The Violent Variations of Black Holes

Artist impression of a black hole. Credit: ESO/L. Calçada

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What is the environment around a black hole really like? Astronomers are getting a better idea by observing the light coming from the accretion disk surrounding black holes. The light is not constant — it flares, sputters and sparkles – and this flickering provides new and surprising insights into the colossal amount of energy emanating from around black holes. By mapping out how well the variations in visible light match those in X-rays on very short timescales, astronomers have shown that magnetic fields must play a crucial role in the way black holes swallow matter.

“The rapid flickering of light from a black hole is most commonly observed at X-ray wavelengths,” says Poshak Gandhi, who led the international team that reports these results. “This new study is one of only a handful to date that also explore the fast variations in visible light, and, most importantly how these fluctuations relate to those in X-rays.”

The observations tracked the flickering of the black holes simultaneously using two different instruments, one on the ground and one in space. The X-ray data were taken using NASA’s Rossi X-ray Timing Explorer satellite. The visible light was collected with the high speed camera ULTRACAM, a visiting instrument at ESO’s Very Large Telescope (VLT), recording up to 20 images a second. ULTRACAM was developed by team members Vik Dhillon and Tom Marsh. “These are among the fastest observations of a black hole ever obtained with a large optical telescope,” says Dhillon.

To their surprise, astronomers discovered that the brightness fluctuations in the visible light were even more rapid than those seen in X-rays. In addition, the visible-light and X-ray variations were found not to be simultaneous, but to follow a repeated and remarkable pattern: just before an X-ray flare the visible light dims, and then surges to a bright flash for a tiny fraction of a second before rapidly decreasing again.

Watch a movie of the fluctuations.

None of this radiation emerges directly from the black hole, but from the intense energy flows of electrically charged matter in its vicinity. The environment of a black hole is constantly being reshaped by a competing forces such as gravity, magnetism and explosive pressure. As a result, light emitted by the hot flows of matter varies in brightness in a muddled and haphazard way. “But the pattern found in this new study possesses a stable structure that stands out amidst an otherwise chaotic variability, and so, it can yield vital clues about the dominant underlying physical processes in action,” says team member Andy Fabian.

The visible-light emission from the neighborhoods of black holes was widely thought to be a secondary effect, with a primary X-ray outburst illuminating the surrounding gas that subsequently shone in the visible range. But if this were so, any visible-light variations would lag behind the X-ray variability, and would be much slower to peak and fade away. “The rapid visible-light flickering now discovered immediately rules out this scenario for both systems studied,” asserts Gandhi. “Instead the variations in the X-ray and visible light output must have some common origin, and one very close to the black hole itself.”

Strong magnetic fields represent the best candidate for the dominant physical process. Acting as a reservoir, they can soak up the energy released close to the black hole, storing it until it can be discharged either as hot (multi-million degree) X-ray emitting plasma, or as streams of charged particles travelling at close to the speed of light. The division of energy into these two components can result in the characteristic pattern of X-ray and visible-light variability.

Papers on this research: Here and Here

Source: ESO

Podcast: Black Hole Surfaces, Magnetic Field Strengths, and the Speed of Gravitons

Artist impression of a black hole.

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As you know, we wanted to answer listener questions regularly, but we found it was taking away from the regular weekly episodes of Astronomy Cast. So we’ve decided to just split it up and run the question shows separately from the regular Astronomy Cast episodes. If this works out, you might be able to enjoy twice the number of Astronomy Cast episodes. So if you’ve got a question on a topic we cover in a recent show, or you just have a general astronomy question, send it in to [email protected]. Either by email, or record your question and email in the audio file.

Click here to download the episode.

Or subscribe to: astronomycast.com/podcast.xml with your podcatching software.

Black Hole Surfaces, Magnetic Field Strengths, and the Speed of Gravitons show notes.

Super-massive and Small Black Holes Both Suck

Artist's impression of material falling into a super-massive black hole together with the average shape of the periodic X-ray signal from REJ1034+396. Credit: Aurore Simonnet, Sonoma State University

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Sorry, couldn’t resist that title. Astronomers studying black holes are able to “see” them due to the fact that the gas getting sucked in gets extremely hot and emits X-rays. These X-ray pulses are commonly seen among smaller black holes, but until now, had not been detected from super-massive black holes. But astronomers using the XMM Newton X-ray satellite have discovered a strong X-ray pulse emitting from a giant black hole in a galaxy 500 million light years from Earth, created by gas being sucked in by gravity. “Scientists have been looking for such behaviour for the past 20 years and our discovery helps us begin to understand more about the activity around such black holes as they grow,” said Dr. Marek Gierlinski from Durham University. Gierlinski and his colleagues say this finding is the “missing link” between small and super-massive black holes.

The astronomers were looking at the center of the galaxy REJ1034+396 galaxy and found that X-rays are being emitted as a regular signal from the super-massive black hole. They say the frequency of the pulse is related to the size of the black hole. “Such signals are a well known feature of smaller black holes in our Galaxy when gas is pulled from a companion star,” said Gierlinski. “The really interesting thing is that we have now established a link between these light-weight black holes and those millions of times as heavy as our Sun.”

The scientists hope future research will tell them why some super-massive black holes show this behavior while others do not. Most galaxies, including the Milky Way, are believed to contain super-massive black holes at their centers.

The researchers, who publish their findings in the journal Nature on September 18, say their discovery will increase the understanding of how gas behaves before falling on to a black hole as it feeds and develops.

Source: Durham University

Black Holes Can Only Get So Big

Ultra-massive black holes, which lurk in the centers of huge galaxy clusters like the one above, seem to have an upper mass limit of 10 billion times that of the Sun. (Credit: NASA)

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Black holes are thought to exist throughout the universe, with the largest and most massive found at the centers of the largest galaxies. These supermassive black holes have been shown to have masses upwards of one billion times that of our own Sun. But an astronomer studying black holes says there’s an upper limit to how big a black hole can get. Priyamvada Natarajan, an associate professor of astronomy and physics at Yale University has shown that even the biggest of these gravitational monsters can’t keep growing forever. Instead, they appear to curb their own growth – once they accumulate about 10 billion times the mass of the Sun.

These ultra-massive black holes, found at the centers of giant elliptical galaxies in huge galaxy clusters, are the biggest in the known universe. Even the large black hole at the center of our own Milky Way galaxy is thousands of times less massive than these behemoths. But these gigantic black holes, which accumulate mass by sucking in matter from neighboring gas, dust and stars, seem unable to grow beyond this limit regardless of where – and when – they appear in the universe. “It’s not just happening today,” said Natarajan. “They shut off at every epoch in the universe.”

Natarajan’s study is the first time an upper mass limit has been derived for black holes. Natarajan used existing optical and X-ray data of these ultra-massive black holes to show that, in order for those various observations to be consistent, the black holes must essentially shut off at some point in their evolution.

Artist's conception of a black hole.  Credit:  U of Tel Aviv
Artist's conception of a black hole. Credit: U of Tel Aviv

One possible explanation, says Natarajan, is that the black holes eventually reach the point when they radiate so much energy as they consume their surroundings that they end up interfering with the very gas supply that feeds them, which may interrupt nearby star formation. The new findings have implications for the future study of galaxy formation, since many of the largest galaxies in the universe appear to co-evolve along with the black holes at their centers.

“Evidence has been mounting for the key role that black holes play in the process of galaxy formation,” said Natarajan. “But it now appears that they are likely the prima donnas of this space opera.”

Source: PhysOrg

Astronomers Link Telescopes to Zoom In On Milky Way’s Black Hole

Computer simulation of what a "hot spot" of gas orbiting a black hole would look like in an extremely high-resolution image. Credit: Avery Broderick (CITA) & Avi Loeb (CfA)

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An international team of astronomers has obtained the closest views ever of what is believed to be a super-massive black hole at the center of the Milky Way galaxy. The astronomers linked together radio dishes in Hawaii, Arizona and California to create a virtual telescope more than 2,800 miles across that is capable of seeing details more than 1,000 times finer than the Hubble Space Telescope. The target of the observations was the source known as Sagittarius A* (“A-star”), long thought to mark the position of a black hole whose mass is 4 million times that of the sun.

Using a technique called Very Long Baseline Interferometry (VLBI), the astronomers studied the radio waves coming from Sagittarius A*. In VLBI, signals from multiple astronomy telescopes are combined to create the equivalent of a single giant telescope, as large as the separation between the facilities. As a result, VLBI yields exquisitely sharp resolution.

They detected structure at a tiny angular scale of 37 micro-arcseconds – the equivalent of a baseball seen on the surface of the moon, 240,000 miles distant. These observations are among the highest resolution ever done in astronomy.

“This technique gives us an unmatched view of the region near the Milky Way’s central black hole,” said Sheperd Doeleman of MIT, first author of the study that will be published in the Sept. 4 issue of the journal Nature.

Computer animation illustrating a spinning black hole.  Credit:  NASA
Computer animation illustrating a spinning black hole. Credit: NASA

Though Sagittarius A* was discovered three decades ago, the new observations for the first time have an angular resolution, or ability to observe small details, that is matched to the size of the black hole “event horizon” — the region inside of which nothing, including light, can ever escape.

With three telescopes, the astronomers could only vaguely determine the shape of the emitting region. Future investigations will help answer the question of what, precisely, they are seeing: a glowing corona around the black hole, an orbiting “hot spot,” or a jet of material. Nevertheless, their result represents the first time that observations have gotten down to the scale of the black hole itself, which has a “Schwarzschild radius” of 10 million miles.

The concept of black holes, objects so dense that their gravitational pull prevents anything including light itself from ever escaping their grasp, has long been hypothesized, but their existence has not yet been proved conclusively. Astronomers study black holes by detecting the light emitted by matter that heats up as it is pulled closer to the event horizon. By measuring the size of this glowing region at the Milky Way center, the new observations have revealed the highest density yet for the concentration of matter at the center of our galaxy, which “is important new evidence supporting the existence of black holes,” said Doeleman.

“This result, which is remarkable in and of itself, also confirms that the 1.3-mm VLBI technique has enormous potential, both for probing the galactic center and for studying other phenomena at similar small scales,” said co-author Jonathan Weintroub.

The team plans to expand their work by developing novel instrumentation to make more sensitive 1.3-mm observations possible. They also hope to develop additional observing stations, which would provide additional baselines (pairings of two telescope facilities at different locations) to enhance the detail in the picture. Future plans also include observations at shorter, 0.85-mm wavelengths; however, such work will be even more challenging for many reasons, including stretching the capabilities of the instrumentation, and the requirement for a coincidence of excellent weather conditions at all sites.

Source: Harvard Smithsonian press release

Is This Black Hole Available in Size Medium?

Can Medium Sized Black Holes be Found in Globular Clusters? Credit: NASA/JPL-Caltech/ NOAO/AURA/NSF

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Black holes are sometimes huge – supermassive as they are called, billions of times the mass of our sun. Other times they are petite with just a few times the sun’s mass. But do black holes also come in size medium? A new study suggests that, most likely, the answer is no. Astronomers have long suspected that the best place to find a medium-mass black hole would be at the core of a miniature galaxy-like object called a globular cluster. Yet nobody has been able to find one conclusively. And now, a team of astronomers has thoroughly examined a globular cluster called RZ2109 and determined that it cannot possess a medium black hole, leading researchers to believe that black holes don’t come in medium, or at most are very rare.

“Some theories say that small black holes in globular clusters should sink down to the center and form a medium-sized one, but our discovery suggests this isn’t true,” said Daniel Stern of NASA’s Jet Propulsion Laboratory. Stern is second author of a study detailing the findings in the Aug. 20 issue of Astrophysical Journal. The lead author is Stephen Zepf of Michigan State University, East Lansing.

Black holes are incredibly dense points of matter, whose gravity prevents even light from escaping. The least massive black holes known are about 10 times the mass of the sun and form when massive stars blow up in supernova explosions. The heftiest black holes are up to billions of times the mass of the sun and lie deep in the bellies of almost all galaxies.

That leaves black holes of intermediate mass, which were thought to be buried at the cores of globular clusters. Globular clusters are dense collections of millions of stars, which reside within galaxies containing hundreds of billions of stars. Theorists argue that a globular cluster should have a scaled down version of a galactic black hole. Such objects would be about 1,000 to 10,000 times the mass of the sun, or medium in size on the universal scale of black holes.

The research team used the Keck Observatory on Mauna Kea in Hawaii to look at the spectrum of the cluster, which revealed that the black hole is petite, with roughly 10 times the mass of our sun.
According to theory, a cluster with a small black hole cannot have a medium one, too. Medium black holes would be quite hefty with a lot of gravity, so if one did exist in a globular cluster, scientists argue that it would quickly drag any small black holes into its grasp.

“If a medium black hole existed in a cluster, it would either swallow little black holes or kick them out of the cluster,” said Stern. In other words, the small black hole in RZ2109 rules out the possibility of a medium one.

The researchers believe other globular clusters would have a similar makeup and the likelihood for finding a medium black hole is not good. Zepf said it is possible such objects are hiding in the outskirts and of galaxies like our Milky Way, either in surrounding so-called dwarf galaxies or in the remnants of dwarf galaxies being swallowed by a bigger galaxy. If so, the black holes would be faint and difficult to find.

News Source: JPL

New “Sunglasses” Help Astronomers See Light Near Black Holes

Looking at the sunset on Mauna Kea through IRPOL. Credit: U of Hawaii

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Although we can’t actually see a black hole, we can see the black hole’s effect on nearby matter. But even that is difficult because infrared light from clouds of dust and gas usually pollutes the view. But astronomers have found a way to get a clean view of the disks surrounding black holes by using a polarizing filter in the infrared. This technique works in particular when the region immediately surrounding the black hole emits a small amount of scattered light. Since scattered light is polarized, astronomers can use a filter that works like polarized sunglasses on large telescopes to detect this small amount of scattered light and measure it with unprecedented accuracy. Scientists have theorized these luminous disks existed around black holes, but until now have not been able to observe them.

The United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii has such an infrared filter, called a polarimeter (IRPOL). Astronmers have been using UKIRT and IRPOL and other telescopes for many years to search for proof that such a luminous supermassive black hole is accreting materials in a particular form of disk, where the disk shines directly using the gravitational binding energy of the black hole. Theorists have long thought that such disks should exist, and while there is a well-developed theory for it, until now theory and observations have been contradictory.

Dr. Makoto Kishimoto of the Max Planck Institute, principal investigator of this project, says: “After many years of controversy, we finally have very convincing evidence that the expected disk is truly there. However, this doesn’t answer all of our questions. While the theory has now been successfully tested in the outer region of the disk, we have to proceed to develop a better understanding of the regions of the disk closer to the black hole. But the outer disk region is important in itself – our method may provide answers to important questions for the outer boundary of the disk.”

A polarizing filter allows the colors of disk to be seen. Figure by M. Kishimoto, with cloud image by Schartmann
A polarizing filter allows the colors of disk to be seen. Figure by M. Kishimoto, with cloud image by Schartmann

Dr. Robert Antonucci of the University of California at Santa Barbara, a fellow investigator, says: “Our understanding of the physical processes in the disk is still rather poor, but now at least we are confident of the overall picture.”

Astronomers are hoping this new method will provide more information about the disks surrounding black holes in the near future.

Now, next on the agenda should be developing a suitable gravitational wave detector to confirm the existence of black holes!

Original News Source: University of Hawaii

How do you Weigh a Supermassive Black Hole? Take its Temperature

A composite image of Chandra and Hubble Space Telescope observations of giant elliptical galaxy NGC 4649 (ASA/STScI/NASA/CXC/UCI/P. Humphrey et al.)

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Working out the mass of huge black holes, like the ones hiding in the centre of galactic nuclei, is no easy task and attempts are being made to find novel ways to weigh them. Using data from the Chandra X-ray Observatory, two scientists have confirmed a theory they conceived ten years ago, that the supermassive black holes in the centre of galaxies strongly influence the nature of the gases surrounding them. So, acting like a remote thermometer, Chandra is being used to probe deep into the neighbourhood of these exotic objects, gauging their masses very accurately…

The supermassive black hole at the centre of NGC 4649 is a monster. It is about 3.4 billion times the mass of the Sun and a thousand times bigger than the black hole at the centre of the Milky Way. This fact makes it an ideal candidate to test new methods of measuring the mass of black holes to see how the results correlate with traditional methods. With a high degree of accuracy, scientists have proven that a previously untested theory of weighing black holes works by using the Chandra X-ray telescope.

Until now, supermassive black hole masses have been measured by observing the motions of stars and gas deep inside galactic nuclei, now astronomers are using the gravitational influence of the black hole over the hot gas trapped around the singularity. As the gas is pulled slowly toward the black hole, it is compressed and heated. The bigger the black hole, the higher the peak temperature. Chandra has been used to measure the peak temperature of the gas right in the centre of NGC 4649 to find the derived mass is identical to the mass previously measured by traditional means.

Fabrizio Brighenti from the University of Bologna in Italy, and William Mathews from the University of California at Santa Cruz have been working on this research for the past decade. It is only now, with the availability of a telescope as powerful as Chandra that these observations have been possible.

It was wonderful to finally see convincing evidence of the effects of the huge black hole that we expected. We were thrilled that our new technique worked just as well as the more traditional approach for weighing the black hole.” – Fabrizio Brighenti

The black hole inside NGC 4649 appears to be in a dormant state; it doesn’t seem to be pulling in material toward its event horizon very rapidly and it isn’t generating much light as it slowly grows. Therefore, using Chandra to indirectly measure its mass by sensing the peak temperature of surrounding matter is required to weigh it. In the early universe, huge black holes such as these will have generated dramatic displays of light. Now, in the local Universe, such black holes lead a more retiring life, making them difficult to observe. This prospect excites the lead scientist on the project, Philip Humphrey. “We can’t wait to apply our new method to other nearby galaxies harboring such inconspicuous black holes,” he said.

Source: Physorg.com

Can Light be “Squeezed” to Improve Sensitivity of Gravitational Wave Detectors?

Visualization of a massive body generating gravitational waves (UWM)

The search is on to detect the first evidence of gravitational waves travelling around the cosmos. How can we do this? The Laser Interferometer Gravitational-Wave Observatory (LIGO) uses a system of laser beams fired over a distance of 4 km (2.5 miles) and reflected back and forth by a system of mirrors. Should a gravitational wave pass through the volume of space-time surrounding the Earth, in theory the laser beam will detect a small change as the passing wave slightly alters the distance between mirrors. It is worth noting that this slight change will be small; so small in fact that LIGO has been designed to detect a distance fluctuation of less than one-thousandth of the width of a proton. This is impressive, but it could be better. Now scientists think they have found a way of increasing the sensitivity of LIGO; use the strange quantum properties of the photon to “squeeze” the laser beam so an increase in sensitivity can be achieved…

LIGO was designed by collaborators from MIT and Caltech to search for observational evidence of theoretical gravitational waves. Gravitational waves are thought to propagate throughout the Universe as massive objects disturb space-time. For example, if two black holes collided and merged (or collided and blasted away from each other), Einstein’s theory of general relativity predicts that a ripple will be sent throughout the fabric of space-time. To prove gravitational waves do exist, a totally different type of observatory needed to be built, not to observe electromagnetic emissions from the source, but to detect the passage of these perturbations travelling through our planet. LIGO is an attempt to measure these waves, and with a gargantuan set-up cost of $365 million, there is huge pressure for the facility to discover the first gravitational wave and its source (for more information on LIGO, see “Listening” for Gravitational Waves to Track Down Black Holes). Alas, after several years of science, none have been found. Is this because there are no gravitational waves out there? Or is LIGO simply not sensitive enough?

The first question is quickly answered by LIGO scientists: more time is needed to collect a longer period of data (there needs to be more “exposure time” before gravitational waves are detected). There is also strong theoretical reasons why gravitational waves should exist. The second question is something scientists from the US and Australia hope to improve; perhaps LIGO needs a boost in sensitivity.

The laser "squeezer" equipment (Keisuke Goda)

To make gravitational wave detectors more sensitive, Nergis Mavalvala leader of this new research and MIT physicist, has focused on the very small to help detect the very big. To understand what the researchers are hoping to achieve, a very brief crash course in quantum “fuzziness” is needed.

Detectors such as LIGO depend on highly accurate laser technology to measure perturbations in space-time. As gravitational waves travel through the Universe, they cause tiny changes in the distance between two positions in space (space is effectively being “warped” by these waves). Although LIGO has the ability to detect a perturbation of less than a thousandth of the width of a proton, it would be great if even more sensitivity is acquired. Although lasers are inherently accurate and very sensitive, laser photons are still governed by quantum dynamics. As the laser photons interact with the interferometer, there is a degree of quantum fuzziness meaning the photon is not a sharp pin-point, but slightly blurred by quantum noise. In an effort to reduce this noise, Mavalvala and her team have been able to “squeeze” laser photons.

Laser photons possess two quantities: phase and amplitude. Phase describes the photons position in time and amplitude describes the number of photons in the laser beam. In this quantum world, if the laser amplitude is reduced (removing some of the noise); quantum uncertainties in laser phase will increase (adding some noise). It is this trade-off that this new squeezing technique is base on. What is important is accuracy in the measurement of amplitude, not the phase, when trying to detect a gravitational wave with lasers.

It is hoped that this new technique can be applied to the multi-million dollar LIGO facility, possibly increasing LIGO’s sensitivity by 44%.

The significance of this work is that it forced us to confront and solve some of the practical challenges of squeezed state injection—and there are many. We are now much better positioned to implement squeezing in the kilometer-scale detectors, and catch that elusive gravitational wave.” – Nergis Mavalvala.

Source: Physorg.com

Double Your Science: Starburst Galaxies Found with Active Quasars

Astronomers now know that essentially every galaxy has a supermassive black hole at its center. When the black hole is actively feeding on material, the surrounding region can blaze brightly – this is a quasar, aka an active galaxy. The Hubble Space Telescope has been used to image a set of exotic active galaxies, known as post-starburst quasars.

What’s the relationship between galaxies and their supermassive black holes? Astronomers have been trying to work that out since these monster black holes were first discovered. One theory is that the growth of both go hand in hand through successive galactic mergers. Each merger adds new stars to the galaxy, as well as additional mass to feed the black hole.

With the galactic mergers, there are intense periods of new star formation. Gravitational interactions collapse clouds of gas and dust that go on to form stellar nurseries. The new star formation is hidden in the beginning, but the active quasar at the middle of the galaxy blows with a powerful wind that eventually blows out the obscuring dust.

Starburst galaxies aren’t bright for long, because all the hottest, most-luminous stars only last a few million years before detonating as supernovae. Astronomers were hoping to see galaxies right in the middle, where starburst activity is fading, at the same time that the quasar is blasting out radiation.

One transition galaxy like this had been discovered in the late 1990s. It possessed both the characteristics of a quasar and an older starburst galaxy. At the time it was discovered, the starburst period had happened 400 million years ago – that’s why it’s a post-starburst galaxy.

An international team of researchers used the Hubble Space Telescope to find another 29 examples of these post-starburst quasars. They searched through a candidate list of 15,000 quasars, and found the signatures of 600 post-starburst objects. With ground-based telescopes, these would just be smudges, but the full galactic shapes can be seen in the Hubble images.

Our galaxy will be colliding with Andromeda in about 3 billion years. When this happens, the Milky Way will burst with star formation. One day, we’ll be living in a post-starburst galaxy.

Original Source: Hubble News Release