paper

Magnetically-dominated jets inside collapsing stars as a model for gamma-ray bursts and supernova explosions

arXiv:0707.0576 · doi:10.1063/1.2721969

Abstract

It has been suggested that magnetic fields play a dynamically-important role in core-collapse explosions of massive stars. In particular, they may be important in the collapsar scenario for gamma-ray bursts (GRB), where the central engine is a hyper-accreting black hole or a millisecond magnetar. The present paper is focussed on the magnetar scenario, with a specific emphasis on the interaction of the magnetar magnetosphere with the infalling stellar envelope. First, the ``Pulsar-in-a-Cavity'' problem is introduced as a paradigm for a magnetar inside a collapsing star. The basic set-up of this fundamental plasma-physics problem is described, outlining its main features, and simple estimates are derived for the evolution of the magnetic field. In the context of a collapsing star, it is proposed that, at first, the ram pressure of the infalling plasma acts to confine the magnetosphere, enabling a gradual build-up of the magnetic pressure. At some point, the growing magnetic pressure overtakes the (decreasing) ram pressure of the gas, resulting in a magnetically-driven explosion. The explosion should be highly anisotropic, as the hoop-stress of the toroidal field, confined by the surrounding stellar matter, collimates the magnetically-dominated outflow into two beamed magnetic-tower jets. This creates a clean narrow channel for the escape of energy from the central engine through the star, as required for GRBs. In addition, the delayed onset of the collimated-explosion phase can explain the production of large quantities of Nickel-56, as suggested by the GRB-Supernova connection. Finally, the prospects for numerical simulations of this scenario are discussed.

Invited paper in the "Physics of Plasmas" (May 2007 special issue), based on an invited talk at the 48th Annual Meeting of the APS Division of Plasma Physics (Oct. 30 - Nov. 3, 2006, Philadelphia, PA); 24 pages, 7 figures

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