Jet launching and field advection in quasi-Keplerian discs

TL;DR

This paper reviews jet launching mechanisms in accretion discs, emphasizing the role of magnetic fields and disc structure, and discusses conditions for jet formation and their observational implications.

Contribution

It demonstrates that steady-state jet launching requires near-equipartition magnetic fields and explores how disc thickness influences jet power, also linking magnetic field variations to observed hysteresis cycles.

Findings

Steady jet launching needs strong magnetic fields close to equipartition.

Thick ADAF-like discs cannot produce super-Alfvénic jets.

Magnetic field variations may explain hysteresis in LMXBs.

Abstract

The fact that self-confined jets are observed around black holes, neutron stars and young forming stars points to a jet launching mechanism independent of the nature of the central object, namely the surrounding accretion disc. The properties of Jet Emitting Discs (JEDs) are briefly reviewed. It is argued that, within an alpha prescription for the turbulence (anomalous viscosity and diffusivity), the steady-state problem has been solved. Conditions for launching jets are very stringent and require a large scale magnetic field $B_z$ close to equipartition with the total (gas and radiation) pressure. The total power feeding the jets decreases with the disc thickness: fat ADAF-like structures with $h\sim r$ cannot drive super-Alfv\'enic jets. However, there exist also hot, optically thin JED solutions that would be observationally very similar to ADAFs. Finally, it is argued that variations in the large scale magnetic $B_z$ field is the second parameter required to explain hysteresis cycles seen in LMXBs (the first one would be $\dot M_a$).