Spectral Properties of the Prompt X-ray Emission and Afterglow from the Gamma-Ray Burst of 1997 February 28

We report high-energy spectral data of the prompt emission of GRB 970228 and its X-ray afterglow. We establish that the nature of the X-ray afterglow emission is nonthermal and similar to the later portion of GRB 970228. Our data can be used to discriminate different emission models of GRB afterglows. While cooling of excited compact objects can be ruled out, fireball models are constrained in the physics of the radiation emission processes and their evolution.