A Central Engine for Cosmic Gamma‐Ray Burst Sources

Abstract

One of a family previously proposed engines for cosmic gamma-ray burst sources is considered in some detail. A steadily accreting 106 G magnetic white dwarf should ultimately collapse to a strongly differentially rotating, millisecond-rotation-period neutron star for a wide range of steady accretion rates and initial masses if the accreting white dwarf has an evolved O-Ne-Mg composition. A similar neutron star could also result from an initial C-O white dwarf but only for more constrained accretion rates. Because the collapsing white dwarf begins as a γ = polytrope, the final neutron star's spin rate increases strongly with cylindrical radius. A stable windup of the neutron star's poloidal magnetic field then produces buoyant magnetic toroids which grow, break loose, rise, and partly penetrate the neutron star surface to form a transient, B ≈ 1017 G millisecond-spin-period pulsar with a powerful pulsar wind. This pulsar wind emission is then rapidly suppressed by the surface shear motion from the strong stellar differential rotation. This windup and transient pulsar formation can occur at other times on different cylinders and/or repeat on the same one, with (re-)windup and surface penetration timescales hugely longer than the neutron star's millisecond spin period. In this way, differential rotation both opens and closes the doors which allow neutron star spin energy to be emitted in powerful bursts of pulsar wind. Predictions of this model compare favorably to needed central engine properties of gamma-ray burst sources (total energy, birth rate, duration, subburst fluctuations and timescales, variability among burst events, and baryon loading).