Implications of Neutron Decoupling in Short Gamma‐Ray Bursts

Roughly half of the observed gamma-ray bursts (GRBs) may arise from theshocking of an ultra-relativistic shell of protons with the interstellar medium(ISM). Any neutrons originally present in the GRB fireball may, depending onthe characteristics of the central engine, dynamically decouple as the fireballaccelerates. This leads to outflow consisting of separate fast proton and slowneutron components. We derive detailed implications of neutron decoupling forthe observed lightcurves of short bursts. We show that the collision of aneutron decayed shell with a decelerating outer shell is expected to result inan observable second peak in the GRB lightcurve. There may be substantialoptical emission associated with such an event, so the upcoming Swift satellitemay be able to place constraints on models for short bursts. We also discussinteresting inferences about central engine characteristics allowed by existingBATSE data and a consideration of neutron decoupling.Comment: 12 pages, 3 figure