Black Hole Overview

Black Hole Schematic

The Black Hole Engine. Note the black hole diagram to the left. The event horizon is the point of no escape. It is a theoretical mathematical sphere of zero width because in real black holes there is nothing there to see. It would be like entering a maximum speeding zone in a car, there is no hard boundary. But in this case, there is no way to turn back.

The "singularity" is at the center of the black hole where all inside materials head towards and whose density and gravity approaches infinitely. The appearance of a singularity in General Relativity indicates the limit of this theory. It illustrates places and/or conditions where the theory breaks down and one can not know the details of exactly what is happening.

All "rotating" black holes pull along a region of spacetime with it due to a phenomenon called Frame Dragging. General Relativity predicts that a fast rotating object will drag any nearby object along with it. The region where it is not possible for an object to remain stationary is called the "ergosphere". Its shape is that of an oblate spheroid - bulging at the equator, and flattened at the poles of the rotating black hole. Since the ergosphere is outside the event horizon, it's possible for an object within it to escape the black hole. When it does, it will leave with extra energy accumulated from the spinning black hole. Not all scientists believe that the ergosphere is real, it may be just a mathematical entity.

The red ring above labeled the photon sphere is also called the photon ring. The photon sphere is also called the innermost stable circular orbit (ISCO). This is another mathematical theoretical boundary. As the gas and other material in the accretion disk spin closer and closer to the black hole, they travel faster and faster approaching the speed of light. The photon sphere is the point where only light photons can can travel fast enough at the full speed of light to continuously orbit the black hole. Again, this is a theoretical radius equal to one and a half times the event horizon radius - the Schwarzschild Radius.

In real life even photons would have a tough time orbiting the ISCO for any length of time because they would eventually collide with another photon and either travel off into the future or fall into the event horizon and be lost. Therefore the photon sphere is in real life an unstable reference point. However it is very useful for mathematical calculations of the innermost ring of the accretion disk. The important take away here is that there is "space" between the black hole and the accretion disk which we shall discuss further below. See Inner Edge Accretion Spacing.

Black Hole Model

Model Of A Supermassive Black Hole. Plasma gas (electrons and protons), falling very close to a black hole, orbits it and accumulates into a flattened Accretion Disk. The gas in the disk spirals inward, becoming compressed and heated as it nears the center. Ultimately reaching temperatures up to 20 million degrees Fahrenheit (12 million C), the gas shines brightly in low-energy "soft" x-rays.

Over 40 years, observations show that black holes also produce "hard" x-rays with energy tens to hundreds of times greater than the soft x-rays. This implies the presence of a much hotter gas above and below the disk, the Corona, with temperatures reaching billions of degrees. The Corona is believed to be the source of the "hard" x-rays and gamma rays. This very hot gas is a phenomenon similar to the hot corona that surrounds our sun.

Keep in mind though, a Corona has never been directly detected and we have no real knowledge of what a Corona is shaped like or what its composition is. Most scientists believe Coronas are real because of the "hard" x-rays and gamma rays that are somehow being generated. Many astro-physicists think the Corona is the source of jets, but at this point it is speculation. Only about 10% of black holes have jets.

Around the edge of a super-massive black hole there is a huge ring of very dusty gas called a "Torus". The Torus is very dense and lumpy. If one is looking at a black hole edge-wise, all you can see is the Torus as it is so large it blocks the view of the actual black hole. One has to be located on quite an angle to see over the Torus to witness the black hole itself. See the NASA artists' redition of NGC 1068 to the left below as a result of NuSTAR, the very high x-ray/gamma ray satellite, taking very detailed images of the NGC 1068 Torus. The Torus prevents many black holes from being seen from earth as they are edge on to us. However, they can still be detected by x-rays.

Lumpy Disk NGC 1068

Between the Torus and the Accretion Disk lies a region called the Broad Line Region (BLR). It consists of clouds of swirling gases that are not part of the Accretion Disk, but are dense enough to radiate broad optical emission lines. These come from cold material close to the Accretion Disk. The lines are broad because the emitting material is revolving around the black hole with high speeds causing a range of Doppler shifts of the emitted photons.

Outside of the Torus there exists more cold swirling dust clouds. This area is called the Narrow Line Region (NLR) because they emit narrow optical lines.

There is no single signature of a black hole. The features described here cover the most important features that charcterize a black hole.  Top

Black Holes Warp Space-time

Warped Space Time

Black holes essentially start out as spheres of mass with strong gravitational pulls. As gas, stars and other material fall into them, black holes absorb not just the mass of the objects but also their magnetic fields. Black holes become more and more magnetically charged as time goes on because the angular momentum of incoming material must be preserved.

When a black hole has acquired a very strong magnetic field, the black hole's spin will warp space-time around it (see the image at the left), causing the magnetic field lines to twist into spirals along the black hole's axis of rotation.

Ordinarily it takes just a millisecond for these particles to cross the final horizon, but astro-physicists now know that this chaotic region is threaded by magnetic fields that provide some particles and radiation a way out. This evidence points to the magnetic fields being so strong as to tear away some particles from the black hole’s gravitational clutches and funnel them outwards. These magnetic fields can possibly create jets of matter with spiral trajectories that can shoot hundreds of thousands of light years into outer space. Scientists have developed computer models to simulate jet formation and the models confirm that spinning magnetic fields can create jets. Current research is aimed at studying the origins of jet streams as close as possible.  Top

Inner Edge Accretion Spacing

Black Hole Rotation

The NASA chart to the left illustrates the basic model for determining the spin rates of black holes. The three artist's images represent the different types of black hole spin: retrograde rotation at the top of the chart, where the accretion disk moves in the opposite direction of the black hole; no spin in the middle of the chart; and prograde rotation at the bottom of the chart, where the accretion disk spins in the same direction as the black hole.

Note that for the retrograde rotation (opposite spins), the edge of the accretion disk is a considerable distance from the black hole. Also note in the blue x-ray chart to the right there is an absence of x-ray energy at the lower energy levels.

In the case of a prograde rotation (both spin in the same direction), the edge of the accretion disk is very close to the event horizon. The green energy chart to the right shows positive values for the low energy levels and also that they are somewhat distorted compared to the no rotation light blue graph in the middle. The x-rays in this case are so close to the event horizon that the strong gravity from the black hole distorts the x-rays according to Einstein's theory of Relativity.

The x-ray chart can be used first of all to "identify" a black hole (in addition to gravitational effects methods), then to determine its spin "direction", and finally the "rate" of spin. This is done by comparing various models to the actual data. One proposed theory is that black hole jets (that occur in only 10% of the black holes) originate in the large spaces that exist in retrograde disks. This interesting theory has not been confirmed.  Top

Black Hole Jet Formation

Black Hole Corona

Physicists still do not know in detail "exactly" how black hole jets are formed. Astro-physicists believe the power source for black hole jets comes from the accretion disc, not the black hole itself. "Real" black holes with jets are a far cry from simplistic mathematical formulations. The big differences between the original simplistic mathematical equations and the real universe are the magnetic fields that are ignored in the simple math formulations (which do not incorporate accretion disks and jets). Recent black hole computer models do incorporate jets and are being constantly revised as scientists better understand black holes and their jets.

The jet formation "hypothesis" is that the plasma gas and dust which forms the accretion disk is made up of protons, electrons and radiation at extremely high temperatures - several million degrees C. These elements are in a wild state, spinning and traveling close to the speed of light roughly in synch with the spinning black hole because of Frame Dragging.

These spinning particles also have associated magnetic fields spinning along with them. The spinning magnetic fields collimate into humongous rising columns which propagate some of the plasma from the accretion disks into huge jets with terrific outward force and speed. The jets of plasma line up with the rotating axis of the black hole and emerge from each face of the disk.

The jets form either on the accretion disk close to the inner edge or in the space between the event horizon and the accretion disk. Only about 10% of black holes have jets, and as mentioned above, one theory is that jets form in the space between the the event horizon and accretion disk in retrograde type black holes which are very much in the minority. Another theory suggests that black holes must have a "minimum spin" level in order to form jets. The ultimate theory of just how jets form is still evolving so stay tuned.  Top

What Do We Know About Black Hole Coronas?

Reflected X-Rays X-Rays

Note the white section at the bottom of the jet in the illustration above left, this is the black hole corona. Scientists associate the corona of a black hole to be something like the corona around the sun. The formulation and make up of the black hole corona is not understood very well. Compact regions above and below black holes contain sources of super hot high energy x-ray radiation and could possibly be the sources of black hole jets. In addition to its own light, the Corona lights up the accretion disk, which then reflects it, making the accretion disk an additional source of x-rays. See the illustration to the left.

Observations have shown that black holes produce  "hard" x-rays, with energy tens to hundreds of times greater than the "soft" x-rays normally produced by the accretion disk. This implies the presence of a much hotter gas, with temperatures reaching billions of degrees - hence the corona. Astronomers at NASA, Johns Hopkins University and the Rochester Institute of Technology have recently (June, 2013) completed a supercomputer simulation of a "non-spinning" black hole and confirmed that stellar-mass black holes due produce a corona. The researchers are currently expanding the simulation to "prograde" spinning black holes, where the rotation pulls the inner edge of the accretion disk further inward and conditions become even more extreme.

The rising temperature, density and speed of the in-falling gas dramatically amplify magnetic fields threading through the disk. This results in a turbulent froth orbiting the black hole at speeds approaching the speed of light. The simulation simultaneously tracked the fluid, electrical and magnetic properties of the gas while also taking into account Einstein's theory of relativity. The team demonstrated for the first time a direct connection between magnetic turbulence in the disk, the formation of a billion-degree corona, and the production of hard X-rays around an actively "feeding" black hole. The reflected light from the disk enables astronomers to see how fast matter is swirling in the inner region of the disk which ultimately allows them to calculate the black hole's spin rate.  Top

Black Hole X-Ray Modeling

Black Hole Cygnus X-1

Cygnus X-1 (Cyg X-1) is a dual (binary) set of celestial objects - a black hole that orbits a supergiant star 8,000 light years away. See a real Cyg X-1 photo to the left, the black hole is the smaller object. (The name X-1 means it was the first x-ray source found in the constellation Cygnus.)

To the left below is a plot of how bright the black hole Cyg X-1 is in x-rays. The vertical axis is brightness and the horizontal axis is the energy of the x-rays. The round black points labeled "X-ray data" represent how bright Cyg X-1 measured at each frequency of x-ray. It is the brightest around 2 keV and has emissions all the way up to 100 keV. The solid line line, a curve with a hump-like shape, is the result of mathematical modeling.

Mathematical models are based on the size, composition and temperature of the accretion disk plus the mass of the black hole. Depending on how hot the disk is, astronomers can estimate how bright its accretion disk will be at different frequencies. When the components of the model are summed up, they make up a curve that can be compared to the original data.

Black Hole X-Ray Model

The model result of Cyg X-1 is the solid black line and lies approximately on top of the real data - an exceptionally good fit. The final model is made up of three parts.

The light the accretion disk emits is shown by the long-dash line (and labeled in blue).

There is also a corona, and much like the corona of our sun, it is a region of hot gas that is much less dense than the accretion disk, but emits very powerful "hard" x-rays. The light the corona emits is shown with the short-dash line (labeled in red). Note that the corona has very strong x-rays all the way up to and beyond 100 keV. 100 keV is the beginning range of gamma rays.

The third component is the light the accretion disk reflects from the corona, its emission is shown with the dotted line (labeled in green). Why is the green reflection line so complicated ? This complex dotted line is a result of all the different kinds of materials that are rotating in the accretion disk.  

At the end of the day, a good model has to agree with observations. The model that best matched the data for Cyg X-1 required a black hole 12 times the mass of the sun. This is one way astronomers can determine the mass of a black hole in a binary system. This is also a way to tell if a binary has a black hole or a neutron star in it. Neutron stars cannot be more massive than 3 times the mass of the sun. So if you find an object that is more than 10 times the mass of the sun, it definitely has to be a black hole.  Top

Estimating The Size Of Distant Black Holes

NGC 1097 Black Hole

A black hole in the heart of distant galaxy NGC 1097 (pictured to the left) has a mass of 140 million suns. Galaxy NGC 1097 is 47 million light years away from earth, too far to determine the mass of its central black hole by the circulation of the stars around it. A new way of estimating the mass of distant black holes was needed.

By tracking the movements of two types of molecular gases around the galaxy's center, researchers using the Atacama Large Millimeter Array (ALMA) in Chile were able to work backwards and determine the black hole's gravitational pull and its mass. The results show that this black hole is significantly larger than the one at the center of the Milky Way, i.e. 140 million suns in mass versus the Milky Way's 4 million suns.

NGC 1097 Gases

The ALMA telescopes tracked the radiation emitted from the two gases, hydrogen cyanide and formyl cation, as they swirled around the galaxy center. The gases don't interact strongly with local conditions within the galaxy, such as ionized gases flowing inward or outward. This means the movements of the gases paint an accurate picture of the effects of the black hole's gravity pull only.

Pictured to the left is the distribution of the two gases, hydrogen cyanide (red) and formyl cation (yellowish green), shown overlaid on an image of galaxy NGC 1097 taken by the Hubble Space Telescope. One gas is moving towards us (green) and one is moving away (red).

By tracking these movements the researchers were able to estimate the central black hole's mass. With just two hours of observational data, they learned enough about the distribution and velocities of the gases to fit them to a model and calculate the pull of the galaxy's core black hole.

The mass of a central black hole affects the physical properties of its host galaxy, and recent work has shown that those effects are different for different types of galaxies. Because of its ability to do fast precision measurements, "ALMA will enable us to observe a large number of galaxies in a practical length of time," said Kyoko Onishi, who is doing her research at the National Astronomical Observatory of Japan (NAOJ). "To understand the different effects, it is important to measure the mass of central black holes in various galaxy types", Onishi said.  Top

Measuring How Fast Supermassive Black Holes Spin

Galaxy NGC 1365

While astronomers have long been able to measure black hole masses effectively, determining their spins has been much more difficult. Imagine trying to measure the spin rate of something you can't even "see" - a formidable task. Two x-ray space observatories, NASA's NuSTAR and the EPA's XMM-Newton, have teamed up to accurately measure, for the first time, the spin rate of a black hole with a mass two million times that of our sun. The supermassive black hole  lies at the heart of a spiral galaxy called NGC 1365 (shown at the left).

Einstein's theory predicts that the faster a black hole spins, the closer the accretion disk aligns to the black hole. The closer the accretion disk is, the more gravity from the black hole will warp x-ray light streaming off the disk. While XMM-Newton determined that X-ray light emitted by iron in the accretion disk was being warped, NuSTAR proved that this distortion was coming from the the black hole and not the gas clouds in the vicinity.

With obscuring clouds ruled out, scientists can use the spectral iron signature to measure the black hole's spin rate, which can be determined from the widths of the iron lines. By zeroing in on high-energy light emitted by iron atoms, scientists were able to trace the motion of the rotating accretion disk that circles the black hole

Astronomers found that the inner edge of the accretion disk was quite close to the black hole - close enough for gravitational effects to wreak havoc with the X-rays streaming from the disk. This in turn implies a rapidly rotating black hole, since general relativity states that the faster a black hole is spinning, the closer its disk can come to it.

The research team, led by Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics and the Italian National Institute for Astrophysics, calculated the rotation rate to be 84 percent of that allowed by general relativity. It is tough to comprehend this figure, since it doesn't translate well into miles per hour. But it's safe to say that the black hole is spinning incredibly fast.

By measuring the spin of distant black holes researchers have discovered important clues about how these objects grow over time. If black holes grow mainly from collisions and mergers between galaxies, they should accumulate material in a stable disk, and the steady supply of new material from the disk should lead to rapidly spinning black holes. In contrast, if black holes grow through many small accretions, they will accumulate material from random directions. Like a small merry go round that is pushed both backwards and forwards, this would make the black hole spin more slowly.  Top

Markarian Corona Movement

Blurred X-Rays

Recently NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) captured an extremely rare event in the region immediately surrounding supermassive black hole Markarian (Mrk) 335. The corona, a compact source of x-rays that sits above and close by the black hole, has moved closer to the black hole due to the pull of the black hole's gravity over a period of just a few days. The exact shape and nature of coronas are not known.

As the corona shifted closer to the black hole, the gravity of the black hole exerted a stronger tug on the x-rays emitted by it. The result was an extreme blurring and stretching of the x-ray light. Such events had been observed previously but never to this degree and in such detail.

The plot of Mrk 335 data to the left, captured by NuSTAR, shows x-ray light streaming from regions near the supermassive black hole. The x-ray light is coming from two areas: the superheated disk of accretion material and a cloud of particles traveling near the speed of light originating from the corona.

The accretion disk's and corona's x-ray light are mapped over a range of energies. The blue line shows what the plot should look like before the effects of the moved corona. The yellow line shows what the data is predicted to look like if the "corona" x-ray light has been stretched or blurred. The white dots show the actual NuSTAR data, indicating the light is extremely blurred and as predicted. What's blurring the light? The enormous gravity of the black hole pulls on the x-ray light, making it harder to escape its grasp and in the process the light loses some energy.

Extremely hot matter has also been observed by the European Space Agency’s (ESA's) Integral Gamma Ray Observatory just a few milliseconds before it plunges into the oblivion of a black hole. That study also revealed that twisting magnetic fields do form an escape tunnel for some of the accretion disk particles. Just a few hundred miles from the deadly event horizon space is a violent storm of hot particles and radiation. Vast amounts of particles are falling to their doom at close to the speed of light and raising the surrounding temperatures to millions of degrees.  Top

Supermassive Black Holes And Galaxies

Black Hole vs Galaxy Bulge

At the heart of virtually every large galaxy is a supermassive black hole (SMBH) with a mass up to about 18 billion times our sun. The size of the SMBH appears to have a direct correlation with the size of the galaxy where it coexists. About ten years ago, researchers calculated that the mass of a SMBH had a relationship of 1 to 700 to the mass of its central galaxy bulge. See the chart to the left. This ratio relationship supports the idea that the evolution of a galaxy is closely tied to the scale of its black hole and vice-versa.

Other studies found another strong correlation. This one is between the mass of a SMBH and the orbital speed of stars in the outer regions of the galaxy. The larger the black hole, the faster the outer stars travel. It is now believed that black holes have played a critical role in the evolution of the universe.

Two independent teams found that the black hole, jets, and newborn stars are all parts of a self-regulating cycle. High-energy jets shooting from the black hole heat a halo of surrounding gas, controlling the rate at which the gas cools and falls into the galaxy.

"Think of the gas surrounding a galaxy as an atmosphere," explained the lead of the first study, Megan Donahue of Michigan State University. "That atmosphere can contain material in different states, just like our own atmosphere has gas, clouds, and rain. As the jets propel gas outward from the center of the galaxy, some of that gas cools and precipitates into cold clumps that fall back toward the galaxy's center like raindrops."

"The raindrops eventually cool enough to become star-forming clouds of cold molecular gas. The far-ultraviolet capabilities of Hubble allowed us to directly observe these 'showers' of star formation," explained the lead of the second study, Grant Tremblay of Yale University. "We know that these showers are linked to the jets because they're found in filaments and tendrils that wrap around the jets or hug the edges of giant bubbles that the jets have inflated," said Tremblay. "And they end up making a swirling 'puddle' of star-forming gas around the central black hole." See the NASA photo below showing cold gas in purple and new stars in white.


But what should be a monsoon of raining gas is reduced to a mere drizzle by the black hole. While some outwardly flowing gas will cool, the black hole heats the rest of the gas around a galaxy, which prevents the whole gaseous envelope from cooling more quickly. The entire cycle is a self-regulating feedback mechanism because the cloud of gas around the black hole provides the fuel that powers the jets. If too much cooling happens, the jets become more powerful and add more heat. And if the jets add too much heat, they reduce their fuel supply and eventually weaken.

For many years, the question has persisted as to why galaxies awash in gas don't turn all of that gas into stars. Theoretical models of galaxy evolution predict that present-day galaxies, more massive than the Milky Way, should be bursting with star formation. But, that is not the case. Now, scientists understand how this cycle of heating and cooling keeps star birth in check. A light drizzle of cooling gas provides enough fuel for the central black hole's jets to keep the rest of the galaxy's gas hot. The researchers show that galaxies don't need catastrophic events, such as galaxy collisions, to explain the star birth rates that they see.

The researchers were aided by a new set of computer simulations of the hydrodynamics of the gas flows developed by Yuan Li of the University of Michigan. "This is the first time we now have models in hand that predict how these things ought to look," explained Donahue. "And when we compare the models to the data, there's a stunning similarity between the star-forming showers we observe and ones that occur in simulations. We're getting physical insights that we can apply to models."  Top

Steven Hawking - No Event Horizon!

Galaxy IC 342

At 71 in January, 2014, Steven Hawking again shook up the astro-physics world by a paper he submitted to the Cornell University web site "arXiv". “There is no escape from a black hole in classical theory,” Hawking told Nature. Quantum theory, however, “enables energy and information to escape from a black hole.” A full explanation of the process, the physicist admits, would require a theory that successfully merges gravity with the other fundamental forces of nature. But that's a goal that has eluded physicists for nearly a century. “The correct treatment,” Hawking says, “remains a mystery.”

Hawking’s proposal is a much more benign “apparent horizon”, which only temporarily holds matter and energy before eventually releasing them, but in a more garbled form. Hawking proposes that Quantum Mechanics and General Relativity remain intact, but black holes simply do not have an event horizon. The key is that quantum effects around the black hole cause spacetime to fluctuate too wildly for a sharp boundary surface to exist.

In place of the event horizon, Hawking suggests an apparent horizon, a surface along which radiation attempting to rush away from the black hole’s core will be suspended. Hawking’s theory is that the apparent horizon is the real boundary. “The absence of event horizons mean that there are no "black" holes - in the sense of regimes from which light can't escape” Hawking writes. Hawking invokes an apparent horizon that changes shape according to quantum fluctuations inside the black hole - like a “grey area” for extreme physics.

An apparent horizon wouldn’t violate either general relativity or quantum dynamics if the region around the apparent horizon is a tangled, chaotic mess of information.  “Thus, information will effectively be lost, although there would be no loss of unitarity,” writes Hawking. This basically means that although the information can escape from the black hole, its chaotic nature ensures it cannot be interpreted, sidestepping the firewall paradox all together.

Also, according to Hawking, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, and would gradually move inwards due to the pull of the black hole, but would never quite crunch down to an infinite density point at the center.

Comments From Astro-physicists

Since the original article by Steven Hawking was published in January, 2014, several noted astro-physicists have commented on his paper based on the technical detail of a black hole's event horizon. Below are a few key points from a paper by Sabine Hossenfelder, Assistant Professor for High Energy Physics at Nordita in Stockholm, Sweden.