Gamma‐Ray Bursts Have Millisecond Variability

Abstract

We have performed searches for isolated Ñares and for steady Ñickering in the initial D1 s of gamma-ray burst light curves on the microsecond to millisecond timescales. Two bursts among our sample of 20 revealed four isolated Ñares with timescales from 256 to 2048 ks. A wavelet analysis for our sample showed low-level Ñickering for all bursts on timescales from 256 ks to 33 ms, with the majority of bursts containing rise times faster than 4 ms and 30% having rise times faster than 1 ms. These results show that millisecond variability is common in classical bursts and not some exceptional activity by a possibly separate class of bursts. These fast rise times can be used to place the following severe limits on burst models. (1) The characteristic thickness of the energy generation region must be less 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 42A. (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 external shock scenario, the characteristic dimension of the impacted cloud must be smaller than 16 AU on average. (5) Within the colli-mated jet scenario, the collimation angle must be smaller than 42A.