Astronomers Detect Signs of an Invisible Black Hole at the Center of the Milky Way

A team of astronomers led by Tomoharu Oka, a professor at Keio University in Japan, has found an enigmatic gas cloud, called CO-0.40-0.22, only 200 light years away from the center of the Milky Way. What makes CO-0.40-0.22 unusual is its surprisingly wide velocity dispersion: the cloud contains gas with a very wide range of speeds. The team found this mysterious feature with two radio telescopes, the Nobeyama 45-m Telescope in Japan and the ASTE Telescope in Chile, both operated by the National Astronomical Observatory of Japan.

To investigate the detailed structure, the team observed CO-0.40-0.22 with the Nobeyama 45-m Telescope again to obtain 21 emission lines from 18 molecules. The results show that the cloud has an elliptical shape and consists of two components: a compact but low density component with a very wide velocity dispersion of 100 km/s, and a dense component extending 10 light years with a narrow velocity dispersion.

What makes this velocity dispersion so wide? There are no holes inside of the cloud. Also, X-ray and infrared observations did not find any compact objects. These features indicate that the velocity dispersion is not caused by a local energy input, such as supernova explosions.

Figure. (a) The center of the Milky Way seen in the 115 and 346 GHz emission lines of carbon monoxide (CO). The white regions show the condensation of dense, warm gas. (b) Close-up intensity map around CO-0.40-0.22 seen in the 355 GHz emission line of HCN molecules. The ellipses indicate shell structures in the gas near C0-0.40-0.22. (c) Velocity dispersion diagram taken along the dotted line shown above. The wide velocity dispersion of 100 km/s in CO-0.40-0.22 stands out.

The team performed a simple simulation of gas clouds flung by a strong gravity source. In the simulation, the gas clouds are first attracted by the source and their speeds increase as they approach it, reaching maximum at the closest point to the object. After that the clouds continue past the object and their speeds decrease. The team found that a model using a gravity source with 100 thousand times the mass of the Sun inside an area with a radius of 0.3 light years provided the best fit to the observed data. “Considering the fact that no compact objects are seen in X-ray or infrared observations,” Oka, the lead author of the paper that appeared in the Astrophysical Journal Letters, explains “as far as we know, the best candidate for the compact massive object is a black hole.”

If that is the case, this is the first detection of an intermediate mass black hole. Astronomers already know about two sizes of black holes: stellar-mass black holes, formed after the gigantic explosions of very massive stars; and supermassive black holes (SMBH) often found at the centers of galaxies. The mass of SMBH ranges from several million to billions of times the mass of the Sun. A number of SMBHs have been found, but no one knows how the SMBHs are formed. One idea is that they are formed from mergers of many intermediate mass black holes. But this raises a problem because so far no firm observational evidence for intermediate mass black holes has been found. If the cloud CO-0.40-0.22, located only 200 light years away from Sgr A* (the 400 million solar mass SMBH at the center of the Milky Way), contains an intermediate mass black hole, it might support the intermediate mass black hole merger scenario of SMBH evolution.

(Left Top) CO-0.40-0.22 seen in the 87 GHz emission line of SiO molecules. (Left Bottom) Position-velocity diagram of CO-0.04-0.22 along the magenta line in the top panel. (Right Top) Simulation results for two moving clouds affected by a strong compact gravity source. The diagram shows changes in the positions and shapes of the clouds over a period of 900 thousand years (starting from t=0) at intervals of 100 thousand years. The axes are in parsecs (1 parsec = 3.26 light years). (Right Bottom) Comparison of observational results (in gray) and the simulation (red, magenta, and orange) in terms of the shape and velocity structure. The shapes and velocities of the clouds at 700 thousand years in the simulation match the observational results well.

These results open a new way to search for black holes with radio telescopes. Recent observations have revealed that there are a number of wide-velocity-dispersion compact clouds similar to CO-0.40-0.22. The team proposes that some of those clouds might contain black holes. A study suggested that there are 100 million black holes in the Milky Way Galaxy, but X-ray observations have only found dozens so far. Most of the black holes may be “dark” and very difficult to see directly at any wavelength. “Investigations of gas motion with radio telescopes may provide a complementary way to search for dark black holes” said Oka. “The on-going wide area survey observations of the Milky Way with the Nobeyama 45-m Telescope and high-resolution observations of nearby galaxies using the Atacama Large Millimeter/submillimeter Array (ALMA) have the potential to increase the number of black hole candidates dramatically.”

The observation results were published as Oka et al. “Signature of an Intermediate-Mass Black Hole in the Central Molecular Zone of Our Galaxy” in the Astrophysical Journal Letters issued on January 1, 2016. The research team members are Tomoharu Oka, Reiko Mizuno, Kodai Miura, Shunya Takekawa, all at Keio University.

This research is supported by the Japanese Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (C) No. 24540236.

PDF Copy of the Study: Signature of an Intermediate-Mass Black Hole in the Central Molecular Zone of Our Galaxy

NuSTAR finds cosmic clumpy doughnut around black hole

Galaxy 1068 can be seen in close-up in this view from NASA's Hubble Space Telescope. NuSTAR's high-energy X-rays eyes were able to obtain the best view yet into the hidden lair of the galaxy's central, supermassive black hole.

The most massive black holes in the universe are often encircled by thick, doughnut-shaped disks of gas and dust. This deep-space doughnut material ultimately feeds and nourishes the growing black holes tucked inside.

Until recently, telescopes weren't able to penetrate some of these doughnuts, also known as tori.

"Originally, we thought that some black holes were hidden behind walls or screens of material that could not be seen through," said Andrea Marinucci of the Roma Tre University in Italy, lead author of a new Monthly Notices of the Royal Astronomical Society study describing results from NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM-Newton space observatory.

With its X-ray vision, NuSTAR recently peered inside one of the densest of these doughnuts known to surround a supermassive black hole. This black hole lies at the center of a well-studied spiral galaxy called NGC 1068, located 47 million light-years away in the Cetus constellation.

The observations revealed a clumpy, cosmic doughnut.

"The rotating material is not a simple, rounded doughnut as originally thought, but clumpy," said Marinucci.

Doughnut-shaped disks of gas and dust around supermassive black holes were first proposed in the mid-1980s to explain why some black holes are hidden behind gas and dust, while others are not. The idea is that the orientation of the doughnut relative to Earth affects the way we perceive a black hole and its intense radiation. If the doughnut is viewed edge-on, the black hole is blocked. If the doughnut is viewed face-on, the black hole and its surrounding, blazing materials can be detected. This idea is referred to as the unified model because it neatly joins together the different black hole types, based solely upon orientation.

In the past decade, astronomers have been finding hints that these doughnuts aren't as smoothly shaped as once thought. They are more like defective, lumpy doughnuts that a doughnut shop might throw away.

The new discovery is the first time this clumpiness has been observed in an ultra-thick doughnut, and supports the idea that this phenomenon may be common. The research is important for understanding the growth and evolution of massive black holes and their host galaxies.

"We don't fully understand why some supermassive black holes are so heavily obscured, or why the surrounding material is clumpy," said co-author Poshak Gandhi of the University of Southampton in the United Kingdom. "This is a subject of hot research."

Both NuSTAR and XMM-Newton observed the supermassive black hole in NGC 1068 simultaneously on two occasions between 2014 to 2015. On one of those occasions, in August 2014, NuSTAR observed a spike in brightness. NuSTAR observes X-rays in a higher-energy range than XMM-Newton, and those high-energy X-rays can uniquely pierce thick clouds around the black hole. The scientists say the spike in high-energy X-rays was due to a clearing in the thickness of the material entombing the supermassive black hole.

"It's like a cloudy day, when the clouds partially move away from the sun to let more light shine through," said Marinucci.

NGC 1068 is well known to astronomers as the first black hole to give birth to the unification idea. "But it is only with NuSTAR that we now have a direct glimpse of its black hole through such clouds, albeit fleeting, allowing a better test of the unification concept," said Marinucci.

The team says that future research will address the question of what causes the unevenness in doughnuts. The answer could come in many flavors. It's possible that a black hole generates turbulence as it chomps on nearby material. Or, the energy given off by young stars could stir up turbulence, which would then percolate outward through the doughnut. Another possibility is that the clumps may come from material falling onto the doughnut. As galaxies form, material migrates toward the center, where the density and gravity is greatest. The material tends to fall in clumps, almost like a falling stream of water condensing into droplets as it hits the ground.

"We'd like to figure out if the unevenness of the material is being generated from outside the doughnut, or within it," said Gandhi.

"These coordinated observations with NuSTAR and XMM-Newton show yet again the exciting science possible when these satellites work together," said Daniel Stern, NuSTAR project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.

For more information on NuSTAR, visit:

http://www.nasa.gov/nustar

http://www.nustar.caltech.edu/

Monster black hole discovered at cosmic dawn

This is an artist's impression of a quasar with a supermassive black hole in the distant universe.

Scientists have discovered the brightest quasar in the early universe, powered by the most massive black hole yet known at that time. The international team led by astronomers from Peking University in China and from the University of Arizona announce their findings in the scientific journal Nature on Feb. 26.

The discovery of this quasar, named SDSS J0100+2802, marks an important step in understanding how quasars, the most powerful objects in the universe, have evolved from the earliest epoch, only 900 million years after the Big Bang, which is thought to have happened 13.7 billion years ago. The quasar, with its central black hole mass of 12 billion solar masses and the luminosity of 420 trillion suns, is at a distance of 12.8 billion light-years from Earth.
The discovery of this ultraluminous quasar also presents a major puzzle to the theory of black hole growth at early universe, according to Xiaohui Fan, Regents' Professor of Astronomy at the UA's Steward Observatory, who co-authored the study.
"How can a quasar so luminous, and a black hole so massive, form so early in the history of the universe, at an era soon after the earliest stars and galaxies have just emerged?" Fan said. "And what is the relationship between this monster black hole and its surrounding environment, including its host galaxy?
"This ultraluminous quasar with its supermassive black hole provides a unique laboratory to the study of the mass assembly and galaxy formation around the most massive black holes in the early universe."
The quasar dates from a time close to the end of an important cosmic event that astronomers referred to as the "epoch of reionization": the cosmic dawn when light from the earliest generations of galaxies and quasars is thought to have ended the "cosmic dark ages" and transformed the universe into how we see it today.
Discovered in 1963, quasars are the most powerful objects beyond our Milky Way galaxy, beaming vast amounts of energy across space as the supermassive black hole in their center sucks in matter from its surroundings. Thanks to the new generation of digital sky surveys, astronomers have discovered more than 200,000 quasars, with ages ranging from 0.7 billion years after the Big Bang to today.
Shining with the equivalent of 420 trillion suns, the new quasar is seven times brighter than the most distant quasar known (which is 13 billion years away). It harbors a black hole with mass of 12 billion solar masses, proving it to be the most luminous quasar with the most massive black hole among all the known high redshift (very distant) quasars.
"By comparison, our own Milky Way galaxy has a black hole with a mass of only 4 million solar masses at its center; the black hole that powers this new quasar is 3,000 time heavier," Fan said.
Feige Wang, a doctoral student from Peking University who is supervised jointly by Fan and Prof. Xue-Bing Wu at Peking University, the study's lead author, initially spotted this quasar for further study.
"This quasar was first discovered by our 2.4-meter Lijiang Telescope in Yunnan, China, making it the only quasar ever discovered by a 2-meter telescope at such distance, and we're very proud of it," Wang said. "The ultraluminous nature of this quasar will allow us to make unprecedented measurements of the temperature, ionization state and metal content of the intergalactic medium at the epoch of reionization."
Following the initial discovery, two telescopes in southern Arizona did the heavy lifting in determining the distance and mass of the black hole: the 8.4-meter Large Binocular Telescope, or LBT, on Mount Graham and the 6.5-meter Multiple Mirror Telescope, or MMT, on Mount Hopkins. Additional observations with the 6.5-meter Magellan Telescope in Las Campanas Observatory, Chile, and the 8.2-meter Gemini North Telescope in Mauna Kea, Hawaii, confirmed the results.
"This quasar is very unique," said Xue-Bing Wu, a professor of the Department of Astronomy, School of Physics at Peking University and the associate director of the Kavli Institute of Astronomy and Astrophysics. "Just like the brightest lighthouse in the distant universe, its glowing light will help us to probe more about the early universe."
Wu leads a team that has developed a method to effectively select quasars in the distant universe based on optical and near-infrared photometric data, in particular using data from the Sloan Digital Sky Survey and NASA's Wide-Field Infrared Explorer, or WISE, satellite.
"This is a great accomplishment for the LBT," said Fan, who chairs the LBT Scientific Advisory Committee and also discovered the previous record holders for the most massive black hole in the early universe, about a fourth of the size of the newly discovered object. "The especially sensitive optical and infrared spectrographs of the LBT provided the early assessment of both the distance of the quasars and the mass of the black hole at the quasar's center."
For Christian Veillet, director of the Large Binocular Telescope Observatory, or LBTO, this discovery demonstrates both the power of international collaborations and the benefit of using a variety of facilities spread throughout the world.
"This result is particularly gratifying for LBTO, which is well on its way to full nighttime operations," Veillet said. "While in this case the authors used two different instruments in series, one for visible light spectroscopy and one for near-infrared imaging, LBTO will soon offer a pair of instruments that can be used simultaneously, effectively doubling the number of observations possible in clear skies and ultimately creating even more exciting science."
To further unveil the nature of this remarkable quasar, and to shed light on the physical processes that led to the formation of the earliest supermassive black holes, the research team will carry out further investigations on this quasar with more international telescopes, including the Hubble Space Telescope and the Chandra X-ray Telescope.

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Story Source:
The above story is based on materials provided by University of Arizona. The original article was written by Christian Veillet/LBTO and Daniel Stolte/University Relations – Communications. Note: Materials may be edited for content and length.

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Journal Reference:

1. Xue-Bing Wu, Feige Wang, Xiaohui Fan, Weimin Yi, Wenwen Zuo, Fuyan Bian, Linhua Jiang, Ian D. McGreer, Ran Wang, Jinyi Yang, Qian Yang, David Thompson, Yuri Beletsky. An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30. Nature, 2015; 518 (7540): 512 DOI: 10.1038/nature14241

Yes, black holes exist in gravitational theories with unbounded speeds of propagation

Lorentz invariance (LI) is a cornerstone of modern physics, and strongly supported by observations.

In fact, all the experiments carried out so far are consistent with it, and no evidence to show that such a symmetry needs to be broken at a certain energy scale. Nevertheless, there are various reasons to construct gravitational theories with broken LI. In particular, our understanding of space-times at Plank scale is still highly limited, and the renomalizability and unitarity of gravity often lead to the violation of LI.
One concrete example is the Horava theory of quantum gravity, in which the LI is broken in the ultraviolet (UV), and the theory can include higher-dimensional spatial derivative operators, so that the UV behavior is dramatically improved and can be made (power-counting) renormalizable.
On the other hand, the exclusion of high-dimensional time derivative operators prevents the ghost instability, whereby the unitarity of the theory -- a problem that has been faced since 1977 [ K.S. Stelle, Phys. Rev. D16, 953 (1977)] -- is assured. In the infrared (IR) the lower dimensional operators take over, whereby a healthy low-energy limit is presumably resulted.
However, once LI is broken different species of particles can travel with different velocities, and in certain theories , such as the Horava theory mentioned above, they can be even arbitrarily large. This suggests that black holes may not exist at all in such theories, as any signal initially trapped inside a horizon can penetrate it and propagate to infinity, as long as the signal has sufficiently large velocity (or energy). This seems in a sharp conflict with current observations, which strongly suggest that black holes exist in our universe [R. Narayan and J.E. MacClintock, Mon. Not. R. Astron. Soc., 419, L69 (2012)].
A potential breakthrough was made recently by Blas and Sibiryakov [D. Blas and S. Sibiryakov, Phys. Rev. D84, 124043 (2011)], who found that there still exist absolute causal boundaries, the so-called universal horizons, and particles even with infinitely large velocities would just move around on these boundaries and cannot escape to infinity.
This has immediately attracted lot of attention. In particular, it was shown that the universal horizon radiates like a blackbody at a fixed temperature, and obeys the first law of black hole mechanics [P. Berglund, J. Bhattacharyya, and D. Mattingly, Phys. Rev. D85, 124019 (2012); Phys. Rev. Lett. 110, 071301 (2013)]. The main idea is as follows: In a given space-time, a globally timelike foliation parametrized by a scalar field, the so-called khronon, might exist.
Then, there is a surface at which the khronon diverges, while physically nothing singular happens there, including the metric and the space-time. Given that the khronon defines an absolute time, any object crossing this surface from the interior would necessarily also move back in absolute time, which is something forbidden by the definition of the causality of the theory. Thus, even particles with superluminal velocities cannot penetrate this surface, once they are trapped inside it.
In all studies of universal horizons carried out so far the khronon is part of the gravitational theory involved. To generalize the conception of the universal horizons to any gravitational theory with broken LI, recently Lin, Abdalla, Cai and Wang promoted the khronon to a test field, a similar role played by a Killing vector, so its existence does not affect the given space-time, but defines the properties of it.
By this way, such a field is no longer part of the underlaid gravitational theory and it may or may not exist in a given space-time, depending on the properties of the space-time considered. Then, they showed that the universal horizons indeed exist, by constructing concrete static charged solutions of the Horava gravity. More important, they showed that such horizons exist not only in the IR limit of the theory, as has been considered so far in the literature, but also in the full Horava theory of gravity, that is, when high-order operators are not negligible.

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Story Source:
The above story is based on materials provided by World Scientific. Note: Materials may be edited for content and length.

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Journal Reference:

1. Kai Lin, Elcio Abdalla, Rong-Gen Cai, Anzhon Wang. Universal horizons and black holes in gravitational theories with broken Lorentz symmetry. International Journal of Modern Physics D, 2014; 1443004 DOI: 10.1142/S0218271814430044

Yes, black holes exist in gravitational theories with unbounded speeds of propagation

Lorentz invariance (LI) is a cornerstone of modern physics, and strongly supported by observations.

In fact, all the experiments carried out so far are consistent with it, and no evidence to show that such a symmetry needs to be broken at a certain energy scale. Nevertheless, there are various reasons to construct gravitational theories with broken LI. In particular, our understanding of space-times at Plank scale is still highly limited, and the renomalizability and unitarity of gravity often lead to the violation of LI.
One concrete example is the Horava theory of quantum gravity, in which the LI is broken in the ultraviolet (UV), and the theory can include higher-dimensional spatial derivative operators, so that the UV behavior is dramatically improved and can be made (power-counting) renormalizable.
On the other hand, the exclusion of high-dimensional time derivative operators prevents the ghost instability, whereby the unitarity of the theory -- a problem that has been faced since 1977 [ K.S. Stelle, Phys. Rev. D16, 953 (1977)] -- is assured. In the infrared (IR) the lower dimensional operators take over, whereby a healthy low-energy limit is presumably resulted.
However, once LI is broken different species of particles can travel with different velocities, and in certain theories , such as the Horava theory mentioned above, they can be even arbitrarily large. This suggests that black holes may not exist at all in such theories, as any signal initially trapped inside a horizon can penetrate it and propagate to infinity, as long as the signal has sufficiently large velocity (or energy). This seems in a sharp conflict with current observations, which strongly suggest that black holes exist in our universe [R. Narayan and J.E. MacClintock, Mon. Not. R. Astron. Soc., 419, L69 (2012)].
A potential breakthrough was made recently by Blas and Sibiryakov [D. Blas and S. Sibiryakov, Phys. Rev. D84, 124043 (2011)], who found that there still exist absolute causal boundaries, the so-called universal horizons, and particles even with infinitely large velocities would just move around on these boundaries and cannot escape to infinity.
This has immediately attracted lot of attention. In particular, it was shown that the universal horizon radiates like a blackbody at a fixed temperature, and obeys the first law of black hole mechanics [P. Berglund, J. Bhattacharyya, and D. Mattingly, Phys. Rev. D85, 124019 (2012); Phys. Rev. Lett. 110, 071301 (2013)]. The main idea is as follows: In a given space-time, a globally timelike foliation parametrized by a scalar field, the so-called khronon, might exist.
Then, there is a surface at which the khronon diverges, while physically nothing singular happens there, including the metric and the space-time. Given that the khronon defines an absolute time, any object crossing this surface from the interior would necessarily also move back in absolute time, which is something forbidden by the definition of the causality of the theory. Thus, even particles with superluminal velocities cannot penetrate this surface, once they are trapped inside it.
In all studies of universal horizons carried out so far the khronon is part of the gravitational theory involved. To generalize the conception of the universal horizons to any gravitational theory with broken LI, recently Lin, Abdalla, Cai and Wang promoted the khronon to a test field, a similar role played by a Killing vector, so its existence does not affect the given space-time, but defines the properties of it.
By this way, such a field is no longer part of the underlaid gravitational theory and it may or may not exist in a given space-time, depending on the properties of the space-time considered. Then, they showed that the universal horizons indeed exist, by constructing concrete static charged solutions of the Horava gravity. More important, they showed that such horizons exist not only in the IR limit of the theory, as has been considered so far in the literature, but also in the full Horava theory of gravity, that is, when high-order operators are not negligible.

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Story Source:
The above story is based on materials provided by World Scientific. Note: Materials may be edited for content and length.

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Journal Reference:

1. Kai Lin, Elcio Abdalla, Rong-Gen Cai, Anzhon Wang. Universal horizons and black holes in gravitational theories with broken Lorentz symmetry. International Journal of Modern Physics D, 2014; 1443004 DOI: 10.1142/S0218271814430044

Record-breaking black hole outburst detected

An image of a simulation of the gas cloud's encounter with Sgr A*. The blue lines mark the orbits of the so-called "S" stars that are in close orbits around the supermassive black hole

Last September, after years of watching, a team of scientists led by Amherst College astronomy professor Daryl Haggard observed and recorded the largest-ever flare in X-rays from a supermassive black hole at the center of the Milky Way. The astronomical event, which was detected by NASA's Chandra X-ray Observatory, puts the scientific community one step closer to understanding the nature and behavior of supermassive black holes.

Haggard and her colleagues discussed the flare today during this year's meeting of the American Astronomical Society in Seattle.
Supermassive black holes are the largest of black holes, and all large galaxies have one. The one at the center of our galaxy, the Milky Way, is called Sagittarius A* (or, Sgr A*, as it is called), and scientists estimate that it contains about four and a half million times the mass of our Sun.
Scientists working with Chandra have observed Sgr A* repeatedly since the telescope was launched into space in 1999. Haggard and fellow astronomers were originally using Chandra to see if Sgr A* would consume parts of a cloud of gas, known as G2.
"Unfortunately, the G2 gas cloud didn't produce the fireworks we were hoping for when it got close to Sgr A*," she said. "However, nature often surprises us and we saw something else that was really exciting."
Haggard and her team detected an X-ray outburst last September that was 400 times brighter than the usual X-ray output from Sgr A*. This "megaflare" was nearly three times brighter than the previous record holder that was seen in early 2012. A second enormous X-ray flare, 200 times brighter than Sgr A* in its quiet state, was observed with Chandra on October 20, 2014.
Haggard and her team have two main ideas about what could be causing Sgr A* to erupt in this extreme way. One hypothesis is that the gravity of the supermassive black hole has torn apart a couple of asteroids that wandered too close. The debris from such a "tidal disruption" would become very hot and produce X-rays before disappearing forever across the black hole's point of no return (called the "event horizon").
"If an asteroid was torn apart, it would go around the black hole for a couple of hours -- like water circling an open drain -- before falling in," said colleague and co-principal investigator Fred Baganoff of the Massachusetts Institute of Technology in Cambridge, MA. "That's just how long we saw the brightest X-ray flare last, so that is an intriguing clue for us to consider."
If that theory holds up, it means astronomers have found evidence for the largest asteroid ever to be torn apart by the Milky Way's black hole.
Another, different idea is that the magnetic field lines within the material flowing towards Sgr A* are packed incredibly tightly. If this were the case, these field lines would occasionally interconnect and reconfigure themselves. When this happens, their magnetic energy is converted into the energy of motion, heat and the acceleration of particles -- which could produce a bright X-ray flare. Such magnetic flares are seen on the Sun, and the Sgr A* flares have a similar pattern of brightness levels to the solar events.
"At the moment, we can't distinguish between these two very different ideas," said Haggard. "It's exciting to identify tensions between models and to have a chance to resolve them with present and future observations."
In addition to the giant flares, Haggard and her team also collected more data on a magnetar -- a neutron star with a strong magnetic field -- located close to Sgr A*. This magnetar is undergoing a long X-ray outburst, and the Chandra data are allowing astronomers to better understand this unusual object.
As for the G2: Astronomers estimate that the gas cloud made its closest approach -- still about 15 billion miles away from the edge of the black hole -- in the spring of 2014. The researchers estimate the record breaking X-ray flares were produced about a hundred times closer to the black hole, making it very unlikely that the Chandra flares were associated with G2.

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Story Source:
The above story is based on materials provided by Amherst College. Note: Materials may be edited for content and length.

Black holes spins faster

black holes spins faster | sci-english.blogspot.com

Two UK astronomers have found that the giant black holes in the centre of galaxies are on average spinning faster than at any time in the history of the Universe. Dr Alejo Martinez-Sansigre of the University of Portsmouth and Prof. Steve Rawlings of the University of Oxford made the new discovery by using radio, optical and X-ray data. They publish their findings in the journal Monthly Notices of the Royal Astronomical Society.

There is strong evidence that every galaxy has a black hole in its centre. These black holes have masses of between a million and a billion Suns and so are referred to as 'supermassive'. They cannot be seen directly, but material swirls around the black hole in a so-called accretion disk before its final demise. That material can become very hot and emit radiation including X-rays that can be detected by space-based telescopes whilst associated radio emission can be detected by telescopes on the ground.
As well as radiation, twin jets are often associated with black holes and their accretion disks. There are many factors that can cause these jets to be produced, but the spin of the supermassive black hole is believed to be important. However, there are conflicting predictions about how the spins of the black holes should be evolving and until now this evolution was not well understood.
Dr Martinez-Sansigre and Professor Rawlings compared theoretical models of spinning black holes with radio, optical and X-ray observations made using a variety of instruments and found that the theories can explain very well the population of supermassive black holes with jets.
Using the radio observations, the two astronomers were able to sample the population of black holes, deducing the spread of the power of the jets. By estimating how they acquire material (the accretion process) the two scientists could then infer how quickly these objects are spinning.
The observations also give information on how the spins of supermassive black holes have evolved. In the past, when the Universe was half its the present size, practically all of the supermassive black holes had very low spins, whereas nowadays a fraction of them have very high spins. So on average, supermassive black holes are spinning faster than ever before.
This is the first time that the evolution of the spin of the supermassive black holes has been constrained and it suggests that those supermassive black holes that grow by swallowing matter will barely spin, while those that merge with other black holes will be left spinning rapidly.
Commenting on the new results, Dr Martinez-Sansigre said: "The spin of black holes can tell you a lot about how they formed. Our results suggest that in recent times a large fraction of the most massive black holes have somehow spun up. A likely explanation is that they have merged with other black holes of similar mass, which is a truly spectacular event, and the end product of this merger is a faster spinning black hole."
Professor Rawlings adds: "Later this decade we hope to test our idea that these supermassive black holes have been set spinning relatively recently. Black hole mergers cause predictable distortions in space and time -- so-called gravitational waves. With so many collisions, we expect there to be a cosmic background of gravitational waves, something that will change the timing of the pulses of radio waves that we detect from the remnants of massive stars known as pulsars.
If we are right, this timing change should be picked up by the Square Kilometre Array, the giant radio observatory due to start operating in 2019."

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Story Source:
The above story is based on materials provided by Royal Astronomical Society (RAS). Note: Materials may be edited for content and length.

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Journal Reference:

1. Alejo Martinez-Sansigre and Steve Rawlings. Observational constraints on the spin of the most massive black holes from radio observations. Monthly Notices of the Royal Astronomical Society, 2011 (in press) [link]

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