Blandford–Znajek process

The Blandford–Znajek process, proposed in 1977 by Roger Blandford and Roman Znajek, describes how energy can be drawn from a rotating black hole. It is the leading explanation for the formation of astrophysical jets around spinning supermassive black holes and is one of the mechanisms that energize quasars—supermassive black holes that are rapidly accreting matter. Studies have shown that the power output from the accretion disk is much greater than that extracted directly from the black hole’s ergosphere, so the presence of a poloidal magnetic field around the hole does not significantly affect the total power output. The process has also been suggested to act as the central engine for gamma-ray bursts.

The ergosphere is key to the Blandford–Znajek process, similar to its role in the Penrose process. To extract energy and angular momentum from the black hole, magnetospheric currents must alter the electromagnetic field around it. These currents require an unscreened electric field, which is most likely provided by an electron-positron pair cascade in a strong electric and radiation field. Because the ergosphere forces the magnetosphere inside it to rotate, the outgoing flux of angular momentum leads to energy extraction from the black hole. The process requires an accretion disk 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 its area, given by \( P = B^2 (r / r_c)^4 r_c c = B^2 r^4 \omega^2 / c \), where \( B \) is the magnetic field strength, \( r_c \) is the Schwarzschild radius, and \( \omega \) is the angular velocity.