Implications of Neutron Decoupling in Short Gamma Ray Bursts

Roughly half of the observed gamma-ray bursts (GRBs) may arise from the shocking of an ultra-relativistic shell of protons with the interstellar medium (ISM). Any neutrons originally present in the GRB fireball may, depending on the characteristics of the central engine, dynamically decouple as the fireball accelerates. This leads to outflow consisting of separate fast proton and slow neutron components. We derive detailed implications of neutron decoupling for the observed lightcurves of short bursts. We show that the collision of a neutron decayed shell with a decelerating outer shell is expected to result in an observable second peak in the GRB lightcurve. There may be substantial optical emission associated with such an event, so the upcoming Swift satellite may be able to place constraints on models for short bursts. We also discuss interesting inferences about central engine characteristics allowed by existing BATSE data and a consideration of neutron decoupling.