Gamma‐Ray Bursts Have Millisecond Variability

We have performed searches for isolated flares and for steady flickering inGamma Ray Burst light curves on the microsecond to millisecond time scales. Twobursts out of our sample of 20 revealed four isolated flares with time scalesfrom $256 \mu s$ to $2048 \mu s$. A wavelet analysis for our sample showed lowlevel flickering for all bursts on time scales from $256 \mu s$ to 33 ms, withthe majority of bursts containing rise times faster than four milliseconds and30% having rise times faster than one millisecond. These results show thatmillisecond variability is common in classical bursts and not some exceptionalactivity by a possibly separate class of bursts. These fast rise times can beused to place severe limits on burst models: (1) The characteristic thicknessof the energy generation region must be less than 1200 km along the line ofsight. (2) The angular size of the gamma ray emission region as subtended fromthe central source must be less than 42 arc-seconds. (3) The expanding ejectamust have a range of Lorentz factors along a radius line with a dispersion ofless than roughly 2%. (4) Within the external shock scenario, thecharacteristic dimension of the impacted cloud must be smaller than 16Astronomical Units on average. (5) Within the collimated jet scenario, thecollimation angle must be smaller than 42 arc-seconds.