Radiation from Collision‐dominated Relativistic Pair Fireballs

Abstract

It is generally accepted that γ-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 γγ 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 milliseconds) 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.