-positron pairs. The term "fireball" refers here to an opaque radiation-plasma whose initial energy is significantly greater than its rest mass. Goodman [217] considered the sudden release of a large amount of energy, E, in a small volume, characterized by a radius, R_i. Such a situation could occur in an explosion. Paczynski [53] considered a steady radiation and electron-positron plasma wind that emerges from a compact region of size R_i with an energy, E, released on a time scale significantly larger than R_i / c. Such a situation could occur if there is a continuous source that operates for a while. As it will become clear later both configurations display, in spite of the seemingly large difference between them, the same qualitative behavior. Both Goodman [217] and Paczynski [53] considered a pure radiation fireballs in which there are no baryons. Later Shemi & Piran [220] and Paczynski [221] considered the effect of a baryonic load. They showed that under most circumstances the ultimate outcome of a fireball with a baryonic load will be the transfer of all the energy of the fireball to the kinetic energy of the baryons. If the baryonic load is sufficiently small the baryons will be accelerated to a relativistic velocity with E / M. If it is large the net result will be a Newtonian flow with v (2E / M)^1/2.