Blandford–Znajek process
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The Blandford–Znajek process is a mechanism for the extraction of
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
from a
rotating black hole A rotating black hole is a black hole that possesses angular momentum. In particular, it rotates about one of its axes of symmetry. All celestial objects – planets, stars (Sun), galaxies, black holes – spin. Types of black holes There a ...
, introduced by
Roger Blandford Roger David Blandford, FRS, FRAS (born 1949) is a British theoretical astrophysicist, best known for his work on black holes. Early life Blandford was born in Grantham, England and grew up in Birmingham. Career Blandford is famous in the ast ...
and
Roman Znajek Roman or Romans most often refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a letter ...
in 1977. This mechanism is the most preferred description of how
astrophysical jets An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets bec ...
are formed around spinning
supermassive black holes A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions of times the mass of the Sun (). Black holes are a class of astronomical obj ...
. This is one of the mechanisms that power
quasar A quasar is an extremely Luminosity, luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a m ...
s, or rapidly accreting supermassive black holes. 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 its ergosphere. 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 a
gamma-ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten milli ...
.


Physics of the mechanism

As in the
Penrose process The Penrose process (also called Penrose mechanism) is theorised by Sir Roger Penrose as a means whereby energy can be extracted from a rotating black hole. The process takes advantage of the ergosphere --- a region of spacetime around the black ho ...
, the
ergosphere file:Ergosphere_and_event_horizon_of_a_rotating_black_hole_(no_animation).gif, 300px, In the ergosphere (shown here in light gray), the component ''gtt'' is negative, i.e., acts like a purely spatial metric component. Consequently, timelike or ligh ...
plays an important role in the Blandford–Znajek process. In order to extract energy and
angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
from the black hole, the
electromagnetic field An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field produced by (stationary or moving) electric charges. It is the field described by classical electrodynamics (a classical field theory) and is the classical c ...
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 favored way to provide this is an e± pair cascade in a strong electric and radiation field.Camenzind, M.: "Compact objects in Astrophysics" (Springer 2007, ), p. 500, 505. 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 An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other f ...
with a strong
poloidal The terms toroidal and poloidal refer to directions relative to a torus of reference. They describe a three-dimensional coordinate system in which the poloidal direction follows a small circular ring around the surface, while the toroidal direct ...
magnetic field around a spinning black hole. The
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
extracts spin energy, and the power can be estimated as the energy density at the speed of light cylinder times area: : P = B^2\left(\frac\right)^4 r_c c = \frac, where ''B'' is the magnetic field strength, r_c is the
Schwarzschild radius The Schwarzschild radius or the gravitational radius is a physical parameter in the Schwarzschild solution to Einstein's field equations that corresponds to the radius defining the event horizon of a Schwarzschild black hole. It is a characteristic ...
, and ''ω'' is the
angular velocity In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an objec ...
.


See also

*
Penrose process The Penrose process (also called Penrose mechanism) is theorised by Sir Roger Penrose as a means whereby energy can be extracted from a rotating black hole. The process takes advantage of the ergosphere --- a region of spacetime around the black ho ...
, another mechanism to extract energy from a black hole *
Hawking radiation Hawking radiation is theoretical black body radiation that is theorized to be released outside a black hole's event horizon because of relativistic quantum effects. It is named after the physicist Stephen Hawking, who developed a theoretical arg ...
, another mechanism to extract mass, hence energy from a black hole *
Astrophysical jets An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets bec ...
, large structures seen around some quasars and created by the Blandford–Znajek process


References


External links

Physicists Identify the Engine Powering Black Hole Energy Beams
a
Quanta Magazine ''Quanta Magazine'' is an editorially independent online publication of the Simons Foundation covering developments in physics, mathematics, biology and computer science. ''Undark Magazine'' described ''Quanta Magazine'' as "highly regarded for ...
article on the SANE vs MAD scenarios of the Blandford–Znajek process {{DEFAULTSORT:Blandford-Znajek process Black holes *