The Shrinking Doughnut Around a Black Hole

GX 339-4, illustrated here, is a binary system of a black hole and a star. Astronomers were able to measure how the disk around the black hole shrinks for the first time. Image Credit: Credit: ESO/L. Calcada

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Homer Simpson would be sad: recent observations of the binary system of a black hole and its companion star have shown the retreat of the doughnut-shaped accretion disk around the black hole. This shrinking ‘doughnut’ was seen in observations of the binary system GX 339-4, a system composed of a star similar in mass to the Sun, and a black hole of ten solar masses.

As the black hole feeds on gas flowing out from the orbiting star, the change in flow of the gas produces a varying size in the disk of matter that piles up around the black hole in a torus shape. For the first time, the changes in the size of this disk have been measured, showing just how much smaller the doughnut becomes.

GX-339-4 lies 26,000 light-years away in the constellation Ara. Every 1.7 days in the system, a star orbits around the more massive black hole. This system, and others like it, show periodic flares of X-ray activity when gas that is being stolen from the star by the black hole gets heated up in the accretion disk that piles up around the black hole. Over the last seven years, the system has had four energetic outbursts in the last seven years, making it a quite active black hole/stellar binary system.

The material falling into the hole forms jets of highly energized photons and gas, one of which is pointed in the direction of the Earth. It is these jets that a team of international astronomers observed using the Suzaku X-ray observatory, operated jointly by the Japan Aerospace Exploration Agency and NASA, and NASA’s X-ray Timing Explorer satellite. The results of their observations were published in the Dec. 10 issue of The Astrophysical Journal Letters.

Though the system was faint when they took their measurements with the telescopes, it was producing steady jets of X-rays. The team was looking for the signature of X-ray spectral lines produced by the fluorescence of iron atoms in the disk. The strong gravity of the black hole shifts the energy of the X-rays produced by the iron, leaving a characteristic spectral line. By measuring these spectral lines, they were able to determine with rather high confidence the size of the shrinking disk.

Here’s how the shrinking occurs: the part of the disk that is closer to the black hole is denser when there is more gas flowing out from the star that accompanies it. But when this flow is reduced, the inner part of the disk heats up and evaporates. During the brightest periods of the black hole’s output, the disk was calculated to be within about 30 km (20 miles) of the black hole’s event horizon, while during lower periods of luminosity the disk retreats to greater than 27 times further, or to 1,000 km (600 miles) from the edge of the black hole.

This has an important implication in the study of how black holes form their jets; even though the accretion disk evaporates close to the black hole, these jets remain at a steady output.

John Tomsick of the Space Sciences Laboratory at the University of California, Berkeley said in a NASA press-release, “This doesn’t tell us how jets form, but it does tell us that jets can be launched even when the high-density accretion flow is far from the black hole. This means that the low-density accretion flow is the most essential ingredient for the formation of a steady jet in a black hole system.”

Read the pre-print version of the teams’ letter. If you want more information on how the X-rays from the disks around black holes can help determine their shape and spin, check out an article from Universe Today from 2003, Iron Can Help Determine if a Black Hole is Spinning.

Source: NASA/Suzaku press release

Quasar Caught Building Future Home Galaxy

An artist's impression of how quasars may be able to construct their own galaxies. Image Credit: ESO/L. Calcada

The birth of galaxies is quite a complicated affair, and little is known about whether the supermassive black holes at the center of most galaxies formed first, or if the matter in the galaxy accreted first, and formed the black hole later. Observations of the quasar HE0450-2958, which is situated outside of a galaxy, show the quasar aiding a nearby galaxy in the formation of stars. This provides evidence for the idea that supermassive black holes can ‘build’ their own galaxies.

The quasar HE0450-2958 is an odd entity: normally, supermassive black holes – also known as quasars – form at the center of galaxies. But HE0450-2958 doesn’t appear to have any host galaxy out of which it formed. This was a novel discovery in its own right when it was made back in 2005. Here’s our original story on the quasar, Rogue Supermassive Black Hole Has No Galaxy.

The formation of the quasar still remains a mystery, but current theories suggest that it formed out of cold interstellar gas filaments that accreted over time, or was somehow ejected from its host galaxy by a strong gravitational interaction with another galaxy.

The other oddity about the object is its proximity to a companion galaxy, which it may be aiding to form stars. The companion galaxy lies directly in the sights of one of the quasar’s jets, and is forming stars at a frantic rate. A team of astronomers from France, Germany and Belgium studied the quasar and companion galaxy using the Very Large Telescope at the European Southern Observatory. The astronomers were initially looking to find an elusive host galaxy for the quasar.

The phenomenon of ‘naked quasars’ has been reported before, but each time further observations are made, a host galaxy is found for the object. Energy streaming from the quasars can obscure a faint galaxy that is hidden behind dust, so the astronomers used the VLT spectrometer and imager for the mid-infrared (VISIR). Mid-infrared observations readily detect dust clouds. They combined these observations with new images obtained from the Hubble Space Telescope in the near-infrared.A color composite image of the quasar in HE0450-2958 obtained using the VISIR instrument on the Very Large Telescope and the Hubble Space Telescope. Image Credit: ESO

Observations of HE0450-2958, which lies 5 billion light years from Earth, confirmed that the quasar is indeed without a host galaxy, and that the energy and matter streaming out of the jets is pointed right at the companion galaxy. This scenario is ramping up star formation in that galaxy: 340 solar masses of stars a year are formed in the galaxy, one-hundred times more than for a typical galaxy in the Universe. The quasar and the galaxy are close enough that they will eventually merge, finally giving the quasar a home.

David Elbaz of the Service d’Astrophysique, who is the lead author of the paper which appeared in Astronomy & Astrophysics, said “The ‘chicken and egg’ question of whether a galaxy or its black hole comes first is one of the most debated subjects in astrophysics today. Our study suggests that supermassive black holes can trigger the formation of stars, thus ‘building’ their own host galaxies. This link could also explain why galaxies hosting larger black holes have more stars.”

‘Quasar feedback’ could be a potential explanation for how some galaxies form, and naturally the study of other systems is needed to confirm whether this scenario is unique, or a common feature in the Universe.

Source: ESO, Astronomy & Astrophysics

First Black Holes May Have Formed in “Cocoons”

Artist concept of a view inside a black hole. Credit: April Hobart, NASA, Chandra X-Ray Observatory
Artist concept of a view inside a black hole. Credit: April Hobart, NASA, Chandra X-Ray Observatory

Very likely, the last image that comes to mind when thinking of black holes is that they need to be nurtured, coddled and protected when young. But new research reveals the first large black holes in the universe likely formed and grew deep inside gigantic, starlike cocoons that smothered their powerful x-ray radiation and prevented surrounding gases from being blown away.

“Until recently, the thinking by many has been that supermassive black holes got their start from the merging of numerous, small black holes in the universe,” said Mitchell Begelman, from the University of Colorado-Boulder. “This new model of black hole development indicates a possible alternate route to their formation.”
Ordinary black holes are thought to be remnants of stars slightly larger than our sun that used up their fuel and died.

But the first big black holes likely formed from very large stars that formed early in the Universe, probably within the first few hundred million years after the Big Bang. The unique process of these large stars becoming black holes includes the formation of a protective cocoon, made of gas.

“What’s new here is we think we have found a new mechanism to form these giant supermassive stars, which gives us a new way of understanding how big black holes may have formed relatively fast,” said Begelman.
These early supermassive stars would have grown to a huge size — as much as tens of millions of times the mass of our sun — and would have been short-lived, with its core collapsing in just in few million years.

The main requirement for the formation of supermassive stars is the accumulation of matter at a rate of about one solar mass per year, said Begelman. Because of the tremendous amount of matter consumed by supermassive stars, subsequent seed black holes that formed in their centers may have started out much bigger than ordinary black holes.

Begelman said the hydrogen-burning supermassive stars would had to have been stabilized by their own rotation or some other form of energy like magnetic fields or turbulence in order to facilitate the speedy growth of black holes at their centers.

After the seed black holes formed, the process entered its second stage, which Begelman has dubbed the “quasistar” stage. In this phase, black holes grew rapidly by swallowing matter from the bloated envelope of gas surrounding them, which eventually inflated to a size as large as Earth’s solar system and cooled at the same time, he said.

Once quasistars cooled past a certain point, radiation began escaping at such a high rate that it caused the gas envelope to disperse and left behind black holes up to 10,000 times or more the mass of Earth’s sun. With such a big head start over ordinary black holes, they could have grown into supermassive black holes millions or billions of times the mass of the sun either by gobbling up gas from surrounding galaxies or merging with other black holes in extremely violent galactic collisions.

Begelman said big black holes formed from early supermassive stars could have had a huge impact on the evolution of the universe, including galaxy formation, possibly going on to produce quasars — the very bright, energetic centers of distant galaxies that can be a trillion times brighter than our sun.

Begelman’s paper will be published in Monthly Notices of the Royal Astronomical Society.

Source: EurekAlert

Finding the Mama Bear of Black Holes

While astronomers have studied both big and little black holes for decades, evidence for those middle-sized black holes has been much harder to come by. Now, astronomers at NASA’s Goddard Space Flight Center in Greenbelt, Md., find that an X-ray source in galaxy NGC 5408 represents one of the best cases for a middleweight black hole to date. “Intermediate-mass black holes contain between 100 and 10,000 times the sun’s mass,” explained Tod Strohmayer, an astrophysicist at Goddard. “We observe the heavyweight black holes in the centers of galaxies and the lightweight ones orbiting stars in our own galaxy. But finding the ‘tweeners’ remains a challenge.”

Several nearby galaxies contain brilliant objects known as ultraluminous X-ray sources (ULXs). They appear to emit more energy than any known process powered by stars but less energy than the centers of active galaxies, which are known to contain million-solar-mass black holes.

“ULXs are good candidates for intermediate-mass black holes, and the one in galaxy NGC 5408 is especially interesting,” said Richard Mushotzky, an astrophysicist at the University of Maryland, College Park. The galaxy lies 15.8 million light-years away in the constellation Centaurus.

Artists concept of a medium sized black hole. Credit: NASA
Artists concept of a medium sized black hole. Credit: NASA

XMM-Newton detected what the astronomers call “quasi-periodic oscillations,” a nearly regular “flickering” caused by the pile-up of hot gas deep within the accretion disk that forms around a massive object. The rate of this flickering was about 100 times slower than that seen from stellar-mass black holes. Yet, in X-rays, NGC 5408 X-1 outshines these systems by about the same factor.

Based on the timing of the oscillations and other characteristics of the emission, Strohmayer and Mushotzky conclude that NGC 5408 X-1 contains between 1,000 and 9,000 solar masses. This study appears in the October 1 issue of The Astrophysical Journal.

“For this mass range, a black hole’s event horizon — the part beyond which we cannot see — is between 3,800 and 34,000 miles across, or less than half of Earth’s diameter to about four times its size,” said Strohmayer.

If NGC 5408 X-1 is indeed actively gobbling gas to fuel its prodigious X-ray emission, the material likely flows to the black hole from an orbiting star. This is typical for stellar-mass black holes in our galaxy.

Strohmayer next enlisted the help of NASA’s Swift satellite to search for subtle variations of X-rays that would signal the orbit of NGC 5408 X-1’s donor star. “Swift uniquely provides both the X-ray imaging sensitivity and the scheduling flexibility to enable a search like this,” he added. Beginning in April 2008, Swift began turning its X-Ray Telescope toward NGC 5408 X-1 a couple of times a week as part of an on-going campaign.

Swift detects a slight rise and fall of X-rays every 115.5 days. “If this is indeed the orbital period of a stellar companion,” Strohmayer said, “then it’s likely a giant or supergiant star between three and five times the sun’s mass (1 solar mass is the mass of the Sun).” This study has been accepted for publication in a future issue of The Astrophysical Journal.

The Swift observations cover only about four orbital cycles, so continued observation is needed to confirm the orbital nature of the X-ray modulation.

“Astronomers have been studying NGC 5408 X-1 for a long time because it is one of the best candidates for an intermediate-mass black hole,” adds Philip Kaaret at the University of Iowa, who has studied the object at radio wavelengths but is unaffiliated with either study. “These new results probe what is happening close to the black hole and add strong evidence that it is unusually massive.”

Paper: Evidence for an Intermediate-Mass Black Hole in NGC 5804

Source: NASA

Could a Black Hole Fit in Your Computer or In Your Pocket?

Artist's illustration of a supermassive black hole. Image credit: NASA

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Some of the most frequently asked questions we get here at Universe Today and Astronomy Cast deal with black holes. Everyone wants to know what conditions would be like at the event horizon, or even inside a black hole. Answering those questions is difficult because so much about black holes is unknown. Black holes can’t be observed directly because their immense gravity won’t let light escape. But in just the past week, three different research teams have released their findings in their attempts to create black holes – or at least conditions analogous to them to advance our understanding.

Make Your Own Accretion Disk

A team of researchers from Osaka University in Japan wanted to sharpen their insights into the behavior of matter and energy in extreme conditions. What could be more extreme than the conditions of the swirling cloud of matter surrounding a black hole, known as the accretion disk? Their unique approach was to blast a plastic pellet with high-energy laser beams.

Accretion disks get crunched and heated by a black hole’s gravitational energy. Because of this, the disks glow in x-ray light. Analyzing the spectra of these x-rays gives researchers clues about the physics of the black hole.

However, scientists don’t know precisely how much energy is required to produce such x-rays. Part of the difficulty is a process called photoionization, in which the high-energy photons conveying the x-rays strip away electrons from atoms within the accretion disk. That lost energy alters the characteristics of the x-ray spectra, making it more difficult to measure precisely the total amount of energy being emitted.
After being hit with laser beams, a small plastic pellet (sunlike object) emits x-rays, some of which bombard a pellet of silicon (blue and purple).  Credit: Adapted from S. Fujioka et al., Nature Physics, Advance Online Publication
To get a better handle on how much energy those photoionized atoms consume, researchers zapped a tiny plastic pellet with 12 laser beams fired simultaneously and allowed some of the resulting radiation to blast a pellet of silicon, a common element in accretion disks.

The synchronized laser strikes caused the plastic pellet to implode, creating an extremely hot and dense core of gas, or plasma. That turned the pellet into “a source of [immensely powerful] x-rays similar to those from an accretion disk around a black hole,” says physicist and lead author Shinsuke Fujioka. The team said the x-rays photoionized the silicon, and that interaction mimicked the emissions observed in accretion disks. By measuring the energy lost from the photoionization, the researchers could measure total energy emitted from the implosion and use it to improve their understanding of the behavior of x-rays emitted by accretion disks.

The Portable Black Hole

Another group of physicists created a tiny device that can create a black hole by sucking up microwave light and converting it into heat. At just 22 centimeters across, the device can fit in your pocket.

The device uses ‘metamaterials’, specially engineered materials that can bend light in unusual ways. Previously, scientists have used such metamaterials to build ‘invisibility carpets’ and super-clear lenses. This latest black hole was made by Qiang Chen and Tie Jun Cui of Southeast University in Nanjing, China.

Real black holes use their huge mass to warp space around it. Light that travels too close to it can become trapped forever.

Metamaterial device that can create a black hole. Credit: Qiang Chen and Tie Jun Cui
Metamaterial device that can create a black hole. Credit: Qiang Chen and Tie Jun Cui

The new meta-black hole also bends light, but in a very different way. Rather than relying on gravity, the black hole uses a series of metallic ‘resonators’ arranged in 60 concentric circles. The resonators affect the electric and magnetic fields of a passing light wave, causing it to bend towards the centre of the hole. It spirals closer and closer to the black hole’s ‘core’ until it reaches the 20 innermost layers. Those layers are made of another set of resonators that convert light into heat. The result: what goes in cannot come out. “The light into the core is totally absorbed,” Cui said.

Not only is the device useful in studying black holes, but the research team hopes to create a version of the device that will suck up light of optical frequencies. If it works, it could be used in applications such as solar cells.

Read their paper here.

Black holes in your computer?

A supercomputer.
A supercomputer.

Could you create a black hole in your computer? Maybe if you had a really big one. Scientists at Rochester Institute of Technology (RIT) hope to make use of two of the fastest supercomputers in the world in their quest to “shine light” on black holes. The team was approved for grants and computing time to study the evolution of black holes and other objects with the “NewHorizons,” a cluster consisting of 85 nodes with four processors each, connected via an Infiniband network that passes data at 10-gigabyte-per-second speeds.

The team has created computer algorithms to simulate with mathematics and computer graphics what cannot be seen directly.

“It is a thrilling time to study black holes,” said Manuela Campanelli, center director. “We’re nearing the point where our calculations will be used to test one of the last unexplored aspects of Einstein’s General Theory of Relativity, possibly confirming that it properly describes the strongest gravitational fields in the universe.”

Sources: Science, Astronomy Magazine Technology Review Blog

What is an Event Horizon?

The Event Horizon Telescope (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — was designed to capture images of a black hole. In coordinated press conferences across the globe, EHT researchers revealed that they succeeded, unveiling the first direct visual evidence of the supermassive black hole in the centre of Messier 87 and its shadow. The shadow of a black hole seen here is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole’s boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across. While this may sound large, this ring is only about 40 microarcseconds across — equivalent to measuring the length of a credit card on the surface of the Moon. Although the telescopes making up the EHT are not physically connected, they are able to synchronize their recorded data with atomic clocks — hydrogen masers — which precisely time their observations. These observations were collected at a wavelength of 1.3 mm during a 2017 global campaign. Each telescope of the EHT produced enormous amounts of data – roughly 350 terabytes per day – which was stored on high-performance helium-filled hard drives. These data were flown to highly specialised supercomputers — known as correlators — at the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory to be combined. They were then painstakingly converted into an image using novel computational tools developed by the collaboration. Credit: Event Horizon Telescope Collaboration

The event horizon of a black hole is the boundary (‘horizon’) between its ‘outside’ and its ‘inside’; those outside cannot know anything about things (‘events’) which happen inside.

What an event horizon is – its behavior – is described by applying the equations of Einstein’s theory of General Relativity (GR); as of today, the theoretical predictions concerning event horizons can be tested in only very limited ways. Why? Because we don’t have any black holes we can study up close and personal (so to speak) … which is perhaps a very good thing!

If the black hole is not rotating, its event horizon has the shape of a sphere; it’s like a 2D surface over a 3D ball. Except, not quite; GR is a theory about spacetime, and contains many counter-intuitive aspects. For example, if you fall freely into a black hole (one sufficiently massive that tidal forces don’t rip you to pieces and smear you into a plastic-wrap thin layer of goo, a supermassive black hole for example), you won’t notice a thing as you pass through the event horizon … and that’s because it’s not the event horizon to you! In other words, the location of the event horizon of a black hole depends upon who is doing the observing (that word ‘relativity’ really does some heavy lifting, if you’ll excuse the pun), and as you fall (freely) into a black hole, the event horizon is always ahead of you.

You’ll often read that the event horizon is where the escape velocity is c, the speed of light; that’s a not-too-bad description, but it’s better to say that the path of any ray of light, inside the event horizon, can never make it beyond that horizon.

If you watch – from afar! – something fall into a black hole, you’ll see that it gets closer and closer, and light from it gets redder and redder (increasingly redshifted), but it never actually reaches the event horizon. And that’s the closest we’ve come to testing the theoretical predictions of event horizons; we see stuff – mass ripped from the normal star in a binary, say – heading down into its massive companion, but we never see any sign of it hitting anything (like a solid surface). In the next decade or so it might be possible to study event horizons much more closely, by imaging SgrA* (the supermassive black hole – SMBH – at the center of our galaxy), or the SMBH in M87, with extremely high resolution.

The Universe Today article Black Hole Event Horizon Measured is about just this kind of black hole-normal star binary, Black Hole Flares as it Gobbles Matter is about observations of matter falling into a SMBH, and Maximizing Survival Time Inside the Event Horizon of a Black Hole describes some of the weird things about event horizons.

There’s more on event horizons in the Astronomy Cast Relativity, Relativity and More Relativity episode, and the Black Hole Surfaces one.

Sources: NASA Science, NASA Imagine the Universe

Two Black Holes Play a Little One on One

NGC 6240. Image credit: X-ray: NASA/CXC/MIT/ C.Canizares, M.Nowak; Optical: NASA/STScI

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If black holes could communicate, there would likely be a lotta in your face trash talkin’ going on between these two merging black holes. This image of NGC 6240 contains new X-ray data from Chandra (shown in red, orange, and yellow) that has been combined with an optical image from the Hubble Space Telescope originally released in 2008. The two black holes are a mere 3,000 light years apart and are seen as the bright point-like sources in the middle of the image.

Scientists think these black holes are in such close proximity because they are in the midst of spiraling toward each other – a process that began about 30 million years ago. It is estimated that the two black holes will eventually drift together and merge into a larger black hole some tens or hundreds of millions of years from now.

Finding and studying merging black holes has become a very active field of research in astrophysics. Since 2002, there has been intense interest in follow-up observations of NGC 6240 by Chandra and other telescopes, as well as a search for similar systems. Understanding what happens when these exotic objects interact with one another remains an intriguing question for scientists.

The formation of multiple systems of supermassive black holes should be common in the Universe, since many galaxies undergo collisions and mergers with other galaxies, most of which contain supermassive black holes. It is thought that pairs of massive black holes can explain some of the unusual behavior seen by rapidly growing supermassive black holes, such as the distortion and bending seen in the powerful jets they produce. Also, pairs of massive black holes in the process of merging are expected to be the most powerful sources of gravitational waves in the Universe.

Click here for access to larger versions of this image.

Source: Marshall Space Flight Center

Hawking Radiation

Zero Gravity Flight
Stephen Hawking, weightless (courtesy Zero Gravity Corporation)

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When humans starve, they grow thin and eventually die; when a black hole starves, it too grows thin and dies … but it does so very spectacularly, in a burst of Hawking radiation.

At least that’s the way we understand it today (no black-hole-pining-away has yet been observed), and the theory may be wrong too.

Cosmologist, astrophysicist, and physicist Stephen Hawking showed, in 1974, that black holes should emit electromagnetic radiation with a black body spectrum; this process is also called black hole evaporation. In brief, this theoretical process works like this: particle-antiparticle pairs are constantly being produced and rapidly disappear (through annihilation); these pairs are virtual pairs, and their existence (if something virtual can be said to exist!) is a certain consequence of the Uncertainty Principle. Normally, we don’t ever see either the particle or antiparticle of these pairs, and only know of their existence through effects like the Casimir effect. However, if one such virtual pair pops into existence near the event horizon of a black hole, one may cross it while the other escapes; and the black hole thus loses mass. A long way away from the event horizon, this looks just like black body radiation.

It turns out that the smaller the mass a black hole has, the faster it will lose mass due to Hawking radiation; right at the end, the black hole disappears in an intense burst of gamma radiation (because the black hole’s temperature rises as it gets smaller). We won’t see any of the black holes in the Milky Way explode any time soon though … not only are they likely still gaining mass (from the cosmic microwave background, at least), but a one sol black hole would take over 10^67 years to evaporate (the universe is only 13 billion years old)!

There are many puzzles concerning black holes and Hawking radiation; for example, black hole evaporation via Hawking radiation seems to mean information is lost forever. The root cause of these puzzles is that quantum mechanics and General Relativity – the two most successful theories in physics, period – are incompatible, and we have no experiments or observations to help us work out how to resolve this incompatibility.

Colorado University’s Andrew Hamilton has a good introduction to this topic, as does Usenet Physics FAQ (often recognized by John Baez’ association with it).

Some Universe Today stories which include Hawking radiation are Synthetic Black Hole Event Horizon Created in UK Laboratory, How to Escape from a Black Hole, and When Black Holes Explode: Measuring the Emission from the Fifth Dimension.

Black Holes Big and Small, and The Large Hadron Collider and the Search for the Higgs-Boson are two Astronomy Casts relevant to Hawking radiation.

Sources:
Colorado University
ThinkQuest
University of California – Riverside

Blaming Black Holes for Gamma Ray Bursts

Artist's rendering of a black hole. Image Credit: NASA

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Black holes get a bad rap. Most people are afraid of them, and some think black holes might even destroy Earth. Now, scientists from the University of Leeds are blaming black holes for causing the most energetic and deadly outbursts in the universe: gamma ray bursts.

The conventional model for GRBs is that a narrow beam of intense radiation is released during a supernova event, as a rapidly rotating, high-mass star collapses to form a black hole. This involves plasma being heated by neutrinos in a disk of matter that forms around the black hole. A subclass of GRBs (the “short” bursts) appear to originate from a different process, possibly the merger of binary neutron stars.

But mathematicians at the University of Leeds have come up with a different explanation: the jets come directly from black holes, which can dive into nearby massive stars and devour them.

Their theory is based on recent observations by the Swift satellite which indicates that the central jet engine operates for up to 10,000 seconds – much longer than the neutrino model can explain.

The scientists believe that this is evidence for an electromagnetic origin of the jets, i.e. that the jets come directly from a rotating black hole, and that it is the magnetic stresses caused by the rotation that focus and accelerate the jet’s flow.

For the mechanism to operate the collapsing star has to be rotating extremely rapidly. This increases the duration of the star’s collapse as the gravity is opposed by strong centrifugal forces.

One particularly peculiar way of creating the right conditions involves not a collapsing star but a star invaded by its black hole companion in a binary system. The black hole acts like a parasite, diving into the normal star, spinning it with gravitational forces on its way to the star’s centre, and finally eating it from the inside.

“The neutrino model cannot explain very long gamma ray bursts and the Swift observations, as the rate at which the black hole swallows the star becomes rather low quite quickly, rendering the neutrino mechanism inefficient, but the magnetic mechanism can,” says Professor Komissarov from the School of Mathematics at the University of Leeds.

“Our knowledge of the amount of the matter that collects around the black hole and the rotation speed of the star allow us to calculate how long these long flashes will be – and the results correlate very well with observations from satellites,” he adds.

Source: EurekAlert

What is Sagittarius A*?

Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.

At the very heart of the Milky Way is a region known as Sagittarius A*. This region is known the be the home of a supermassive black hole with millions of times the mass of our own Sun. And with the discovery of this object, astronomers have turned up evidence that there are supermassive black holes at the centers most most spiral and elliptical galaxies.

The best observations of Sagittarius A*, using Very Long Baseline Interferometry (VLBI) radio astronomy have determined that it’s approximately 44 million km across (that’s just the distance of Mercury to the Sun). Astronomers have estimated that it contains 4.31 million solar masses.

Of course, astronomers haven’t actually seen the supermassive black hole itself. Instead, they have observed the motion of stars in the vicinity of Sagittarius A*. After 10 years of observations, astronomers detected the motion of a star that came within 17 light-hours distance from the supermassive black hole; that’s only 3 times the distance from the Sun to Pluto. Only a compact object with the mass of millions of stars would be able to make a high mass object like a star move in that trajectory.

The discovery of a supermassive black hole at the heart of the Milky Way helped astronomers puzzle out a different mystery: quasars. These are objects that shine with the brightness of millions of stars. We now know that quasars come from the radiation generated by the disks of material surrounding actively feeding supermassive black holes. Our own black hole is quiet today, but it could have been active in the past, and might be active again in the future.

Some astronomers have suggested other objects that could have the same density and gravity to explain Sagittarius A, but anything would quickly collapse down into a supermassive black hole within the lifetime of the Milky Way.

We have written many articles about Sagittarius A. Here’s an article about how the Milky Way’s black hole is sending out flares, and even more conclusive evidence after 16 years of observations.

Here’s an article from NASA back in 1996 showing how astronomers already suspected it was a supermassive black hole, and the original ESO press release announcing the discovery.

We have recorded an episode of Astronomy Cast all about the Milky Way. Give it a listen: Episode: 99 – The Milky Way

Source: Wikipedia