Stellar Explosions by Magnetic Towers

We propose a magnetic mechanism for the collimated explosion of massive stars relevant for long-duration gamma-ray bursts (GRBs), X-ray Flashes (XRFs) and asymmetric core collapse supernovae. In particular, we apply Lynden-Bell’s magnetic tower scenario to the interior of a massive rotating star after the core has collapsed to form a collapsar with a black hole accretion disk or a millisecond magnetar as the central engine. The key element of the model is that the pressure of the toroidal magnetic field, continuously generated by differential rotation of the central engine, drives a rapid expansion which becomes vertically collimated after lateral force balance with the surrounding gas pressure is reached. The collimation naturally occurs because hoop stress concentrates magnetic field toward the rotation axis and inhibits lateral expansion without affecting vertical expansion. This leads to the growth of a self-collimated magnetic structure which Lynden-Bell termed a magnetic tower. When embedded in a massive star, the supersonic expansion of the tower drives a strong bow shock behind which an over-pressured cocoon of shocked stellar material forms, as observed in hydrodynamical simulations of collapsar jets. The cocoon confines the tower by supplying collimating pressure support and provides stabilization against disruption due to magnetohydrodynamical instabilities. Because the tower consists of closed field lines starting and ending on the central engine, mixing of baryons from the cocoon into the tower is suppressed. Baryon loading due to magneto-centrifugal winds from the central engine may also be suppressed because of the expected field line geometry. The channel cleared by the growing tower is thus plausibly free of baryons and allows the escape of magnetic energy from the central engine through the star. While propagating down the stellar density gradient, the expansion of the tower accelerates and becomes relativistic. At some point during the expansion fast collisionless reconnection becomes possible. The resulting dissipation of magnetic energy into accelerated particles may be responisble for GRB prompt emission. Subject headings: accretion, accretion disks — magnetic fields — MHD — gamma rays: bursts — supernovae: general — stars: magnetic fields