On the magnetic acceleration and collimation of astrophysical outflows

The axisymmetric 3-D MHD outflow of a cold plasma from a magnetized and rotating astrophysical object is numerically simulated with the purpose of investigating the outflow's magnetocentrifugal acceleration and eventual collimation. Gravity and thermal pressure are neglected while a split-monopole is used to describe the initial magnetic field configuration. It is found that the stationary final state depends critically on a single parameter alpha expressing the ratio of the corotating speed at the Alfven distance to the initial flow speed along the initial monopole-like magnetic fieldlines. Several angular velocity laws have been used for relativistic and nonrelativistic outflows. The acceleration of the flow is most effective at the equatorial plane and the terminal flow speed depends linearly on alpha. Significant flow collimation is found in nonrelativistic efficient magnetic rotators corresponding to relatively larger than 1 values of alpha while very weak collimation occurs in inefficient magnetic rotators with values of alpha smaller than about 1. Part of the flow around the rotation and magnetic axis is cylindrically collimated while the remaining part obtains radial asymptotics. The transverse radius of the jet is inversely proportional to alpha while the density in the jet grows linearly with alpha. For alpha greater than about 5 the magnitude of the flow in the jet remains below the fast MHD wave speed everywhere. In relativistic outflows, no collimation is found in the supersonic region for parameters typical for radio pulsars. All above results verify the main conclusions of general theoretical studies on the magnetic acceleration and collimation of outflows from magnetic rotators and extend previous numerical simulations to large stellar distances.Comment: 15 pages, 13 figures. Accepted for publication, MNRA

Magnetocentrifugal acceleration of plasma in a nonaxisymmetric magnetosphere

Violation of the axial synnetry of a magnetic field essentially modifies the physics of the plasma outflow in the magnetosphere of rotating objects. In comparison to the axisymmetric outflow, two new affects appear: more efficient magnetocentrifugal acceleration of the plasma along particular field lines and generation of MHD waves. Here, we use an ideal MHD approximation to study the dynamics of a cold wind in the nonaxisymmetric magnetosphere. We obtain a self-consistent analytical solution of the problem of cold plasma outflow from a slowly rotating star with a slightly nonaxisymmetric magnetic field using perturbation theory. In the axisymmetric (monopole-like) magnetic field, the first term in the expansion of the terminating energy of the plasama in powers of $\Omega$ is proportional to $\Omega^4$, where $\Omega$ is the angular velocity of the central source. Violation of the axial symmetry of the magnetic field crucially changes this dependence. The first correction to the energy of the plasma becomes proportional to $\Omega$. Efficient magnetocentrifugal acceleration occurs along the field lines curved initially in the direction of the rotation. I argue that all necessary conditions for the efficient magnetocentrifugal acceleration of the plasma exist in the radio pulsar megnetosphere. We calculated the first correction of the rotational losses due to the generation of the MHD waves and analysed the plasma acceleration by these waves

Synthetic synchrotron emission maps from MHD models for the jet of M87

We present self-consistent global, steady-state MHD models and synthetic optically thin synchrotron emission maps for the jet of M87. The model consist of two distinct zones: an inner relativistic outflow, which we identify with the observed jet, and an outer cold disk-wind. While the former does not self-collimate efficiently due to its high effective inertia, the latter fulfills all the conditions for efficient collimation by the magneto-centrifugal mechanism. Given the right balance between the effective inertia of the inner flow and the collimation efficiency of the outer disk wind, the relativistic flow is magnetically confined into a well collimated beam and matches the measurements of the opening angle of M87 over several orders of magnitude in spatial extent. The synthetic synchrotron maps reproduce the morphological structure of the jet of M87, i.e. center-bright profiles near the core and limb-bright profiles away from the core. At the same time, they also show a local increase of brightness at some distance along the axis associated to a recollimation shock in the MHD model. Its location coincides with the position of the optical knot HST-1. In addition our best fitting model is consistent with a number of observational constraints such as the magnetic field in the knot HST-1, and the jet-to-counterjet brightness ratio.Comment: 9 pages, 9 figures, accepted by Ap