A Gamma Ray Burst with a 220 Microsecond Rise Time and a Sharp Spectral Cutoff

The Gamma Ray Burst GRB920229 has four extreme and unprecedented properties; a rise in brightness with an e-folding time scale of 220 ± 30μs, a fall in brightness with an e-folding time scale of 400± 100μs, a large change in spectral shape over a time of 768μs, and a sharp spectral cutoff to high energies with ∆E/E = 18%. The rapid changes occur during a spike in the light curve which was seen 0.164 s after the start of the burst. The spectrum has a peak νFν at 200 keV with no significant flux above 239 keV, although the cutoff energy shifts to less than 100 keV during the decay of the spike. These numbers can be used to place severe limits on fireball models of bursts. The thickness of the energy production region must be smaller than ∼ 66km, ejected shells must have a dispersion of the Lorentz factor of less than roughly 1% along a particular radius, and the angular size of the radiation emission region is of order 1 arc-minute as viewed from the burst center. The physical mechanism that caused the sharp spectral cutoff has not been determined. Subject headings: gamma rays: bursts