The Type Ib supernova SN 2008D in galaxy NGC 2770, shown in X-ray (left) and visible light (right), at the corresponding positions of the images. (NASA image.)
Type Ib and Type Ic supernovae are categories of supernovae that are caused by the stellar corecollapse of massive stars. These stars have shed or been stripped of their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line
of silicon. Compared to Type Ib, Type Ic supernovae are hypothesized to
have lost more of their initial envelope, including most of their
helium. The two types are usually referred to as stripped core-collapse supernovae.
Spectra
When a supernova is observed, it can be categorized in the Minkowski–Zwicky supernova classification scheme based upon the absorption lines that appear in its spectrum. A supernova is first categorized as either a Type I or Type II, then subcategorized based on more specific traits. Supernovae belonging to the general category Type I lack hydrogen
lines in their spectra; in contrast to Type II supernovae which do
display lines of hydrogen. The Type I category is subdivided into
Type Ia, Type Ib and Type Ic.
Type Ib/Ic supernovae are distinguished from Type Ia by the lack of an absorption line of singly ionized silicon at a wavelength of 635.5 nanometres. As Type Ib and Ic supernovae age, they also display lines from elements such as oxygen, calcium and magnesium. In contrast, Type Ia spectra become dominated by lines of iron. Type Ic supernovae are distinguished from Type Ib in that the former also lack lines of helium at 587.6 nm.
Formation
The onion-like layers of an evolved, massive star (not to scale).
Prior to becoming a supernova, an evolved massive star is organized
in the manner of an onion, with layers of different elements undergoing
fusion. The outermost layer consists of hydrogen, followed by helium,
carbon, oxygen, and so forth. Thus when the outer envelope of hydrogen
is shed, this exposes the next layer that consists primarily of helium
(mixed with other elements). This can occur when a very hot, massive
star reaches a point in its evolution when significant mass loss is
occurring from its stellar wind. Highly massive stars (with 25 or more
times the mass of the Sun) can lose up to 10−5solar masses (M☉) each year—the equivalent of 1 M☉ every 100,000 years.
Type Ib and Ic supernovae are hypothesized to have been produced
by core collapse of massive stars that have lost their outer layer of
hydrogen and helium, either via winds or mass transfer to a companion. The progenitors of Types Ib and Ic have lost most of their outer envelopes due to strong stellar winds or else from interaction with a close companion of about 3–4 M☉. Rapid mass loss can occur in the case of a Wolf–Rayet star,
and these massive objects show a spectrum that is lacking in hydrogen.
Type Ib progenitors have ejected most of the hydrogen in their outer
atmospheres, while Type Ic progenitors have lost both the hydrogen and
helium shells; in other words, Type Ic have lost more of their envelope
(i.e., much of the helium layer) than the progenitors of Type Ib.
In other respects, however, the underlying mechanism behind Type Ib and
Ic supernovae is similar to that of a Type II supernova, thus placing
Types Ib and Ic between Type Ia and Type II. Because of their similarity, Type Ib and Ic supernovae are sometimes collectively called Type Ibc supernovae.
There is some evidence that a small fraction of the Type Ic supernovae may be the progenitors of gamma ray bursts
(GRBs); in particular, type Ic supernovae that have broad spectral
lines corresponding to high-velocity outflows are thought to be strongly
associated with GRBs. However, it is also hypothesized that any
hydrogen-stripped Type Ib or Ic supernova could be a GRB, dependent upon
the geometry of the explosion.
In any case, astronomers believe that most Type Ib, and probably
Type Ic as well, result from core collapse in stripped, massive stars,
rather than from the thermonuclear runaway of white dwarfs.
As they are formed from rare, very massive stars, the rate of
Type Ib and Ic supernovae occurrence is much lower than the
corresponding rate for Type II supernovae. They normally occur in regions of new star formation, and are extremely rare in elliptical galaxies.
Because they share a similar operating mechanism, Type Ibc and the
various Type II supernovae are collectively called core-collapse
supernovae. In particular, Type Ibc may be referred to as stripped core-collapse supernovae.
Light curves
The light curves
(a plot of luminosity versus time) of Type Ib supernovae vary in form,
but in some cases can be nearly identical to those of Type Ia
supernovae. However, Type Ib light curves may peak at lower luminosity
and may be redder. In the infrared portion of the spectrum, the light curve of a Type Ib supernova is similar to a Type II-L light curve. Type Ib supernovae usually have slower decline rates for the spectral curves than Ic.
Type Ia supernovae light curves are useful for measuring distances on a cosmological scale. That is, they serve as standard candles.
However, due to the similarity of the spectra of Type Ib and Ic
supernovae, the latter can form a source of contamination of supernova
surveys and must be carefully removed from the observed samples before
making distance estimates.
ESO image of hypernova SN 1998bw in a spiral arm of galaxy ESO 184-G82
A hypernova is a type of stellar explosion which ejects material with an unusually high kinetic energy, an order of magnitude higher than most supernovae. They usually appear similar to a type Ic supernova,
but with unusually broad spectral lines indicating an extremely high
expansion velocity. Hypernovae are one of the mechanisms for producing
long gamma ray bursts (GRBs), which range from 2 seconds to over a minute in duration.
History
In the 1980s, the term hypernova was used to describe a theoretical type of supernova now known as a pair-instability supernova. It referred to the extremely high energy of the explosion compared to typical core collapse supernovae. The term had previously been used to describe hypothetical explosions from diverse events such as hyperstars, extremely massive population III stars in the early universe, or from events such as black hole mergers.
GRBs were initially detected on July 2, 1967 by US military
satellites in high orbit, which were meant to detect gamma radiation.
The US had suspected the USSR of conducting secret nuclear tests despite signing the Nuclear Test Ban Treaty of 1963, and the Vela satellites were capable of detecting explosions behind the moon.
The satellites detected a signal, but it was unlike that of a nuclear
weapon signature, nor could it be correlated to solar flares.
Over the next few decades, the GRBs posed a compelling mystery. Gamma
rays require highly energetic events to be produced, yet GRBs could not
be correlated to supernovae, solar flares, or any other activity in the
sky. Their brevity made them difficult to trace. Once their direction
could be determined, it was found that they were evenly spread across
the sky. Thus they were not originating in the Milky Way or nearby
galaxies, but from deep space.
In February 1997, Dutch-Italian satellite BeppoSAX was able to trace GRB 970508 to a faint galaxy roughly 6 billion light years away.
From analyzing the spectroscopic data for both the GRB 970508 and its
host galaxy, Bloom et al. concluded in 1998 that a hypernova was the
likely cause.[7] That same year, hypernovae were hypothesized in greater detail by Polish astronomer Bohdan Paczyński as supernovae from rapidly-spinning stars.
The usage of the term hypernova from the late 20th century has since been refined to refer to those supernovae with unusually large kinetic energy. The first hypernova observed was SN 1998bw, with a luminosity 100 times higher than a standard Type Ib.
This supernova was the first to be associated with a gamma-ray burst
(GRB) and it produced a shockwave containing an order of magnitude more
energy than a normal supernova. Other scientists prefer to call these
objects simply broad-lined type Ic supernovae. Since then the term has been applied to a variety of objects, not all of them meeting the standard definition, for example ASASSN-15lh.
Properties
Hypernovae are now widely accepted to be supernovae with ejecta having a kinetic energy larger than about 1052erg,
an order of magnitude higher than a typical core collapse supernova.
The ejected nickel masses are large and the ejection velocity up to 99%
of the speed of light.
These are typically of type Ic, and some are associated with
long-duration gamma-ray bursts. The electromagnetic energy released by
these events varies from comparable to other type Ic supernova, to some
of the most luminous supernovae known such as SN 1999as.
The archetypal hypernova SN 1998bw was associated with GRB 980425.
Its spectrum showed no hydrogen and no clear helium features, but
strong silicon lines identified it as a type Ic supernova. The main absorption lines were extremely broadened and the light curve showed a very rapid brightening phase, reaching the brightness of a type Ia supernova at day 16. The total ejected mass was about 10 M☉ and the mass of nickel ejected about 0.4 M☉. All supernovae associated with GRBs have shown the high-energy ejecta that characterises them as hypernovae.
Unusually bright radio supernovae have been observed as counterparts to hypernovae, and have been termed radio hypernovae.
Astrophysical models
Models for hypernova focus on the efficient transfer of energy into the ejecta. In normal core collapse supernovae,
99% of neutrinos generated in the collapsing core escape without
driving the ejection of material. It is thought that rotation of the
supernova progenitor drives a jet that accelerates material away from
the explosion at close to the speed of light. Binary systems are
increasingly being studied as the best method both for stripped of
stellar envelopes to leave a bare carbon-oxygen core, and for inducing
the necessary spin conditions to drive a hypernova.
Collapsar model
The collapsar model describes a type of supernova that produces a gravitationally collapsed object, or black hole. The word "collapsar", short for "collapsed star", was formerly used to refer to the end product of stellar gravitational collapse, a stellar-mass black hole.
The word is now sometimes used to refer to a specific model for the
collapse of a fast-rotating star. When core collapse occurs in a star
with a core at least around fifteen times the sun's mass (M☉)—though
chemical composition and rotational rate are also significant—the
explosion energy is insufficient to expel the outer layers of the star,
and it will collapse into a black hole without producing a visible
supernova outburst.
A star with a core mass slightly below this level—in the range of 5–15 M☉—will
undergo a supernova explosion, but so much of the ejected mass falls
back onto the core remnant that it still collapses into a black hole. If
such a star is rotating slowly, then it will produce a faint supernova,
but if the star is rotating quickly enough, then the fallback to the
black hole will produce relativistic jets.
The energy that these jets transfer into the ejected shell renders the
visible outburst substantially more luminous than a standard supernova.
The jets also beam high energy particles and gamma rays directly outward
and thereby produce x-ray or gamma-ray bursts; the jets can last for
several seconds or longer and correspond to long-duration gamma-ray
bursts, but they do not appear to explain short-duration gamma-ray
bursts.
Binary models
The
mechanism for producing the stripped progenitor, a carbon-oxygen star
lacking any significant hydrogen or helium, of type Ic supernovae was
once thought to be an extremely evolved massive star, for example a type
WO Wolf-Rayet star whose dense stellar wind
expelled all its outer layers. Observations have failed to detect any
such progenitors. It is still not conclusively shown that the
progenitors are actually a different type of object, but several cases
suggest that lower-mass "helium giants" are the progenitors. These
stars are not sufficiently massive to expel their envelopes simply by
stellar winds, and they would be stripped by mass transfer to a binary
companion. Helium giants are increasingly favoured as the progenitors
of type Ib supernovae, but the progenitors of type Ic supernovae is
still uncertain.
One proposed mechanism for producing gamma-ray bursts is induced gravitational collapse, where a neutron star is triggered to collapse into a black hole
by the core collapse of a close companion consisting of a stripped
carbon-oxygen core. The induced neutron star collapse allows for the
formation of jets and high-energy ejecta that have been difficult to
model from a single star.
Gravitational collapse of a massive star, resulting in a Type II supernova
Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity.
Gravitational collapse is a fundamental mechanism for structure
formation in the universe. Over time an initial, relatively smooth
distribution of matter will collapse to form pockets of higher density, typically creating a hierarchy of condensed structures such as clusters of galaxies, stellar groups, stars and planets.
A star is born through the gradual gravitational collapse of a cloud of interstellar matter. The compression caused by the collapse raises the temperature until thermonuclear fusion occurs at the center of the star, at which point the collapse gradually comes to a halt as the outward thermal pressure balances the gravitational forces. The star then exists in a state of dynamic equilibrium. Once all its energy sources are exhausted, a star will again collapse until it reaches a new equilibrium state.
Star formation
An interstellar cloud of gas will remain in hydrostatic equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force. Mathematically this is expressed using the virial theorem, which states that, to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy.
If a pocket of gas is massive enough that the gas pressure is
insufficient to support it, the cloud will undergo gravitational
collapse. The mass above which a cloud will undergo such collapse is
called the Jeans mass. This mass depends on the temperature and density of the cloud, but is typically thousands to tens of thousands of solar masses.
Stellar remnants
NGC 6745 produces material densities sufficiently extreme to trigger star formation through gravitational collapse
At what is called the death of the star (when a star has burned out
its fuel supply), it will undergo a contraction that can be halted only
if it reaches a new state of equilibrium. Depending on the mass during
its lifetime, these stellar remnants can take one of three forms:
Black hole, in which there is no force strong enough to resist gravitational collapse
White dwarf
The collapse of the stellar core to a white dwarf takes place over
tens of thousands of years, while the star blows off its outer envelope
to form a planetary nebula. If it has a companion star, a white dwarf-sized object can accrete matter from the companion star. Before it reaches the Chandrasekhar limit
(about one and a half times the mass of our Sun, at which point
gravitational collapse would start again), the increasing density and
temperature within a carbon-oxygen white dwarf initiates a new round of
nuclear fusion, which is not regulated because the star's weight is
supported by degeneracy rather than thermal pressure, allowing
temperature to rise exponentially. The resulting runawaycarbon detonation completely blows the star apart in a Type Ia supernova.
Neutron star
Neutron stars are formed by gravitational collapse of the cores of larger stars, and are the remnant of other types of supernova.
They are so compact that a Newtonian description is inadequate for an
accurate treatment, which requires the use of Einstein's general
relativity.
Black holes
Logarithmic
plot of mass against mean density (with solar values as origin) showing
possible kinds of stellar equilibrium state. For a configuration in the
shaded region, beyond the black hole limit line, no equilibrium is
possible, so runaway collapse will be inevitable.
According to Einstein's theory, for even larger stars, above the Landau-Oppenheimer-Volkoff limit, also known as the Tolman–Oppenheimer–Volkoff limit
(roughly double the mass of our Sun) no known form of cold matter can
provide the force needed to oppose gravity in a new dynamical
equilibrium. Hence, the collapse continues with nothing to stop it.
Simulated view from outside black hole with thin accretion disc, by J. A. Marck
Once a body collapses to within its Schwarzschild radius it forms what is called a black hole, meaning a space-time region from which not even light can escape. It follows from general relativity and the theorem of Roger Penrose that the subsequent formation of some kind of singularity is inevitable. Nevertheless, according to Penrose's cosmic censorship hypothesis, the singularity will be confined within the event horizon bounding the black hole, so the space-time region outside will still have a well behaved geometry, with strong but finite curvature, that is expected to evolve towards a rather simple form describable by the historic Schwarzschild metric in the spherical limit and by the more recently discovered Kerr metric if angular momentum is present.
On the other hand, the nature of the kind of singularity to be
expected inside a black hole remains rather controversial. According to
theories based on quantum mechanics, at a later stage, the collapsing object will reach the maximum possible energy density for a certain volume of space or the Planck density
(as there is nothing that can stop it). This is the point at which it
has been hypothesized that the known laws of gravity cease to be valid. There are competing theories as to what occurs at this point. For example loop quantum gravity predicts that a Planck star
would form. Regardless, it is argued that gravitational collapse
ceases at that stage and a singularity, therefore, does not form.
Theoretical minimum radius for a star
The
radii of larger mass neutron stars (about 2.0 solar mass) are estimated
to be about 12-km, or approximately 2.0 times their equivalent
Schwarzschild radius.
It might be thought that a sufficiently massive neutron star
could exist within its Schwarzschild radius (1.0 SR) and appear like a
black hole without having all the mass compressed to a singularity at
the center; however, this is probably incorrect. Within the event horizon,
matter would have to move outward faster than the speed of light in
order to remain stable and avoid collapsing to the center. No physical
force therefore can prevent a star smaller than 1.0 SR from collapsing
to a singularity (at least within the currently accepted framework of general relativity;
this does not hold for the Einstein–Yang–Mills–Dirac system). A model
for nonspherical collapse in general relativity with emission of matter
and gravitational waves has been presented.
The following outline is provided as an overview of and topical guide to black holes:
Black hole – mathematically defined region of spacetime
exhibiting such a strong gravitational pull that no particle or
electromagnetic radiation can escape from inside it. The theory of general relativity
predicts that a sufficiently compact mass can deform spacetime to form a
black hole. The boundary of the region from which no escape is possible
is called the event horizon.
Although crossing the event horizon has enormous effect on the fate of
the object crossing it, it appears to have no locally detectable
features. In many ways a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation,
with the same spectrum as a black body of a temperature inversely
proportional to its mass. This temperature is on the order of billionths
of a kelvin for black holes of stellar mass, making it essentially
impossible to observe.
What type of thing is a black hole?
A black hole can be described as all of the following:
Schwarzschild metric
– In Einstein's theory of general relativity, the Schwarzschild
solution, named after Karl Schwarzschild, describes the gravitational
field outside a spherical, uncharged, non-rotating mass such as a star,
planet, or black hole.
Virtual black hole – black hole that exists temporarily as a result of a quantum fluctuation of spacetime.
Types of black holes, by size
Micro black hole
– predicted as tiny black holes, also called quantum mechanical
black holes or mini black holes, for which quantum mechanical effects
play an important role.
Extremal black hole – black hole with the minimal possible mass that can be compatible with a given charge and angular momentum.
Black hole electron
– if there were a black hole with the same mass and charge as an
electron, it would share many of the properties of the electron
including the magnetic moment and Compton wavelength.
Stellar black hole – black hole formed by the gravitational collapse of a massive star. They have masses ranging from about 3 to several tens of solar masses.
Intermediate-mass black hole – black hole whose mass is significantly more than stellar black holes yet far less than supermassive black holes .
Supermassive black hole – largest type of black hole in a galaxy, on the order of hundreds of thousands to billions of solar masses.
Quasar – very energetic and distant active galactic nucleus.
Active galactic nucleus
– compact region at the centre of a galaxy that has a much higher than
normal luminosity over at least some portion, and possibly all, of the
electromagnetic spectrum.
Blazar – very compact quasar associated with a presumed supermassive black hole at the center of an active, giant elliptical galaxy.
Specific black holes
List of black holes
– incomplete list of black holes organized by size; some items in this
list are galaxies or star clusters that are believed to be organized
around a black hole.
Black hole exploration
Rossi X-ray Timing Explorer – satellite that observes the time structure of astronomical X-ray sources, named after Bruno Rossi.
Formation of black holes
Stellar evolution – process by which a star undergoes a sequence of radical changes during its lifetime.
Neutron star
– type of stellar remnant that can result from the gravitational
collapse of a massive star during a Type II, Type Ib or Type Ic
supernova event.
Compact star – white dwarfs, neutron stars, other exotic dense stars, and black holes.
Quark star – hypothetical type of exotic star composed of quark matter, or strange matter.
Exotic star
– compact star composed of something other than electrons, protons,
and neutrons balanced against gravitational collapse by degeneracy
pressure or other quantum properties.
White dwarf – also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter.
Supernova – stellar explosion that is more energetic than a nova.
Hypernova – also known as a Type Ic Supernova, refers to an immensely large star that collapses at the end of its lifespan.
Gamma-ray burst – flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies.
Properties of black holes
Accretion disk
– structure (often a circumstellar disk) formed by diffused material in
orbital motion around a massive central body, typically a star.
Accretion disks of black holes radiate in the X-ray part of the
spectrum.
Black hole thermodynamics – area of study that seeks to reconcile the laws of thermodynamics with the existence of black hole event horizons.
Schwarzschild radius
– distance from the center of an object such that, if all the mass of
the object were compressed within that sphere, the escape speed from
the surface would equal the speed of light.
M-sigma relation – empirical correlation between the stellar velocity dispersion of a galaxy bulge and the mass M of the supermassive black hole at
Event horizon – boundary in spacetime beyond which events cannot affect an outside observer.
Quasi-periodic oscillation – manner in which the X-ray light from an astronomical object flickers about certain frequencies.
Photon sphere – spherical region of space where gravity is strong enough that photons are forced to travel in orbits.
Ergosphere – region located outside a rotating black hole.
Hawking radiation – black-body radiation that is predicted to be emitted by black holes, due to quantum effects near the event horizon.
Penrose process – process theorised by Roger Penrose wherein energy can be extracted from a rotating black hole.
Spaghettification
– vertical stretching and horizontal compression of objects into long
thin shapes in a very strong gravitational field, and is caused by
extreme tidal forces.
Gravitational lens
– distribution of matter between a distant source and an observer,
that is capable of bending the light from the source, as it travels
towards the observer.
Pierre-Simon Laplace – early mathematical theorist (1796) of the idea of black holes
Albert Einstein – in 1915, arrived at the theory of general relativity
Karl Schwarzschild – described the gravitational field of a point mass in 1915
Subrahmanyan Chandrasekhar
– in 1931, using special relativity, postulated that a non-rotating
body of electron-degenerate matter above a certain limiting mass (now
called the Chandrasekhar limit at 1.4 solar masses) has no stable
solutions.
David Finkelstein – identified the Schwarzschild surface as an event horizon
Roy Kerr – In 1963, found the exact solution for a rotating black hole
Models of black holes
Gravitational singularity
– or spacetime singularity is a location where the quantities that are
used to measure the gravitational field become infinite in a way that
does not depend on the coordinate system.
Penrose–Hawking singularity theorems – set of results in general relativity which attempt to answer the question of when gravitation produces singularities.
Primordial black hole
– hypothetical type of black hole that is formed not by the
gravitational collapse of a large star but by the extreme density of
matter present during the universe's early expansion.
Gravastar – object hypothesized in astrophysics as an alternative to the black hole theory by Pawel Mazur and Emil Mottola.
Dark star (Newtonian mechanics)
– theoretical object compatible with Newtonian mechanics that, due to
its large mass, has a surface escape velocity that equals or exceeds the
speed of light.
Fuzzball (string theory) – theorized by some superstring theory scientists to be the true quantum description of black holes.
White hole
– hypothetical region of spacetime which cannot be entered from the
outside, but from which matter and light have the ability to escape.
Naked singularity – gravitational singularity without an event horizon.
Ring singularity
– describes the altering gravitational singularity of a rotating
black hole, or a Kerr black hole, so that the gravitational singularity
becomes shaped like a ring.
Immirzi parameter – numerical coefficient appearing in loop quantum gravity, a nonperturbative theory of quantum gravity.
Membrane paradigm
– useful "toy model" method or "engineering approach" for visualising
and calculating the effects predicted by quantum mechanics for the
exterior physics of black holes, without using quantum-mechanical
principles or calculations.
Kugelblitz (astrophysics)
– concentration of light so intense that it forms an event horizon and
becomes self-trapped: according to general relativity, if enough
radiation is aimed into a region, the concentration of energy can warp
spacetime enough for the region to become a black hole .
Wormhole – hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime.
Quasi-star – hypothetical type of extremely massive star that may have existed very early in the history of the Universe.
No-hair theorem
– postulates that all black hole solutions of the Einstein-Maxwell
equations of gravitation and electromagnetism in general relativity can
be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum.
Cosmic censorship hypothesis – two mathematical conjectures about the structure of singularities arising in general relativity.
Nonsingular black hole models
– mathematical theory of black holes that avoids certain theoretical
problems with the standard black hole model, including information loss
and the unobservable nature of the black hole event horizon.
Holographic principle
– property of quantum gravity and string theories which states that
the description of a volume of space can be thought of as encoded on a
boundary to the region—preferably a light-like boundary like a
gravitational horizon.
Black hole complementarity – conjectured solution to the black hole information paradox, proposed by Leonard Susskind and Gerard 't Hooft.
Black hole metrics
Schwarzschild metric – describes the gravitational field outside a spherical, uncharged, non-rotating mass such as a star, planet, or black hole.
Kerr metric
– describes the geometry of empty spacetime around an uncharged,
rotating black hole (axially symmetric with an event horizon which is
topologically a sphere)
Reissner–Nordström metric
– static solution to the Einstein-Maxwell field equations, which
corresponds to the gravitational field of a charged, non-rotating,
spherically symmetric body of mass M.
Kerr-Newman metric
– solution of the Einstein–Maxwell equations in general relativity,
describing the spacetime geometry in the region surrounding a charged,
rotating mass.
Astronomical objects including a black hole
Hypercompact stellar system – dense cluster of stars around a supermassive black hole that has been ejected from the centre of its host galaxy.
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 star systems or by dynamic processes and mutual capture, and binary supermassive black holes believed to be a result of galactic mergers.
For many years, proving the existence of binary black holes was
made difficult because of the nature of black holes themselves, and the
limited means of detection available. However, in the event that a pair
of black holes were to merge, an immense amount of energy should be
given off as gravitational waves, with distinctive waveforms that can be calculated using general relativity.
Therefore, during the late 20th and early 21st century, binary black
holes became of great interest scientifically as a potential source of
such waves, and a means by which gravitational waves could be proven to
exist. Binary black hole mergers would be one of the strongest known
sources of gravitational waves in the Universe, and thus offer a good
chance of directly detecting such waves.
As the orbiting black holes give off these waves, the orbit decays, and
the orbital period decreases. This stage is called binary black hole
inspiral. The black holes will merge once they are close enough. Once
merged, the single hole settles down to a stable form, via a stage
called ringdown, where any distortion in the shape is dissipated as more
gravitational waves.
In the final fraction of a second the black holes can reach extremely
high velocity, and the gravitational wave amplitude reaches its peak.
The existence of stellar-mass binary black holes (and gravitational waves themselves) were finally confirmed when LIGO detected GW150914
(detected September 2015, announced February 2016), a distinctive
gravitational wave signature of two merging stellar-mass black holes of
around 30 solar masses each, occurring about 1.3 billion light years
away. In its final 20 ms of spiraling inward and merging, GW150914
released around 3 solar masses as gravitational energy, peaking at a
rate of 3.6×1049watts — more than the combined power of all light radiated by all the stars in the observable universe put together. Supermassive binary black hole candidates have been found, but not yet categorically proven.
Occurrence
Supermassive black-hole binaries are believed to form during galaxy mergers. Some likely candidates for binary black holes are galaxies with double cores still far apart. An example double nucleus is NGC 6240. Much closer black-hole binaries are likely in single core galaxies with double emission lines. Examples include SDSS J104807.74+005543.5 and EGSD2 J142033.66 525917.5. Other galactic nuclei have periodic emissions suggesting large objects orbiting a central black hole, for example in OJ287.
The quasar PG 1302-102 appears to have a binary black hole with an orbital period of 1900 days.
Stellar mass binary black holes have been demonstrated to exist, by the first detection of a black hole merger event GW150914 by LIGO.
Final parsec problem
When two galaxies collide, the supermassive black holes at their centers do not hit head-on, but would shoot past each other on hyperbolic trajectories if some mechanism did not bring them together. The most important mechanism is dynamical friction, which transfers kinetic energy from the black holes to nearby matter. As a black hole passes a star, the gravitational slingshot accelerates the star while decelerating the black hole.
This slows the black holes enough that they form a bound, binary, system, and further dynamical friction steals orbital energy from the pair until they are orbiting within a few parsecs
of each other. However, this process also ejects matter from the
orbital path, and as the orbits shrink, the volume of space the black
holes pass through reduces, until there is so little matter remaining
that it could not cause merger within the age of the universe.
Gravitational waves can be a significant contributor, but not
until the separation shrinks to a much smaller value, roughly
0.01–0.001 parsec.
Nonetheless, supermassive black holes appear to have merged, and
what appears to be a pair in this intermediate range has been observed,
in PKS 1302-102. The question of how this happens is the "final parsec problem".
A number of solutions to the final parsec problem have been
proposed. Most involve mechanisms to bring additional matter, either
stars or gas, close enough to the binary pair to extract energy from the
binary and cause it to shrink. If enough stars pass close by to the
orbiting pair, their gravitational ejection can bring the two black
holes together in an astronomically plausible time.
One mechanism that is known to work, although infrequently, is a third supermassive black hole from a second galactic collision. With three black holes in close proximity, the orbits are chaotic and allow three additional energy loss mechanisms:
The black holes orbit through a substantially larger volume of
the galaxy, interacting with (and losing energy to) a much greater
amount of matter,
The orbits can become highly eccentric, allowing energy loss by gravitational radiation at the point of closest approach, and
Two of the black holes can transfer energy to the third, possibly ejecting it.
Lifecycle
Inspiral
The first stage of the life of a binary black hole is the inspiral,
a gradually shrinking orbit. The first stages of the inspiral take a
very long time, as the gravitation waves emitted are very weak when the
black holes are distant from each other. In addition to the orbit
shrinking due to the emission of gravitational waves, extra angular
momentum may be lost due to interactions with other matter present, such
as other stars.
As the black holes’ orbit shrinks, the speed increases, and
gravitational wave emission increases. When the black holes are close
the gravitational waves cause the orbit to shrink rapidly.
The last stable orbit or innermost stable circular orbit (ISCO) is the innermost complete orbit before the transition from inspiral to merger.
Merger
This is
followed by a plunging orbit in which the two black holes meet, followed
by the merger. Gravitational wave emission peaks at this time.
Ringdown
Immediately
following the merger, the now single black hole will “ring” –
oscillating in shape between a distorted, elongated spheroid and a
flattened spheroid. This ringing is damped in the next stage, called the
ringdown, by the emission of gravitational waves. The
distortions from the spherical shape rapidly reduce until the final
stable sphere is present, with a possible slight distortion due to
remaining spin.
Observation
The first observation of stellar mass binary black holes merging was performed by the LIGO detector.
As observed from Earth, a pair of black holes with estimated masses
around 36 and 29 times that of the Sun spun into each other and merged
to form a 62 solar mass black hole (approximate) on 14 September 2015,
at 09:50 UTC. Three solar masses were converted to gravitational radiation in the final fraction of a second, with a peak power 3.6×1056 ergs/second (200 solar masses per second), which is 50 times the total output power of all the stars in the observable universe. The merger took place at 1.3 billion light years from Earth, and therefore 1.3 billion years ago. The observed signal is consistent with the predictions of numerical relativity.
Dynamics modelling
Some simplified algebraic models can be used for the case where the black holes are far apart, during the inspiral stage, and also to solve for the final ringdown.
Post-Newtonian approximations
can be used for the inspiral. These approximate the general relativity
field equations adding extra terms to equations in Newtonian gravity.
Orders used in these calculations may be termed 2PN (second order post
Newtonian) 2.5PN or 3PN (third order post Newtonian). Effective-one-body
(EOB) solves the dynamics of the binary black hole system by
transforming the equations to those of a single object. This is
especially useful where mass ratios are large, such as a stellar mass
black hole merging with a galactic core black hole, but can also be used for equal mass systems.
For the ringdown, black hole perturbation theory can be used. The final Kerr black hole is distorted, and the spectrum of frequencies it produces can be calculated.
To solve for the entire evolution, including merger, requires
solving the full equations of general relativity. This can be done in numerical relativity
simulations. Numerical relativity models space-time and simulates its
change over time. In these calculations it is important to have enough
fine detail close into the black holes, and yet have enough volume to
determine the gravitation radiation that propagates to infinity. In
order to make this have few enough points to be tractable to calculation
in a reasonable time, special coordinate systems can be used such as Boyer-Lindquist coordinates or fish-eye coordinates.
Numerical relativity techniques steadily improved from the initial attempts in
the 1960s and 1970s.
Long-term simulations of orbiting black holes,
however, were not possible until three groups independently developed
groundbreaking new methods to model the inspiral, merger, and ringdown of
binary black holes in 2005.
In the full calculations of an entire merger, several of the
above methods can be used together. It is then important to fit the
different pieces of the model that were worked out using different
algorithms. The Lazarus Project linked the parts on a spacelike
hypersurface at the time of the merger.
Results from the calculations can include the binding energy. In a
stable orbit the binding energy is a local minimum relative to
parameter perturbation. At the innermost stable circular orbit the local
minimum becomes an inflection point.
The gravitational waveform produced is important for observation
prediction and confirmation. When inspiralling reaches the strong zone
of the gravitational field, the waves scatter within the zone producing
what is called the post Newtonian tail (PN tail).
In the ringdown phase of a Kerr black hole, frame-dragging
produces a gravitation wave with the horizon frequency. In contrast the
Schwarzschild black-hole ringdown looks like the scattered wave from
the late inspiral, but with no direct wave.
The radiation reaction force can be calculated by Padé resummation
of gravitational wave flux. A technique to establish the radiation is
the Cauchy characteristic extraction technique CCE which gives a close
estimate of the flux at infinity, without having to calculate at larger
and larger finite distances.
The final mass of the resultant black hole depends on the definition of mass in general relativity. The Bondi mass MB is calculated from the Bondi-Sach mass loss formula. . With f(U) the gravitational wave flux at retarded time U. f is a surface integral of the News function at null infinity varied by solid angle. The Arnowitt-Deser-Misner (ADM) energy or ADM mass is the mass as measured at infinite distance and includes all the gravitational radiation emitted. .
Angular momentum
is also lost in the gravitational radiation. This is primarily in the z
axis of the initial orbit. It is calculated by integrating the product
of the multipolar metric waveform with the news function complement over
retarded time.
Shape
One of the problems to solve is the shape or topology of the event horizon during a black-hole merger.
In numerical models, test geodesics are inserted to see if they
encounter an event horizon. As two black holes approach each other, a
‘duckbill’ shape protrudes from each of the two event horizons towards
the other one. This protrusion extends longer and narrower until it
meets the protrusion from the other black hole. At this point in time
the event horizon has a very narrow X-shape at the meeting point. The
protrusions are drawn out into a thin thread. The meeting point expands to a roughly cylindrical connection called a bridge.
Simulations as of 2011 had not produced any event horizons with toroidal
topology (ring-shaped). Some researchers suggested that it would be
possible if, for example, several black holes in the same
nearly-circular orbit coalesce.
Black-hole merger recoil
An
unexpected result can occur with binary black holes that merge, in that
the gravitational waves carry momentum and the merging black-hole pair
accelerates seemingly violating Newton's third law. The center of gravity can add over 1000 km/s of kick velocity.
The greatest kick velocities (approaching 5000 km/s) occur for
equal-mass and equal-spin-magnitude black-hole binaries, when the spins
directions are optimally oriented to be counter-aligned, parallel to the
orbital plane or nearly aligned with the orbital angular momentum.
This is enough to escape large galaxies. With more likely orientations a
smaller effect takes place, perhaps only a few hundred kilometers per
second. This sort of speed will eject merging binary black holes from
globular clusters, thus preventing the formation of massive black holes
in globular cluster cores. In turn this reduces the chances of
subsequent mergers, and thus the chance of detecting gravitational
waves. For non spinning black holes a maximum recoil velocity of
175 km/s occurs for masses in the ratio of five to one. When spins are
aligned in the orbital plane a recoil of 5000 km/s is possible with two
identical black holes.
Parameters that may be of interest include the point at which the black
holes merge, the mass ratio which produces maximum kick, and how much
mass/energy is radiated via gravitational waves. In a head-on collision
this fraction is calculated at 0.002 or 0.2%. One of the best candidates of the recoiled supermassive black holes is CXO J101527.2+625911.
Halo drive for space travel
It has been hypothesized that binary black holes could transfer energy and momentum to a spacecraft using a "halo drive", exploiting the holographic reflection
created by a set of null geodesics looping behind and then around one
of the black holes before returning to the spacecraft. The reflection
passing through these null geodesics would form one end of a laser
cavity, with a mirror on the spacecraft forming the other end of the
laser cavity. Even a planet-sized spacecraft would thereby accelerate to
speeds exceeding the approaching black hole's relative speed. If true, a
network of these binary black holes might permit travel across the
galaxy.
