S. Bogovalov

On the magnetic acceleration and collimation of astrophysical outflows

ABSTRA C T The axisymmetric 3D MHD outflow of cold plasma from a magnetized and rotating astrophysical object is numerically simulated with the purpose of investigating the magnetocentrifugal acceleration and eventual collimation of the outflow. 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 a expressing the ratio of the corotating speed at the Alfven distance to the initial flow speed along the initial monopole-like magnetic field lines. Several angular velocity laws have been used for relativistic and non-relativistic outflows. The acceleration of the flow is most effective at the equatorial plane and the terminal flow speed depends linearly on a. Significant flow collimation is found in non-relativistic efficient magnetic rotators corresponding to relatively large values of a * 1 while very weak collimation occurs in inefficient magnetic rotators with smaller values of a < 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 a while the density in the jet grows linearly with a .F ora * 5 the magnitude of the flow in the jet remains below the fast MHD wave speed everywhere. In relativistic outflows, no collima- tion is found in the supersonic region for parameters typical for radio pulsars. All the above results verify the main conclusions of general theoretical studies on the magnetic accel- eration and collimation of outflows from magnetic rotators and extend previous numerical simulations to large stellar distances.

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 consists 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 magnetocentrifugal 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 magnitudes 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 with 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.

Magnetically collimated jets with high mass flux

ABSTRA C T Recent numerical simulations and analytical models of magnetically collimated plasma outflows from a uniformly rotating central gravitating object and/or a Keplerian accretion disc have shown that relatively low mass and magnetic fluxes reside in the produced jet. Observations, however, indicate that in some cases, as in jets of young stellar objects, the collimated outflow carries higher fluxes than these simulations predict. A solution to this problem is proposed here by assuming that jets with high mass flux originate in a central source which produces a non-collimated outflow provided that this source is surrounded by a rapidly rotating accretion disc. The relatively faster rotating disc produces a collimated wind, which then forces all the enclosed outflow from the central source to be collimated too. This conclusion is confirmed by self-consistent numerical solutions of the full set of the magnetohydrodynamic equations.

MHD MODELS AND SYNTHETIC SYNCHROTRON MAPS FOR THE JET OF M87

We present a self-consistent MHD model 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 for a wide range of parameters and matches the measurements of the opening angle of M87 over several orders of magnitude in spatial extent. In the second part of this work, we present synthetic synchrotron emission maps for our MHD models. In principle, the two-zone model can reproduce the morphological structure seen in radio observations, as central-peaked profiles across the jet close the the source, limb-bright further down the jet, and a bright knot close to the position of HST-1. However it is difficult to reconcile all features with a single set of parameters.

Magnetic collimation of the relativistic jet in M87

The slow collimation of the relativistic jet of M87 which collimates from a wide opening angle of about 60° at sub parscec scales to a smaller angle of about 10° at the pc scale is modelled by means of a two‐component MHD outflow consisting of a relativistic wind‐type outflow from a central source and a non relativistic wind from a surrounding disk. We employ a numerical code for the direct numerical solution of the steady state problem in the zone of super fast magnetized flow and show that in this two‐component model it is possible to easily collimate into a cylindrical jet all the mass and magnetic fluxes which are available from the central source. This magnetic collimation occurs over the scales suggested by observations of the M87 jet after the jet undergoes several oscillations in its width, as also found by earlier analytical solutions. A layer of increased density surrounds the collimated jet, a result consistent with an emissivity which is observed to be much higher in a thin boundary layer than elsewhere in the jet.

Magnetic collimation of relativistic jets

Abstract We briefly outline a scenario for the magnetic collimation of outflows from AGN and other astrophysical compact objects, based on recent numerical simulations of axisymmetric 3-D MHD outflows from the central parts of these objects. It follows from our results that magnetic self-collimation of a relativistic plasma is negligibly small if all the outflow is relativistic. However, a tightly collimated inner relativistic jet can be indirectly formed by an exterior nonrelativistic but collimated disk-wind.