A gamma-ray burst with a 220 microsecond rise time and a sharp spectral cutoff

The gamma-ray burst GRB 920229 has four extreme and unprecedented propertie s: a spike near the end of the burst has a rise in brightness with an e-fol ding timescale of 220 +/- 30 mu s, a fall in brightness with an e- folding timescale of 400 +/- 100 mu s, and a large change in spectral shape over a time of 768 mu s, while the spectrum of the entire burst has a sharp spectr al cutoff to high energies with Delta E/E = 18%. The rapid changes occur du ring a spike in the light curve that was seen 0.164 s after the start of th e burst. The spectrum has a peak nu F-nu at 200 keV with no significant flu x above 239 keV, although the cutoff energy shifts to less than 100 keV dur ing the decay of the spike. These numbers can be used to place severe limit s on fireball models of bursts. The thickness of the energy production regi on must be smaller than similar to 66 km, ejected shells must have a disper sion of the Lorentz factor of less than roughly 1% along a particular radiu s, and the angular size of the radiation emission region is on the order of 1' as viewed from the burst center. The physical mechanism that caused the sharp spectral cutoff has not been determined.