The Central Engine of Gamma-Ray Bursters

GRBs are thought to arise in relativistic blast-wave shocks at distances of10 to 1000 AU from the point where the explosive energy is initially released.To account for the observed duration and variability of the gamma-ray emissionin most GRBs, a central engine powering the shocks must remain active forseveral seconds to many minutes but must strongly fluctuate in its output onmuch shorter timescales. We show how a neutron star differentially rotating atmillisecond periods (DROMP) could be such an engine. A magnetized DROMP wouldrepeatedly wind up toroidal magnetic fields to about 10**17 G and only releasethe corresponding magnetic energy, when each buoyant magnetic field torusfloats up to, and breaks through, the stellar surface. The resulting rapidsub-bursts, separated by relatively quiescent phases, repeat until the kineticenergy of differential rotation is exhausted by these events. Calculated valuesof the energy released and of the various timescales are in agreement withobservations of GRBs. The baryon loading in each sub-burst may also beconsistent with theoretical requirements for a blast wave capable of giving theX-ray, optical and radio afterglows recently observed from cosmologicaldistances. DROMPs could be created in several kinds of astrophysical events;some of these would be expected to occur at about the observed GRB rate. Therequisite differential rotation could be imparted to neutron stars as they areborn or at the end of their existence: some DROMPs may be created close to starforming regions while others may arise far from galaxies.Comment: 6 pages, 1 figur