  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 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 hitherto unexplained bimodal distribution
in GRB time durations. The two burst classes could correspond to MSPs that form
spinning above and below a gravitationally unstable limit respectively. In the
former case, the spin-down time scale is due to gravitational radiation
emission ($<1s$) while the spin-down time scale of the latter is due to
electromagnetic dipole emission ($\gg 1s$). These two time scales 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.

Astrophysics