Blandford–Znajek process

The Blandford–Znajek process is a mechanism for the extraction of energy from a rotating black hole,^[1] introduced by Roger Blandford and Roman Znajek in 1977.^[2] It is one of the best explanations for the way quasars are powered.^[3] As in the Penrose process, the ergosphere plays an important role in the Blandford–Znajek process. In order to extract energy and angular momentum from the black hole, the electromagnetic field around the hole must be modified by magnetospheric currents. In order to drive such currents, the electric field needs to not be screened, and consequently the vacuum field created within the ergosphere by distant sources must have an unscreened component. The most favoured way to provide this is an e^± pair cascade in a strong electric and radiation field.^[4] As the ergosphere causes the magnetosphere inside it to rotate, the outgoing flux of angular momentum results in extraction of energy from the black hole.

The Blandford–Znajek process requires an accretion disc with a strong poloidal magnetic field around a spinning black hole. The magnetic field extracts spin energy, and the power can be estimated as the energy density at the speed of light cylinder times area:

${\displaystyle P=B^{2}\left({\frac {r}{r_{c}}}\right)^{4}r_{c}c={\frac {B^{2}r^{4}\omega ^{2}}{c}},}$

where B is the magnetic field strength, $r_{c}$ is the speed-of-light radius, and ω is the angular velocity.^[5]

References

1. Frolov, Valeri P.; Zelnikov, Andrei (2011), Introduction to Black Hole Physics, Oxford: Oxford University Press, ISBN 978-0-19-969229-3, Zbl 1234.83001, (Chapter 8.9: Black Holes in External Magnetic Field).
2. R. D. Blandford and R. L. Znajek, "Electromagnetic extraction of energy from Kerr black holes", Mon. Not. R. Astr. Soc. 179:433-456 (1977).
3. http://everything2.com/title/Blandford%2520Znajek%2520process.
4. Camenzind, M.: "Compact objects in Astrophysics" (Springer 2007, ISBN 978-3-540-25770-7), p. 500, 505.
5. Relativistic Astrophysics and Cosmology Lecture Notes, A. Lasenby, Cambridge University, 2010–2011.