Acceleration and collimation of relativistic plasmas ejected by fast rotators

A stationary self-consistent outflow of a magnetised relativistic plasma from a rotating object with an initially monopole-like magnetic field is investigated in the ideal MHD approximation under the condition $\sigma/U_0^2 >1$, where $\sigma$ is the ratio of the Poynting flux over the mass energy flux at the equator and the surface of the star, with $U_0=\gamma_0v_0/c$ and $\gamma_0$ the initial four-velocity and Lorentz factor of the plasma. The mechanism of the magnetocentrifugal acceleration and self-collimation of the relativistic plasma is investigated. A jet-like relativistic flow along the axis of rotation is found in the steady-state solution under the condition $\sigma/U_0^2 >1$ with properties predicted analytically. The amount of the collimated matter in the jet is rather small in comparison to the total mass flux in the wind. An explanation for the weak self-collimation of relativistic winds is given.