Magnetar-driven magnetic tower as a model for gamma-ray bursts and asymmetric supernovae

We consider a newly born millisecond magnetar, focusing on its interaction with the dense stellar plasma in which it is initially embedded. We argue that the confining pressure and inertia of the surrounding plasma act to collimate the magnetar's Poynting-flux-dominated outflow into tightly beamed jets and increase its magnetic luminosity. We propose this process as an essential ingredient in the magnetar model for gamma-ray-burst and asymmetric supernova central engines. We introduce the ``pulsar in a cavity'' as an important model problem, representing a magnetized rotating neutron star inside a collapsing star. We describe its essential properties and derive simple estimates for the evolution of the magnetic field and the resulting spin-down power. We find that the infalling stellar mantle confines the magnetosphere, enabling a gradual buildup of the toroidal magnetic field due to continuous twisting. The growing magnetic pressure eventually becomes dominant, resulting in a magnetically driven explosion. The initial phase of the explosion is quasi-isotropic, potentially exposing a sufficient amount of material to Ni-56-producing temperatures to cause a bright supernova. However, if significant expansion of the star occurs prior to the explosion, then very little Ni-56 is produced, and no supernova is expected. In either case, hoop stress subsequently collimates the magnetically dominated outflow, leading to the formation of a magnetic tower. After the star explodes, the decrease in bounding pressure causes the magnetic outflow to become less beamed. However, episodes of late fallback can reform the beamed outflow, which may be responsible for late X-ray flares.