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.