Gamma-ray bursts have millisecond variability

We have performed searches for isolated flares and for steady flickering in the initial similar to 1 s of gamma-ray burst light curves on the microsec ond to millisecond timescales. Two bursts among our sample of 20 revealed f our isolated flares with timescales from 256 to 2048 mu s. A wavelet analys is for our sample showed low-level flickering for all bursts on timescales from 256 mu s to 33 ms, with the majority of bursts containing rise times f aster than 4 ms and 30% having rise times faster than 1 ms. These results s how that millisecond variability is common in classical bursts and not some exceptional activity by a possibly separate class of bursts. These fast ri se times can be used to place the following severe limits on burst models. (1) The characteristic thickness of the energy generation region must be le ss than 1200 km along the line of sight. (2) The angular size of the gamma- ray emission region as subtended from the central source must be less than 42 ". (3) The expanding ejecta must have a range of Lorentz factors along a radius line with a dispersion of less than roughly 2%. (4) Within the exte rnal shock scenario, the characteristic dimension of the impacted cloud mus t be smaller than 16 AU on average. (5) Within the collimated jet scenario, the collimation angle must be smaller than 42 ".