Astronomy and Astrophysics – Astrophysics – High Energy Astrophysical Phenomena
Scientific paper
2011-01-12
Astronomy and Astrophysics
Astrophysics
High Energy Astrophysical Phenomena
4 pages, 1 figure, proceedings of IAU 275 "Jets at all scales" (Gustavo E. Romero, Rashid A. Sunyaev and Tomaso Belloni, eds)
Scientific paper
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 $\dot M_a$).
Ferreira Jonathan
Petrucci Pierre Olivier
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