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 ultrarelativistic 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 light curves of short bursts. In particular, 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 light curve. There may also be substantial optical emission associated with such an event. Two peaked bursts arising from neutron decoupling will generally have an interpeak duration equal to the duration of the second peak. We also discuss interesting inferences about central-engine characteristics allowed by existing BATSE data and a consideration of neutron decoupling.