On the Source and Location of Temporal Variability in Fireballs

Most cosmological models for gamma-ray bursts invoke the production of a fireball'' in a compact region, as indicated by the short time variability of the observed GRBs. The high density of $e^+e^-$ pairs in such fireballs inevitably makes them opaque to gamma-rays and requires the gamma-ray emission to take place only after the fireball has expanded and became optically thin to gamma'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.