Background: BH Jet Formation

Although semi-relativistic winds can be launching along the large-scale field lines threading the accretion disk (by the help of magneto-centrifugal force) [1][2] and powered by rotational energy of the accretion disk, relativistic jets are believed to be powered by the central black holes via the loss of the rotational energy of the black hole.

A spinning black hole stores its rotational energy in the ergosphere, inside which everything should corotate in the same direction that the spin of the black hole has because the effect of rotating spacetime is important there. Large-scale, hole-threading field lines play the major role in extracting the spin energy of black holes. Magnetic fields inside the ergosphere are distorted by the rotation of spacetime as well. Such distortion will cause magnetic tension, and magnetic fields can therefore propagate the perturbation outward (outside the ergosphere) by using the magnetic tension as the restoring force (i.e., in the form of Alfven waves). The extracted energy is used to power the relativistic jet, by finally converted most of their energy into the particle kinetic energy.

If the environment near the black hole horizon is nearly vacuum, the electromagnetic extraction is well described by the “Blandford-Znajek process” [3]. Instead, if  the environment near the black hole horizon is filled of plasma, the magnetohydrodynamic (MHD) theory should be considered to include the additional plasma, which can load on the hole-threading field lines, contribution. The extraction of rotational energy from black holes by MHD flows are described by the “MHD Penrose Process” [4]. When the plasma loading is negligible, the Blandford-Znajek process becomes a good approximation of the MHD Penrose process.

[1] Blandford, R. D., & Payne, D. G. 1982, MNRAS, 199, 883

[2] Sadowski, A., & Sikora, M. 2010, A&A, 517, A18

[3] Blandford, R. D., & Zna jek, R. L. 1977, MNRAS, 179, 433

[4] Takahashi, M., Nitta, S., Tatematsu, Y., & Tomimatsu, A. 1990, ApJ, 363, 206