A black hole spin-flip occurs when the
spin axis
Rotation around a fixed axis is a special case of rotational motion. The fixed-axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's rota ...
of 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 ...
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 mergers, when two
supermassive black holes form a bound pair at the center of the merged
galaxy
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
and coalesce after emitting
gravitational waves
Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that Wave propagation, propagate as waves outward from their source at the speed of light. They were first proposed by Oliv ...
. Spin-flips are significant astrophysically since a number of physical processes are associated with black hole spins; for instance,
jets in
active galaxies
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not pr ...
are believed to be launched parallel to the spin axes of supermassive black holes.
A change in the rotation axis of a black hole due to a spin-flip would therefore result in a change in the direction of the jet.
Physics of spin-flips
A spin-flip is a late stage in the evolution of a
binary black hole
A binary black hole (BBH) is a system consisting of two black holes in close orbit around each other. Like black holes themselves, binary black holes are often divided into stellar binary black holes, formed either as remnants of high-mass binary ...
. The binary consists of two black holes, with masses
and
, that revolve around their common
center of mass
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
. The total
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 ...
of the binary system is the sum of the angular momentum of the
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
,
, plus the
spin angular momenta
Spin is a conserved quantity carried by elementary particles, and thus by composite particles (hadrons) and atomic nuclei.
Spin is one of two types of angular momentum in quantum mechanics, the other being ''orbital angular momentum''. The orbita ...
of the two holes. If we write
as the masses of each hole and
as their
Kerr parameters
Kerr may refer to:
People
*Kerr (surname)
*Kerr (given name) Places
;United States
*Kerr Township, Champaign County, Illinois
*Kerr, Montana, A US census-designated place
*Kerr, Ohio, an unincorporated community
* Kerr County, Texas
Other uses ...
, then use the angle from north of their spin axes as given by
, we can write,
If the orbital separation is sufficiently small, emission of energy and angular momentum in the form of
gravitational radiation
Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
will cause the orbital separation to drop. Eventually, the smaller hole
reaches the innermost stable circular orbit, or ISCO, 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
The Kerr metric or Kerr geometry describes the geometry of empty spacetime around a rotating uncharged axially symmetric black hole with a quasispherical event horizon. The Kerr metric is an exact solution of the Einstein field equations of ge ...
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, 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.
See also
*
*
*
References
{{Reflist
External links
Massive black hole binary evolution
An article on binary black holes.
Scientists Detect "Smoking Gun" of Colliding Black Holes
Supermassive black holes
Black holes