An Explanation for the Bimodal Duration Distribution of Gamma-Ray Bursts: Millisecond Pulsars from Accretion-induced Collapse

Cosmological gamma-ray bursts (GRBs) could be driven by dissipation of pure electromagnetic energy (Poynting flux) extracted from rapidly rotating compact objects with strong magnetic fields. One such possibility is a young millisecond pulsar (MSP) formed from the accretion-induced collapse (AIC) of a white dwarf. The combination of an efficient magnetic dynamo, likely operating during the first seconds of the initially hot and turbulent MSP interior, and the subsequent modest beaming of gamma ray-emitting outflows, would easily account for energy constraints. But the remarkable feature of such models is that they may naturally explain the puzzling bimodal distribution in GRB time durations. The two burst classes could correspond to MSPs that form spinning above and below, respectively, a gravitationally unstable limit. In the former case, the spin-down timescale is caused by gravitational radiation emission (<1 s), while the spin-down timescale of the latter is caused by electromagnetic dipole emission (≫1 s). These two timescales account for the short and long GRB durations, i.e., the observed bimodal GRB duration distribution. A natural prediction is that the short-duration GRBs would be accompanied by strong gravitational radiation emission, which is absent from the longer class. Both would show millisecond variabilities.