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 pureelectromagnetic energy (Poynting flux) extracted from rapidly rotating compactobjects with strong magnetic fields. One such possibility is a youngmillisecond pulsar (MSP) formed from accretion-induced collapse (AIC) of awhite dwarf. The combination of an efficient magnetic dynamo, likely operatingduring the first seconds of the initially hot and turbulent MSP interior, andthe subsequent modest beaming of gamma-ray emitting outflows, would easilyaccount for energy constraints. But the remarkable feature of such models isthat they may naturally explain the hitherto unexplained bimodal distributionin GRB time durations. The two burst classes could correspond to MSPs that formspinning above and below a gravitationally unstable limit respectively. In theformer case, the spin-down time scale is due to gravitational radiationemission ($<1s$) while the spin-down time scale of the latter is due toelectromagnetic dipole emission ($\gg 1s$). These two time scales account forthe short and long GRB durations, i.e. the observed bimodal GRB durationdistribution. A natural prediction is that the short duration GRBs would beaccompanied by strong gravitational radiation emission which is absent from thelonger class. Both would show millisecond variabilities.Comment: 10 pages, Ap