M. Böttcher, R. Schlickeiser, and A. Marra
Astrophysical Journal, 563, 71 (2001)
It is generally accepted that gamma-ray bursts (GRBs) are initiated
by a relativistic pair fireball, converting its internal energy into
kinetic energy of a relativistically moving plasmoid and subsequently
into radiation. Here, we investigate the early stages of this evolution,
after the pair fireball has become optically thin to gamma-gamma
pair production. We show that for a short period of time, ~ 0.1
- a few seconds after the initial explosion, the pair plasmoid
evolution might be dominated by collisional processes prior to
the formation of a collisionless shock. We simulate these processes
during the early pair plasmoid evolution and calculate the expected
radiative signatures. We show that the radiation from the
collision-dominated pair plasmoid phase results in a short
(~ a few ms) flash of thermal soft X-ray emission, followed
by a transition phase of 
1 s
during which the fireball turns Thomson thin, but its radiation
remains dominated by thermal Comptonization, peaking at around
Epk ~ 100 MeV - a few GeV. While the very early thermal
emission could be associated with the quasi-thermal radiation
signatures found in the very early phases of several bright
BATSE GRBs, the predicted subsequent flash of high-energy
emission should be easily detectable with the GLAST satellite.