Spin-flip - AbsoluteAstronomy.com

A black hole spin-flip occurs when the spin

Rotation

A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

axis of a rotating black hole

Rotating black hole

A rotating black hole is a black hole that possesses spin angular momentum.-Types of black holes:There are four known, exact, black hole solutions to Einstein's equations, which describe gravity in General Relativity. Two of these rotate...

undergoes a sudden change in orientation due to absorption of a second (smaller) black hole.
Spin-flips are believed to be a consequence of galaxy merger

Galaxy merger

Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies...

s, when two supermassive black holes form a bound pair at the center of the merged galaxy

Galaxy

A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

and coalesce after emitting gravitational waves.
Spin-flips are significant astrophysically since a number of physical processes are associated with black hole spins; for instance, jets

Relativistic jet

Relativistic jets are extremely powerful jets of plasma which emerge from presumed massive objects at the centers of some active galaxies, notably radio galaxies and quasars. Their lengths can reach several thousand or even hundreds of thousands of light years...

in active galaxies are believed to be launched parallel to the spin axes of supermassive black holes.

Physics of Spin-Flips

A spin-flip is a late stage in the evolution of a binary black hole. The binary consists of two black holes, with masses

and

, that revolve around their common center of mass

Center of mass

In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...

. The total angular momentum

Angular momentum

In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

of the binary system is the sum of the angular momentum of the orbit

Orbit

In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

, plus the spin angular momenta

of the two holes:

If the orbital separation is sufficiently small, emission of energy and angular momentum in the form of gravitational radiation will cause the orbital separation to drop. Eventually, the smaller hole

reaches the innermost stable circular orbit, or ISCO

ISCO

ISCO may refer to:*International Standard Classification of Occupations, an International Labour Organization classification structure*Information Sharing Customer Outreach, a directorate within the office of the Chief Information Officer under the Office of the Director of National Intelligence...

, around the larger hole. Once the ISCO is reached, there no longer exists a stable orbit, and the smaller hole plunges into the larger hole, coalescing with it. The final angular momentum after coalescence is just

the spin angular momentum of the single, coalesced hole. Neglecting the angular momentum that is carried away by gravitational waves during the final plunge—which is small -- conservation of angular momentum implies

is of order

times

and can be ignored if

is much smaller than

. Making this approximation,

This equation states that the final spin of the hole is the sum of the larger hole's initial spin plus the orbital angular momentum of the smaller hole at the last stable orbit. Since the vectors

and

are generically oriented in different directions,

will point in a different direction than

-- a spin-flip.

The angle by which the black hole's spin re-orients itself depends on the relative size of

and

, and on the angle between them. At one extreme, if

is very small, the final spin will be dominated by

and the flip angle can be large. At the other extreme, the larger black hole might be a maximally-rotating Kerr black hole initially. Its spin angular momentum is then of order

The orbital angular momentum of the smaller hole at the ISCO depends on the direction of its orbit, but is of order

Comparing these two expressions, it follows that even a fairly small hole, with mass about one-fifth that of the larger hole, can reorient the larger hole by 90 degrees or more.

Connection with radio galaxies

Black hole spin-flips were first discussed in the context of a particular class of radio galaxy

Radio galaxy

Radio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy that are very luminous at radio wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process...

, the X-shaped radio sources. The X-shaped galaxies exhibit two, misaligned pairs of radio lobes: the "active" lobes and the "wings". It is believed that the wings are oriented in the direction of the jet prior to the spin-flip, and that the active lobes point in the current jet direction. The spin-flip could have been caused by absorption of a second black hole during a galaxy merger

Galaxy merger

External links

Massive black hole binary evolution An article on binary black holes.
Scientists Detect "Smoking Gun" of Colliding Black Holes

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Physics of Spin-Flips

Connection with radio galaxies

See also

External links