The collimation of magnetic jets by disk winds

TL;DR

This paper presents a semi-analytic model demonstrating how disk winds can effectively collimate magnetic jets, with implications for understanding jet structures like that of M87.

Contribution

It introduces a new semi-analytic approach to model jet collimation by disk winds, exploring a wide parameter space and applying it to real astrophysical observations.

Findings

Jet collimation occurs when wind power exceeds about 10% of jet power.

Gradual collimation for moderate winds; rapid focusing for strong winds.

Model reproduces the parabolic shape of M87's jet within the Bondi radius.

Abstract

The collimation of a Poynting-flux dominated jet by a wind emanating from the surface of an accretion flow is computed using a semi-analytic model. The injection of the disk wind is treated as a boundary condition in the equatorial plane, and its evolution is followed by invoking a prescribed geometry of streamlines. Solutions are obtained for a wide range of disk wind parameters. It is found that jet collimation generally occurs when the total wind power exceeds about ten percents of the jet power. For moderate wind powers we find gradual collimation. For strong winds we find rapid collimation followed by focusing of the jet, after which it remains narrow over many Alfv\'en crossing times before becoming conical. We estimate that in the later case the jet's magnetic field may be dissipated by the current-driven kink instability over a distance of a few hundreds gravitational radii. We apply the model to M87 and show that the observed parabolic shape of the radio jet within the Bondi radius can be reproduced provided that the wind injection zone extends to several hundreds gravitational radii, and that its total power is about one third of the jet power. The radio spectrum can be produced by synchrotron radiation of relativistically hot, thermal electrons in the sheath flow surrounding the inner jet.