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We propose a magnetic mechanism for the collimated explosion of a massivestar relevant for GRBs, XRFs and asymmetric supernovae. We apply Lynden-Bell'smagnetic tower scenario to the interior of a massive rotating star after thecore has collapsed to form a black hole with an accretion disk or a millisecondmagnetar acting as a central engine. We solve the force-free Grad-Shafranovequation to calculate the magnetic structure and growth of a tower embedded ina stellar environment. The pressure of the toroidal magnetic field,continuously generated by differential rotation of the central engine, drives arapid expansion which becomes vertically collimated after lateral force balancewith the surrounding gas pressure is reached. The collimation naturally occursbecause hoop stress concentrates magnetic field toward the rotation axis andinhibits lateral expansion. This leads to the growth of a self-collimatedmagnetic tower. When embedded in a massive star, the supersonic expansion ofthe tower drives a strong bow shock behind which an over-pressured cocoonforms. The cocoon confines the tower by supplying collimating pressure andprovides stabilization against disruption due to MHD instabilities. Because thetower consists of closed field lines starting and ending on the central engine,mixing of baryons from the cocoon into the tower is suppressed. The channelcleared by the growing tower is thus plausibly free of baryons and allows theescape of magnetic energy from the central engine through the star. Whilepropagating down the stellar density gradient, the tower accelerates andbecomes relativistic. During the expansion, fast collisionless reconnectionbecomes possible resulting in dissipation of magnetic energy which may beresponsible for GRB prompt emission.Comment: 19 pages, 8 figures, accepted to ApJ, updated references and  additional discussion adde

Stellar Explosions by Magnetic Towers