Models for Gamma-Ray Bursts Free

The discussion of one-step versus two-step models for gamma-ray bursts has been going on for much longer than was indicated in the Physics Today, July 2002 issue of Physics Today (page 18). Observations of delayed GeV gamma rays following GRB940217 led to the hypothesis that it was surrounded by gas from stripping of a progenitor star ¹ (or one component of a progenitor binary). The inferred cloud dimensions and density are consistent with those estimated by James Reeves and colleagues ² from the x-ray line emission they observed. It was also predicted that some or all gamma-ray bursts would be accompanied by visible emission resembling that of a supernova as the gas is heated by collision with the relativistic debris that makes the burst itself. That prediction is now supported by evidence for supernova-like contributions to afterglows. ³