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 purl electromagnetic energy (Poynting Bur) extracted from rapidly rot ating compact objects with strong magnetic fields. One such possibilit y is a young millisecond pulsar (MSP) formed from the accretion-induce d collapse (AIC) of a white dwarf. The combination of an efficient mag netic dynamo, likely operating during the first seconds of the initial ly hot and turbulent MSP interior, and the subsequent modest beaming o f gamma ray-emitting outflows, would easily account for energy constra ints. But the remarkable feature of such models is that they may natur ally explain the puzzling bimodal distribution in GRB time durations. The two burst classes could correspond to MSPs that form spinning abov e and below, respectively, a gravitationally unstable limit. In the fo rmer case, the spin-down timescale is caused by gravitational radiatio n emission (<1 s), while the spin-down timescale of the latter is caus ed by electromagnetic dipole emission (much greater than 1 s). These t wo timescales account for the short and long GRB durations, i.e., the observed bimodal GRB duration distribution. A natural prediction is th at the short-duration GRBs would be accompanied by strong gravitationa l radiation emission, which is absent from the longer class. Both woul d show millisecond variabilities.