On the collimation properties of jets with finite Poynting flux launched from Keplerian accretion discs

TL;DR

This study uses axisymmetric MHD simulations to explore how jets launched from magnetized Keplerian accretion discs are naturally collimated and form steady recollimation shocks, revealing observable features and the influence of external pressure.

Contribution

It demonstrates that steady recollimation shocks are a generic feature of jets from Keplerian discs and investigates their connection to external pressure and jet acceleration.

Findings

Jets exhibit steady recollimation shocks at large distances.

External pressure influences jet shape and acceleration.

Recollimation shocks may produce observable emission knots.

Abstract

It is generally accepted that the launching of astrophysical jets requires a large-scale magnetic field threading a central object (black hole or star) and/or its surrounding accretion disc. However, the collimation mechanism far away from the central object has not yet been fully understood. In a previous work we investigated a mechanism in which the jet is self-collimated due to a dominant hoop stress. We ran numerical simulations in which a Jet-Emitting disc (JED) spans the entire lower computational boundary. Those were the first of their kind to showcase the steady recollimation shocks predicted by steady-state analytical studies of jets. However, the huge size of the JED prevented a complete study of the connection between the accelerating and asymptotic electric circuits, as well as the influence of the outer medium. We performed a set of axisymmetric ideal MagnetoHydroDynamics (MHD) non-relativistic jet simulations. In those, only the innermost region of the accretion disc is a jet-launching zone. The jets of finite radial extent in those simulations also produce steady recollimation shocks at large distances from the central object. Standing recollimation shocks are not a bias of self-similarity, but a generic feature of jets emitted from magnetized Keplerian accretion discs. They may produce observable features, such as a standing emission knots, a decrease of the rotation rate or a change in polarisation. We also recover previous results on the influence of external pressure on jet confinement, such as the relation between pressure profile and jet shape, and jet acceleration efficiency.