On the Source and Location of Temporal Variability in Fireballs

Most cosmological models for γ-ray bursts (GRBs) invoke the production of a “fireball” 1−3 in a compact region, as indicated by the short time variability of the observed GRBs. The high density of ee pairs in such fireballs inevitably makes them opaque to γ-rays and requires the γ-ray emission to take place only after the fireball has expanded and became optically thin to γ’s. Many of the “traditional” scenarios explain the temporal variability of GRBs as being formed by growing instabilities in the expanding fireball. Here we explore this possiblity and examine its validity. Piran et al. have shown that generally, after a short rearrangement phase in relativistic fireballs, most of the matter and energy are concentrated in a narrow shell. Because of the relativistic beaming, only a small angular part of the shell with an angular size of θ ≈ 1/γ is seen by a distant observer, where γ = 1/ √