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

In this Letter, we present the results of two-dimensional axisymmetric relativistic magnetohydrodynamic (MHD) simulations of the interaction between anisotropic pulsar wind and slowly expanding supernova ejecta. They show that the jet-torus pattern recently discovered in the Crab nebula and other pulsar nebulae can be explained within the MHD approximation when the condition of spherical symmetry is no longer enforced. The incorporation of anisotropy consistent with modern theory of pulsar winds results in a highly non-spherical termination shock squeezed along the rotational axis of the pulsar. In fact, the simulations reveal a whole complex of shocks terminating the wind. Downstream of the complex, the outflow proceeds mainly in the equatorial direction, with a typical velocity of 0.6c until it reaches the supernova envelope and gets deflected back into the main body of the nebula. If the global magnetization parameter of the pulsar wind is about σ≈ 0.01 or higher, the magnetic hoop stress halts the outflow in the surface layers of the equatorial disc and prompts formation of pressure-driven, magnetically collimated polar jets. The typical velocity of these transonic jets is about 0.5c. The simulated synchrotron X-ray images of the inner region of the nebulae are subject to strong Doppler beaming and resemble the Chandra data closely.