Indexed on: 01 Jan '98Published on: 01 Jan '98Published in: Astrophysics and Space Science
There is now little doubt that the nuclei of most galaxies contain massive black holes with 106M⊙< M < 1010M⊙and that these are ultimately responsible for most nuclear activity when they are supplied with gas. Parallel observations of Galactic X-ray sources are providing small scale counterparts that have similar properties except that they vary far more rapidly. As we can now determine the masses of these black holes in several instances, we can ask much more quantitative questions about the mode of accretion and the origin and collimation of outflows. In this contribution, I will emphasize the importance of magnetic field for extracting the angular momentum from the flow and explain how the radiative efficiency is controlled by the mass accretion rate and the actual properties of magnetized, plasma.