TheBlandford–Znajek process is a mechanism for the extraction ofenergy from arotating black hole,^[1][2] introduced byRoger Blandford andRoman Znajek in 1977.^[3] This mechanism is the most preferred description of howastrophysical jets are formed around spinningsupermassive black holes. This is one of the mechanisms that powerquasars, or rapidly accreting supermassive black holes.^[4] Generally speaking, it was demonstrated that the power output of the accretion disk is significantly larger than the power output extracted directly from the hole, through itsergosphere.^[5][6] Hence, the presence (or not) of a poloidal magnetic field around the black hole is not determinant in its overall power output. It was also suggested that the mechanism plays a crucial role as a central engine for agamma-ray burst.^[7]

As in thePenrose process, theergosphere plays an important role in the Blandford–Znajek process. In order to extract energy andangular momentum from the black hole, theelectromagnetic field around the hole must be modified bymagnetospheric 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 favored way to provide this is an e^± pair cascade in a strong electric and radiation field.^[8] 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 anaccretion disc with a strongpoloidal magnetic field around a spinning black hole. Themagnetic field extracts spin energy, and the power can be estimated as the energy density at the speed of light cylinder times area:

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

whereB is the magnetic field strength,${\displaystyle r_c}$ is theSchwarzschild radius, andω is theangular velocity.^[4]

• Penrose process, another mechanism to extract energy from a black hole
• Hawking radiation, another mechanism to extract mass, hence energy from a black hole
• Astrophysical jets, large structures seen around some quasars and created by the Blandford–Znajek process and/or the Penrose process

Physicists Identify the Engine Powering Black Hole Energy Beams aQuanta Magazine article on the SANE vs MAD scenarios of the Blandford–Znajek process

• Black holes
• Quasars

• Articles with short description
• Short description is different from Wikidata