  We propose a magnetic mechanism for the collimated explosion of a massive
star relevant for GRBs, XRFs and asymmetric supernovae. We apply Lynden-Bell's
magnetic tower scenario to the interior of a massive rotating star after the
core has collapsed to form a black hole with an accretion disk or a millisecond
magnetar acting as a central engine. We solve the force-free Grad-Shafranov
equation to calculate the magnetic structure and growth of a tower embedded in
a stellar environment. 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. This leads to the growth of a self-collimated
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
forms. The cocoon confines the tower by supplying collimating pressure and
provides stabilization against disruption due to MHD 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. 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 tower accelerates and
becomes relativistic. During the expansion, fast collisionless reconnection
becomes possible resulting in dissipation of magnetic energy which may be
responsible for GRB prompt emission.
