The origin of peculiar jet-torus structure in the Crab nebula

Recent discoveries of the intriguing “jet-torus” structure in the Crab Nebula and other pulsar nebulae prompted calls for re-examining of their theory. The most radical proposals involve abolishing of the MHD approximation alto-gether and developing of purely electromagnetic models. However, the classical MHD models of the Crab Nebula were hampered by the assumption of spherical symmetry made in order to render the flow equations easily integrable. The impressive progress in computational relativistic magnetohydrodynamics in recent years has made it possible to study the Crab nebula via numerical simulations without making such a drastic simplification of the problem. In this letter we present the results of the first study of such kind. They show that the jet-torus pattern can be explained within MHD approximation when anisotropy of pulsar winds is taken into account. They also indicate that the flow in the nebula is likely to be much more intricate than it has been widely believed.