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Researchers Discovered the Mysterious Structure of a Black Hole Seen Edge On

Edge-on view of the accretion disc, as seen from the Earth. A raised structure (like a donut), seen in the interior, causes the light from the inner parts of the disc (those closest to the black hole) to be eclipsed. This 'donut' gyrates around the black hole in a few minutes and produces eclipses over more or less regular intervals. Copyright: Gabriel Pérez Díaz, Servicio MultiMedia (IAC)

Swift J1357.2-0933 is a black hole obscured by a disc of gas with a vertical structure (rather like a donut) that continues to expand

This is the first time a black hole has been seen with this inclination and it's the first time that eclipses in brightness have been detected in this kind of system

The structure described in the study could be present in many other systems, which would make Swift J1357.2-0933 the prototype of a hitherto veiled population with a high inclination"

Like an immense donut (or toroid) that expands daily. That's how Jesús Corral-Santana, a researcher at the Instituto de Astrofísica de Canarias (IAC), describes the odd - hitherto unknown - structure of the binary system Swift J1357.2-0933, consisting of a 'normal' star and a stellar-mass black hole (which feeds off its companion star). The investigation, just published in Science journal with Corral as first author, follows the stages of the outburst of the system, an event that occurs only once in decades or centuries.

The team observed strange eclipses in the system that lasted and were repeated every few minutes. This finding led them to two conclusions: they had to be viewing the black hole edge on (it had an inclination of at least 75 degrees) and it presented an odd vertical structure within the accretion disc of the system. In other words, the matter being drawn from the companion star formed an outflow in the shape of a whirlpool, rather like water flowing down a plughole.

As Jorge Casares, also an IAC researcher, coauthor of the paper and Principal investigator (PI), explains, 'this kind of structure is possibly present in many - or all - X-ray binaries, the class of systems to which Swift J1357.2-0933 belongs. So the object we've observed could be the prototype of a hitherto hidden population of highly inclined systems in which the black hole is obscured.' Statistically, around 20% of systems could be of this type.

Black holes are formed following the death of very massive stars, and their detection is complicated. 'Since they don't emit any light, they're nearly impossible to find if they are alone,' says Casares. 'In cases where they have a companion star, their probability of detection is much higher, since their presence is betrayed by the "cannibalization" of the companion star by the black hole.' This explains why, since the first detection of such a system in 1964, only 18 other black holes have been found in our Galaxy. Swift J1357.2-0933, discovered by the Swift X-ray satellite and studied by the IAC team, is the latest one in the list. There are a further 32 black-hole candidates, but these haven't yet been confirmed.

Many X-ray binaries are characterized by decades-long - even centuries-long - periods of quiescence, and it is easy to confuse them with normal stars when in this state. But, with no prior warning, these system can erupt, brightening dramatically (by as much as a million times their normal brilliance), in any part of the Galaxy. This enables them to be detected by satellites scanning the sky in X-rays. The system fades back to quiescence after a few months.

It's then, adds Corral-Santana, that the scientific community can analyse its structure: a 'normal' star and a compact object (either a black hole, as in this case, or a neutron star). The star transfers material to its companion, forming an accretion disc.

In the case of Swift J1357.2-0933, the IAC researcher continues, more data have been gathered owing to its relative proximity, estimated at 5000 light years and its great distance above the plane of the Milky Way, where most of the matter of the Galaxy is concentrated, meaning that light from the binary system is not dimmed by interstellar dust or glare from nearby stars.

The scientists found that the system has a very short period, a mere 2.8 hours. In that time the companion star completes an orbit around the black hole. They also measured the mass of the black hole to be three times that of the Sun. 'That's a lower limit. In fact, the mass could be very much greater than that. Further observations during the period of quiescence would enable us to get a more accurate value,' Corral-Santana explains.

However, the most unusual find concerns the eclipses of the system. From images taken with various telescopes at the Teide and Roque de los Muchachos Observatories (the IAC-80, Liverpool, Mercator and INT), it was found that eclipses occurred in which the brightness of the system dropped by 30% in only seven seconds, and that they were repeated at longer intervals after a few days. 'It's the first time a phenomenon with these characteristics has been observed. None of the 50 known transitory X-ray binaries (18 with confirmed black holes and 32 candidates)
produces eclipses by the companion star,' the IAC astrophysicist points out.

What could be causing the eclipses? The researchers are clear that they aren't produced by the companion star since these have orbital periods of 2.8 hours and the eclipses are produced every few minutes and are of extremely short duration. Corral-Santana further adds, 'The period in which the eclipses are repeated gets progressively longer each day. This fact suggests that the eclipses are produced by a vertical structure that is initially located close to the black hole and gradually moves outwards like a wave from the inner part of the accretion disc.

This discovery is closely linked with another: 'The simple fact of detecting the eclipses indicates that the system is viewed at a high inclination, even greater than 75 degrees. In effect, we're looking at it edge on,' says the scientist, who further describes the structure as, 'probably like a donut, with the black hole permanently hidden in the middle.'



Elliptical Galaxy Reveals a Spectacular "Ruins" of Black Holes and Neutron Stars

The Chandra X-Ray Observatory image of the elliptical galaxy NGC 4261 revealed dozens of black holes and neutron stars strung out across tens of thousands of light years like beads on a necklace. The spectacular structure, which is not apparent from the optical image of the galaxy, is thought to be the remains of a collision between galaxies a few billion years ago, when a smaller galaxy was captured and pulled apart by the gravitational tidal forces of NGC 4261. As the doomed galaxy fell into the larger galaxy, large streams of gas were pulled out into long tidal tails. Shock waves in these tidal tails triggered the formation of many massive stars.

Over the course of a few million years, these stars evolved into neutron stars or black holes. A few of these collapsed stars had companion stars, and became bright X-ray sources as gas from the companions was captured by their intense gravitational fields.

The currently favored view is that elliptical galaxies are produced by collisions between spiral galaxies. Computer simulations of galaxy collisions support this idea, and optical evidence of tails, shells, ripples, arcs and other structures have been interpreted as evidence for this theory.

However as the image above shows, the optical evidence rather quickly fades into the starry background of the galaxy, whereas the X-ray signature lingers for hundreds of millions of years. Chandra's image of NGC 4261 shows that X-ray observations may be the best way to identify the ancient remains of mergers between galaxies.

The Hubble Space Telescope picture below of the center of NGC 4261 tells a dramatic tale. The gas and dust in this disk are swirling into what is almost certainly a super, perhaps super-super massive black hole. The disk is probably what remains of the smaller galaxy that fell in hundreds of millions of years ago. Collisions like this may be a common way of creating such active galactic nuclei as quasars. Strangely, the center of this fiery whirlpool is offset from the exact center of the galaxy - for a reason that for now remains an astronomical mystery.



The Search for the Missing "Dark Galaxies" of the Universe

The new serach for "dark galaxies" could help answer a paradox in astronomy: why today's galaxies haven't yet run out of gas. According to observations, most galaxies have just enough fuel left to make stars for another billion years or so. Yet galaxies have endured for most of the age of the universe.

Using supercomputers at the National Energy Research Scientific Computing Center (NERSC), Sukanya Chakrabarti, a professor of physics at Florida Atlantic University and an Institute for Theory and Computation (ITC) Fellow at the Harvard-Smithsonian Center for Astrophysics, developed a mathematical method to uncover “dark” galaxies. When she applied this method to our own Milky Way galaxy, Chakrabarti discovered a faint satellite might be lurking on the opposite side of the galaxy from Earth, approximately 300,000 light-years from the galactic center.
“Our approach has broad implications for many fields of physics and astronomy - for the indirect detection of dark matter as well as dark-matter dominated dwarf galaxies, planetary dynamics, and for galaxy evolution dominated by satellite impacts,” said Chakrabarti.
Chakrabarti’s technique involves an analysis of the cold atomic hydrogen gas that comprise the outskirts of a large spiral galaxy’s disk. This cold gas is gravitationally confined to the galactic disk and extends much further out than the visible stars - sometimes up to five times the diameter of the visible spiral. This gas can be mapped by radio telescopes.

According to Chakrabarti, the dark satellite galaxies create disturbances in the cold atomic hydrogen gas at the edges of the spiral galaxy’s disk, and these perturbations reveal the mass, distance and location of the satellite. With the help of NERSC systems, she successfully validated her method by analyzing the radio observations of the Whirlpool Galaxy, which has a visible satellite one-third of its size, and NGC 1512, which has a satellite one-hundredth it's size. Her calculations correctly predicted the mass and location of both of the known satellite galaxies. 

When she applied this analysis to radio observations of our own Milky Way, the analysis revealed a potential dwarf galaxy, or Galaxy X, sitting in the constellation of Norma or Circinus, just west of the galactic center in Sagittarius when viewed from Earth. Because this satellite sits across the Milky Way from Earth, it is obscured in our line of sight by gas and dust, and thus has not been detected.

According to her former colleague Leo Blitz of the University of California at Berkeley, searching for satellite galaxies with this method is like inferring the size and speed of a ship by looking at its wake. “You see the waves from a lot of boats, but you have to be able to separate out the wake of a medium or small ship from that of an ocean liner,” he said.

Simulations of galaxy formation suggest a galaxy the size of the Milky Way should feature about 1000 dwarf galaxies, but only a few dozen have been found so far. Some of the missing dwarfs may be dark galaxies that are all but invisible.

Blitz's models predict that the universe should contain far more dwarf galaxies than the tiny fraction that astronomers can identify.

If so, Blitz thinks he knows how to find the dark galaxies. "Imagine them plopping through the gas of the outer Milky Way," he says. "They might create some sort of splash or ripple."

These distant reaches are relatively calm, making such disturbances possible to detect. Blitz explains, "It's like throwing darts at a board. As these dark galaxies come at the Milky Way, they're likely going to hit the outer parts because there's more surface area there."

To pinpoint any dark galaxy hot spots, Blitz and his research group are mapping the structure of the Milky Way. In the process, they have been able to characterize the warping of our generally flat galaxy: "It's like hitting cymbals; it's held in the middle and the outer parts are free to vibrate," he says.Within this structure, Blitz identified areas of very localized vibrations - an encouraging sign - and is now searching other galaxies for similar characteristics.

"That's exactly the kind of signature we look for if the Milky Way were being hit by these dark matter galaxies," he says. As promising as the mapping looks, Blitz is hedging his bets with a second approach: seeking gassy cores that could be embedded even in dark galaxies.

"We're trying to survey regions of the sky to see if there are concentrations of atomic hydrogen that are not associated with known galaxies," he says. "I'm hoping that by making a large enough survey of the sky, we'll be able to find galaxies that contain only hydrogen and no stars. By looking at the motions of the hydrogen, we'll be able to determine the properties of the dark matter that's within it as well."

The resulting map of interstellar hydrogen could help answer another paradox in astronomy: why today's galaxies haven't yet run out of gas. According to observations, most galaxies have just enough fuel left to make stars for another billion years or so. Yet galaxies have endured for most of the age of the universe, making it unlikely that so many should blink out at once.

Blitz thinks they could be topping up their tanks with interstellar gases. As galaxies interact gravitationally, gases from their edges will get torn loose. These gases may eventually fall onto other galaxies, just as water vapor gets recycled back into rain. "There should be enough material between galaxies to be able to make up for the stars that are currently being formed," he says. "That's measurable with the Allen Telescope." 



Predicting Key Property in Andromeda's Satellites

Andromeda galaxy, or M31. (Credit: NASA/JPL-Caltech)

Using modified laws of gravity, researchers from Case Western Reserve University and Weizmann Institute of Science closely predicted a key property measured in faint dwarf galaxies that are satellites of the nearby giant spiral galaxy Andromeda.

The predicted property in this study is the velocity dispersion, which is the average velocity of objects within a galaxy relative to each other. Astronomers can use velocity dispersion to determine the accelerations of objects within the galaxy and, roughly, the mass of a galaxy, and vice-versa.

To calculate the velocity dispersion for each dwarf galaxy, the researchers utilized Modified Newtonian Dynamics, MOND for short, which is a hypothesis that attempts to resolve what appears to be an insufficient amount of mass in galaxies needed to support their orbital speeds.

MOND suggests that, under a certain condition, Newton's law of gravity must be altered. That hypothesis is less widely accepted than the hypothesis that all galaxies contain unseen dark matter that provides needed mass.

"MOND comes out surprisingly well in this new test," said Stacy McGaugh, astronomy professor at Case Western Reserve. "If we're right about dark matter, this shouldn't happen."

McGaugh teamed with Mordehai Milgrom, the father of MOND and professor of physics and astrophysics at Weizmann Institute in Israel. Their study, "Andromeda Dwarfs in the Light of MOND" will be published in the Astrophysical Journal.

Astronomers and physicists need some way to explain why galaxies rotate faster than predicted by the law of gravity without flying apart. That spurred researchers to theorize that dark matter, first assumed by Dutch astronomer Jan Oort in 1932, is gathered in and around galaxies, adding the mass needed to hold galaxies together.

Dissatisfied with that hypothesis, Milgrom offered MOND, which says that Newton's force law must be tweaked at low acceleration, eleven orders of magnitude lower than what we feel on the surface of Earth. Acceleration above that threshold is linearly proportional to the force of gravity -- as Newton's law states -- but below the threshold, is not, he posits. When the force law is tweaked under that limitation, the modification can resolve the mass discrepancy.

Early in his career, McGaugh believed in dark matter. But, over time, he's found the hypothesis comes up short in a number of aspects while he's found increasing evidence that supports MOND.

In this paper, researchers tested MOND with dwarf spheroidal galaxies. These very low-surface brightness galaxies are satellites of larger galaxies. By the standards of galaxies they are tiny, containing only a few hundred thousand stars.

"These dwarfs are spread exceedingly thin. Their light is spread over hundreds to thousands of light-years. These systems pose a strong test of MOND because their low stellar density predicts low accelerations," McGaugh said.

McGaugh and Milgrom used the luminosity of the galaxies, an indicator of stellar mass, and MOND to make their calculations and predict the velocity dispersions of 17 faint galaxies. In 16 cases, the predictions closely matched the velocity dispersions measured by others. In the last case, the data from independent observers differed from one another.

"Many predictions were bang on," McGaugh said. "Typically, the better the data, the better the agreement."

The scientists also used MOND to predict velocity dispersions for 10 more faint dwarf galaxies in Andromeda. They are awaiting measurements to refute or verify this prediction.



Strange 'Ist-of-Its-Kind' Galaxy --"Reveals Clues to Evolution of the Universe"

A unique galaxy, dubbed Speca by its discoverers, holds clues to the evolution of galaxies billions of years ago.  The galaxy has a combination of characteristics never seen before, giving astronomers a tantalizing peek at processes they believe played key roles in the growth of galaxies and clusters of galaxies early in the history of the Universe.

The galaxy is only the second spiral, as opposed to elliptical, galaxy known to produce large, powerful jets of subatomic particles moving at nearly the speed of light. It also is one of only two galaxies to show that such activity occurred in three separate episodes.

Giant jets of superfast particles are powered by supermassive black holes at the cores of galaxies. Both elliptical and spiral galaxies harbor such black holes, but only Speca and one other spiral galaxy have been seen to produce large jets. The jets pour outward from the poles of rapidly-rotating disks of material orbiting the black hole. The on-and-off jet episodes have been seen in a dozen ellipticals, but only one other elliptical shows evidence, like Speca, for three such distinct episodes.

"This is probably the most exotic galaxy with a black hole ever seen. It has the potential to teach us new lessons about how galaxies and clusters of galaxies formed and developed into what we see today," said Ananda Hota, of the Academia Sinica Institute of Astronomy and Astrophysics, in Taiwan.

The scientists believe that Speca, about 1.7 billion light-years from Earth, and the 60-some other galaxies in a cluster with it are providing a look at what young galaxies and clusters may have been like when the Universe was much younger. In the young Universe, galaxies in such clusters would have been gathering up additional material, colliding with each other, undergoing bursts of star formation, and interacting with primordial material falling into the cluster from outside.

"Speca is showing evidence for many of these phenomena," Ananda said, adding that "We hope to find many more galaxies like it with future observations, and to learn more about the processes and an environment that were much more common when the Universe was a fraction of its current age."

Speca (an acronym for Spiral-host Episodic radio galaxy tracing Cluster Accretion) first came to Ananda's attention in an image that combined data from the visible-light Sloan Digital Sky Surveyand the FIRST survey done with the National Science Foundation's Very Large Array (VLA) radio telescope. Followup observations with the Lulin optical telescope in Taiwan and ultraviolet data from NASA's GALEX satellite confirmed that the giant lobes of radio emission, usually seen coming from elliptical galaxies, were coming from a spiral galaxy with ongoing star formation.

Ananda's team also examined the galaxy in images from the NRAO VLA Sky Survey (NVSS), then made new observations with the Giant Meterwave Radio Telescope (GMRT) in India, which observes at longer wavelengths than the VLA and is the premier telescope for observing at those long wavelengths.
With this impressive variety of data from across the electromagnetic spectrum, the researchers unraveled the galaxy's complex and fascinating history.

The radio images from the VLA FIRST survey had shown one pair of radio-emitting lobes. The VLA's NVSS images showed another, distinct pair of lobes farther from the galaxy. The GMRT images confirmed this second pair, but showed another, smaller pair close to the galaxy, presumably produced by the most-recently ejected jet particles.

"By using these multiple sets of data, we found clear evidence for three distinct epochs of jet activity," Ananda explained.

The biggest surprise -- the low-frequency nature of the oldest, outermost lobes -- gave a valuable clue about the galaxy's -- and the cluster's -- environment. The outermost radio-emitting lobes are old enough that their particles should have lost most of their energy and ceased to produce radio emission.
"We think these old, relic lobes have been 're-lighted' by shock waves from rapidly-moving material falling into the cluster of galaxies as the cluster continues to accrete matter," said Ananda.

"All these phenomena combined in one galaxy make Speca and its neighbors a valuable laboratory for studying how galaxies and clusters evolved billions of years ago," Ananda said.

"The ongoing low-frequency TIFR GMRT Sky Survey will find many more relic radio lobes of past black hole activity and energetic phenomena in clusters of galaxies like those we found in Speca," said Sandeep K. Sirothia of India's National Centre for Radio AstrophysicsTata Institute of Fundamental Research.

Composite image of Speca below : Optical SDSS image of the galaxies in yellow,low resolution radio image from NVSS in blue, high resolution radio image from GMRT in red.