Have researchers discovered the sound of the stars?

Solar flares on our nearest star, the sun.

A chance discovery by a team of researchers, including a University of York scientist, has provided experimental evidence that stars may generate sound.

The study of fluids in motion -- now known as hydrodynamics -- goes back to the Egyptians, so it is not often that new discoveries are made. However when examining the interaction of an ultra-intense laser with a plasma target, the team observed something unexpected.
Scientists including Dr John Pasley, of the York Plasma Institute in the Department of Physics at York, realized that in the trillionth of a second after the laser strikes, plasma flowed rapidly from areas of high density to more stagnant regions of low density, in such a way that it created something like a traffic jam. Plasma piled up at the interface between the high and low density regions, generating a series of pressure pulses: a sound wave.
However, the sound generated was at such a high frequency that it would have left even bats and dolphins struggling! With a frequency of nearly a trillion hertz, the sound generated was not only unexpected, but was also at close to the highest frequency possible in such a material -- six million times higher than that which can be heard by any mammal!
Dr Pasley, who worked with scientists from the Tata Institute of Fundamental Research in Mumbai, India, and the Science and Technology Facilities Council's Central Laser Facility in Oxfordshire, said: "One of the few locations in nature where we believe this effect would occur is at the surface of stars. When they are accumulating new material stars could generate sound in a very similar manner to that which we observed in the laboratory -- so the stars might be singing -- but, since sound cannot propagate through the vacuum of space, no one can hear them."
The technique used to observe the sound waves in the lab works very much like a police speed camera. It allows the scientists to accurately measure how fluid is moving at the point that is struck by the laser on timescales of less than a trillionth of a second.
Dr Alex Robinson from the Plasma Physics Group at STFC's Central Laser Facility developed a numerical model to generate acoustic waves for the experiment. He said, "It was initially hard to determine the origin of the acoustic signals, but our model produced results that compared favorably with the wavelength shifts observed in the experiment. This showed that we had discovered a new way of generating sound from fluid flows. Similar situations could occur in plasma flowing around stars"
The research was funded by the Engineering and Physical Sciences Research Council and the Tata Institute of Fundamental Research. It is published in Physical Review Letters.

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The above story is based on materials provided by University of York. Note: Materials may be edited for content and length.

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Journal Reference:

1. Amitava Adak, A. P. L. Robinson, Prashant Kumar Singh, Gourab Chatterjee, Amit D. Lad, John Pasley, G. Ravindra Kumar. Terahertz Acoustics in Hot Dense Laser Plasmas. Physical Review Letters, 2015; 114 (11) DOI: 10.1103/PhysRevLett.114.115001

Fast-moving unbound star has broken the galactic speed record

Pictorial representation of a fast-moving unbound star

A fast-moving unbound star discovered by astronomers at Queen's University Belfast has broken the galactic speed record.

The unbound star, named US708, is traveling at 1,200 kilometers per second -- the fastest speed ever recorded for such an object in our galaxy -- meaning it is not held back by gravity and will eventually leave the Milky Way.
US708 is believed to have once been part of a double-star solar system, which also included a massive white dwarf star. The white dwarf is thought to have turned into a 'thermonuclear supernovae' and exploded, kicking US708 and sending it hurtling across space.
The discovery of US708 sheds light on the mysterious double-star systems that give rise to thermonuclear explosions. Thermonuclear, or 'type Ia', supernovae have long been used to calculate the distances to faraway galaxies -- a measurement which helps to determine how the universe is changing and expanding.
Dr Rubina Kotak and Ken Smith, from the Astrophysics Centre at Queen's University, were part of a team of scientists from countries across the world who made the ground-breaking discovery using data gathered by the Pan-STARRS1 telescope on Mount Haleakala on the Hawaiian island of Maui. Using a range of data gathered over the last 59 years the team were able to determine the full 3-D motion of the star and measure how quickly it is moving across the plane of the sky.
Dr Rubina Kotak, from the Astrophysics Centre at Queen's University Belfast, said: "It is very exciting to have contributed to this important discovery which is a great example of Queen's commitment to achieving excellence and advancing knowledge for the benefit of society. It brings us a step closer to solving the type Ia puzzle."
European Southern Observatory fellow, Stephan Geier, who led the study, said: "Several types of stars have been suspected of causing the explosion of a white dwarf as supernova of type Ia. Until now, none of them could be confirmed. Now we have found a delinquent on the run bearing traces from the crime scene."
Queen's University Belfast is a full member of the PS1 science consortium, which carried out this research involving astronomers from ten other institutes dotted across the world. The research was led by Dr Stephan Geier, European Southern Observatory fellow, and comprised contributions from scientists from a number of countries including Germany, USA, the Netherlands, China and the UK.

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Story Source:
The above story is based on materials provided by Queen's University, Belfast. Note: Materials may be edited for content and length.

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Journal Reference:

1. S. Geier, F. Furst, E. Ziegerer, T. Kupfer, U. Heber, A. Irrgang, B. Wang, Z. Liu, Z. Han, B. Sesar, D. Levitan, R. Kotak, E. Magnier, K. Smith, W. S. Burgett, K. Chambers, H. Flewelling, N. Kaiser, R. Wainscoat, C. Waters. The fastest unbound star in our Galaxy ejected by a thermonuclear supernova. Science, 2015; 347 (6226): 1126 DOI: 10.1126/science.1259063

Fermi Detects Hints of Starquakes in Magnetar ‘Storm’ | sci-english.blogspot.com

A rupture in the crust of a highly magnetized neutron star, shown here in an artist’s rendering, can trigger high-energy eruptions. Fermi observations of these blasts include information on how the star’s surface twists and vibrates, providing new insights into what lies beneath | sci-english.blogspot.com

Using data from NASA’s Fermi Gamma-ray Space Telescope, astronomers have discovered underlying signals related to the rapid-fire “storm” of high-energy blasts detected in 2009 from a highly magnetized neutron star.
Such signals were first identified during the fadeout of rare giant flares produced by magnetars. Over the past 40 years, giant flares have been observed just three times — in 1979, 1998 and 2004 — and signals related to starquakes, which set the neutron stars ringing like a bell, were identified only in the two most recent events.
“Fermi’s Gamma-ray Burst Monitor (GBM) has captured the same evidence from smaller and much more frequent eruptions called bursts, opening up the potential for a wealth of new data to help us understand how neutron stars are put together,” said Anna Watts, an astrophysicist at the University of Amsterdam in the Netherlands and co-author of a new study about the burst storm. “It turns out that Fermi’s GBM is the perfect tool for this work.”
In the midst of SGR J1550-5418’s 2009 burst storm, Swift’s X-Ray Telescope captured an expanding halo produced by the magnetar’s brightest bursts. The rings formed as X-rays from the brightest bursts scattered off of intervening dust clouds. Clouds closer to Earth produced larger rings. Image Credit: NASA/Swift/Jules Halpern, Columbia University
Neutron stars are the densest, most magnetic and fastest-spinning objects in the universe that scientists can observe directly. Each one is the crushed core of a massive star that ran out of fuel, collapsed under its own weight, and exploded as a supernova. A neutron star packs the equivalent mass of half-a-million Earths into a sphere about 12 miles across, roughly the length of Manhattan Island in New York City.
While typical neutron stars possess magnetic fields trillions of times stronger than Earth’s, the eruptive activity observed from magnetars requires fields 1,000 times stronger still. To date, astronomers have confirmed only 23 magnetars.
Because a neutron star’s solid crust is locked to its intense magnetic field, a disruption of one immediately affects the other. A fracture in the crust will lead to a reshuffling of the magnetic field, or a sudden reorganization of the magnetic field may instead crack the surface. Either way, the changes trigger a sudden release of stored energy via powerful bursts that vibrate the crust, a motion that becomes imprinted on the burst’s gamma-ray and X-ray signals.
It takes an incredible amount of energy to convulse a neutron star. The closest comparison on Earth is the 9.5-magnitude Chilean earthquake of 1960, which ranks as the most powerful ever recorded on the standard scale used by seismologists. On that scale, said Watts, a starquake associated with a magnetar giant flare would reach magnitude 23.
The 2009 burst storm came from SGR J1550−5418, an object discovered by NASA’s Einstein Observatory, which operated from 1978 to 1981. Located about 15,000 light-years away in the constellation Norma, the magnetar was quiet until October 2008, when it entered a period of eruptive activity that ended in April 2009. At times, the object produced hundreds of bursts in as little as 20 minutes, and the most intense explosions emitted more total energy than the sun does in 20 years. High-energy instruments on many spacecraft, including NASA’s Swift and Rossi X-ray Timing Explorer, detected hundreds of gamma-ray and X-ray blasts.
Speaking at the Fifth Fermi International Symposium in Nagoya, Japan, on October 21, Watts said the new study examined 263 individual bursts detected by Fermi’s GBM and confirms vibrations in the frequency ranges previously seen in giant flares. “We think these are likely twisting oscillations of the star where the crust and the core, bound by the super-strong magnetic field, are vibrating together,” she explained. “We also found, in a single burst, an oscillation at a frequency never seen before and which we still do not understand.”
A key element of the research is a new analysis technique developed by University of Amsterdam researcher Daniela Huppenkothen. Normally scientists search for oscillations in high-energy data by looking for variations aligned to a particular frequency. Such methods are best suited for finding a strong signal with little competition rather than a faint signal immersed in a bright and rapidly changing environment, such as a burst.
Huppenkothen likens the problem to detecting ripples from a stone tossed into a quiet pond. “Now imagine you’re in the middle of the North Atlantic during a storm, searching for those ripples amidst huge waves in a churning sea,” she explained. “Our old methods really weren’t appropriate for this, but I have in effect developed a way of accounting for the rough sea so we can find ripples even in stormy conditions.”
While there are many efforts to describe the interiors of neutron stars, scientists lack enough observational detail to choose between differing models. Neutron stars reach densities far beyond the reach of laboratories and their interiors may exceed the density of an atomic nucleus by as much as 10 times. Knowing more about how bursts shake up these stars will give theorists an important new window into understanding their internal structure.
“Right now,” added Watts, “we are waiting for more bursts — and if we’re lucky, a giant flare — to take advantage of GBM’s excellent capabilities.”
Publications:

• D. Huppenkothen, et al., “Quasi-periodic Oscillations in Short Recurring Bursts of the Soft Gamma Repeater J1550–5418,” 2014, ApJ, 787, 128; doi:10.1088/0004-637X/787/2/128
• Daniela Huppenkothen, et al., “Quasi-Periodic Oscillations and Broadband Variability in Short Magnetar Bursts,” 2013, ApJ, 768, 87; doi:10.1088/0004-637X/768/1/87

PDF Copies of the Studies:

• Quasi-Periodic Oscillations and broadband variability in short magnetar bursts
• Quasi-Periodic Oscillations in Short Recurring Bursts of the Soft-Gamma Repeater J1550-5418

Source: Francis Reddy, NASA’s Goddard Space Flight Center
Image: NASA’s Goddard Space Flight Center/S. Wiessinger

Extreme gravity effects revealed by oxygen for the first time in neutron star that 'eats' white dwarf | sci-english.blogspot.com

Neutron star 'eats' oxygen-rich white dwarf in a peculiar binary system | sci-english.blogspot.com

Astronomers from SRON Netherlands Institute for Space Research and Utrecht University have found blurred oxygen signatures in the X-rays from a neutron star that 'eats' a white dwarf. For the first time the effects of extreme gravity are revealed by oxygen instead of iron atoms.

Although strong gravity near neutron stars and black holes has been studied before in a similar way, this result is unique. Until now, only blurred X-ray signatures of iron atoms have been observed in the X-rays from a neutron star. However, the characteristics of these so called 'iron lines' are disputed, which makes them less suited for extreme gravity field measurements.
The neutron star has been studied before but now Oliwia Madej, PhD student at Utrecht University and SRON Netherlands Institute for Space Research, has found blurred oxygen signatures in the X-rays from the star. She made this discovery in an archival observation performed by ESA's XMM-Newton observatory, which is equipped with the SRON reflection grating spectrometer (RGS) that is extemely sensitive in these particular wavelenghts. The research was carried out under supervision of SRON-researcher Peter Jonker.
The neutron star that the astronomers observed is part of a binary system called 4U 0614+091. In the binary, the neutron star and a white dwarf closely orbit each other in roughly 50 minutes. The white dwarf -- basically a burnt out star -- orbits at such a small distance from the neutron star that the oxygen-rich gas is pulled off the dwarf and starts closely swirling around the neutron star in a disk.
Extreme gravity
"Normally, hot oxygen atoms emit X-rays at a specific energy," Madej explains. "But because of the extreme gravity and the hot gas in the disk around the neutron star, this oxygen signature in the X-ray data is blurred." From the shape of the blur Madej tried to estimate the inner radius of the oxygen-rich disk around the neutron star, which should give an idea of the maximum radius that the neutron star could possibly have.
"Unfortunately, the current data are not yet good enough to give a definitive answer on the size of a neutron star," Peter Jonker admits. "To determine this in greater detail we need more observation time. And when we find the signature of iron molecules as well, we can now compare the characteristics of the two emission lines. Measured together, uncertainties about the measurements of the iron line can be taken away, which will guide the interpretation in other systems where only iron has been seen. All in all our observations are definitely an important step on the way towards a better understanding of the extreme conditions around and inside a neutron star."
Neutron stars -- shaped out of the collapsing cores of massive stars -- are the most compact objects with a surface in the universe. A neutron star has a slightly higher mass compared to a white dwarf, but the matter is squeezed into a ball of only 10-20 km in diameter. At these high densities, normal atoms cannot exist anymore. Anything denser would collapse into a black hole. Therefore, astronomers are very interested in the state of the matter inside a neutron star.
The results of the research appear in the Monthly Notices of the Royal Astronomical Society.

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Story Source:
The above story is based on materials provided by SRON Netherlands Institute for Space Research. Note: Materials may be edited for content and length.

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Journal Reference:

1. O.K. Madej, P.G. Jonker, A.C. Fabian, C. Pinto, F. Verbunt, J. de Plaa. A relativistically broadened O VIII Lyalpha line in the ultra-compact X-ray binary 4U 0614 091. Monthly Notices of the Royal Astronomical Society, 2010; (accepted for publication) [link]

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Dark energy is real, say astronomers | sci-english.blogspot.com

A visual impression of the data used in the study. The relevant extra-galactic maps are represented as shells of increasing distance from Earth | sci-english.blogspot.com

Dark energy, a mysterious substance thought to be speeding up the expansion of the Universe is really there, according to a team of astronomers at the University of Portsmouth and LMU University Munich. After a two-year study led by Tommaso Giannantonio and Robert Crittenden, scientists conclude that the likelihood of its existence stands at 99.996 per cent. Their findings are published in the Monthly Notices of the Royal Astronomical Society.

Professor Bob Nichol, a member of the Portsmouth team, said: "Dark energy is one of the great scientific mysteries of our time, so it isn't surprising that so many researchers question its existence.
"But with our new work we're more confident than ever that this exotic component of the Universe is real -- even if we still have no idea what it consists of."
Over a decade ago, astronomers observing the brightness of distant supernovae realised that the expansion of the Universe appeared to be accelerating. The acceleration is attributed to the repulsive force associated with dark energy now thought to make up 73 per cent of the content of the cosmos. The researchers who made this discovery received the Nobel Prize for Physics in 2011, but the existence of dark energy remains a topic of hot debate.
Many other techniques have been used to confirm the reality of dark energy but they are either indirect probes of the accelerating Universe or susceptible to their own uncertainties. Clear evidence for dark energy comes from the Integrated Sachs Wolfe effect named after Rainer Sachs and Arthur Wolfe.
The Cosmic Microwave Background, the radiation of the residual heat of the Big Bang, is seen all over the sky. In 1967 Sachs and Wolfe proposed that light from this radiation would become slightly bluer as it passed through the gravitational fields of lumps of matter, an effect known as gravitational redshift.
In 1996, Robert Crittenden and Neil Turok, now at the Perimeter Institute in Canada, took this idea to the next level, suggesting that astronomers could look for these small changes in the energy of the light, or photons, by comparing the temperature of the radiation with maps of galaxies in the local Universe.
In the absence of dark energy, or a large curvature in the Universe, there would be no correspondence between these two maps (the distant cosmic microwave background and relatively closer distribution of galaxies), but the existence of dark energy would lead to the strange, counter-intuitive effect where the cosmic microwave background photons would gain energy as they travelled through large lumps of mass.
The Integrated Sachs Wolfe effect was first detected in 2003 and was immediately seen as corroborative evidence for dark energy, featuring in the 'Discovery of the year' in Science magazine. But the signal is weak as the expected correlation between maps is small and so some scientists suggested it was caused by other sources such as the dust in our galaxy. Since the first Integrated Sachs Wolfe papers, several astronomers have questioned the original detections of the effect and thus called some of the strongest evidence yet for dark energy into question.
In the new paper, the product of nearly two years of work, the team have re-examined all the arguments against the Integrated Sachs Wolfe detection as well as improving the maps used in the original work. In their painstaking analysis, they conclude that there is a 99.996 per cent chance that dark energy is responsible for the hotter parts of the cosmic microwave background maps (or the same level of significance as the recent discovery of the Higgs boson).
"This work also tells us about possible modifications to Einstein's theory of General Relativity," notes Tommaso Giannantonio, lead author of the present study.
"The next generation of cosmic microwave background and galaxy surveys should provide the definitive measurement, either confirming general relativity, including dark energy, or even more intriguingly, demanding a completely new understanding of how gravity works."

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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. T. Ginnantonio, R. Crittenden, R. Nichol, A. Ross. The significance of the integrated Sachs-Wolfe effect revisited. Monthly Notices of the Royal Astronomical Society, 2012; (in press) [link]

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Astronomers ask 'where are all the dwarf galaxies?' | sci-english.blogspot.com

Cosmic Web Stripping | sci-english.blogspot.com

Astronomers of the international CLUES collaboration have identified "Cosmic Web Stripping" as a new way of explaining the famous missing dwarf problem: the lack of observed dwarf galaxies compared with that predicted by the theory of Cold Dark Matter and Dark Energy.

High-precision observations over the last two decades have indicated that our Universe consists of about 75% Dark Energy, 20% Dark Matter and 5% ordinary matter. Galaxies and matter in the universe clump in an intricate network of filaments and voids, known as the Cosmic Web. Computer experiments on massive supercomputers have shown that in such a Universe a huge number of small "dwarf" galaxies weighing just one thousandth of the Milky Way should have formed in our cosmic neighbourhood. Yet only a handful of these galaxies are observed orbiting around the Milky Way. The observed scarcity of dwarf galaxies is a major challenge to our understanding of galaxy formation.
An international team of researchers has studied this issue within the Constrained Local UniversE Simulations project (CLUES). The CLUES simulations use the observed positions and peculiar velocities of galaxies within Tens of Millions of light years of the Milky Way to accurately simulate the local environment of the Milky Way. "The main goal of this project is to simulate the evolution of the Local Group -- the Andromeda and Milky Way galaxies and their low-mass neighbours -- within their observed large scale environment," said Stefan Gottlöber of the Leibniz Institute for Astrophysics Potsdam.
Analysing the CLUES simulations, the astronomers have now found that some of the far-out dwarf galaxies in the Local Group move with such high velocities with respect to the Cosmic Web that most of their gas can be stripped and effectively removed. They call this mechanism "Cosmic Web Stripping," since it is the pancake and filamentary structure of the cosmos that is responsible for depleting the dwarfs' gas supply.
"These dwarfs move so fast that even the weakest membranes of the Cosmic Web can rip off their gas," explained Alejandro Benítez LLambay, PhD student at the Instituto de Astronomía Teórica y Experimental of the Universidad Nacional de Córdoba in Argentina, and first author of the publication of this study. Without a large gas reservoir out of which to form stars, these dwarf galaxies should be so small and dim that they would be hardly be visible today. The missing dwarfs may simply be too faint to see.

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Story Source:
The above story is based on materials provided by Leibniz-Institut für Astrophysik Potsdam (AIP). Note: Materials may be edited for content and length.

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Journal Reference:

1. Alejandro Benítez-Llambay, Julio F. Navarro, Mario G. Abadi, Stefan Gottlöber, Gustavo Yepes, Yehuda Hoffman, Matthias Steinmetz. Dwarf Galaxies and the Cosmic Web. The Astrophysical Journal, 2013; 763 (2): L41 DOI: 10.1088/2041-8205/763/2/L41

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Best way to measure dark energy just got better | sci-english.blogspot.com

A Type Ia supernova occurs when a white dwarf accretes material from a companion star until it exceeds the Chandrasekhar limit and explodes. By studying these exploding stars, astronomers can measure dark energy and the expansion of the universe. CfA scientists have found a way to correct for small variations in the appearance of these supernovae, so that they become even better standard candles. The key is to sort the supernovae based on their color | sci-english.blogspot.com

Dark energy is a mysterious force that pervades all space, acting as a "push" to accelerate the Universe's expansion. Despite being 70 percent of the Universe, dark energy was only discovered in 1998 by two teams observing Type Ia supernovae. A Type 1a supernova is a cataclysmic explosion of a white dwarf star.

These supernovae are currently the best way to measure dark energy because they are visible across intergalactic space. Also, they can function as "standard candles" in distant galaxies since the intrinsic brightness is known. Just as drivers estimate the distance to oncoming cars at night from the brightness of their headlights, measuring the apparent brightness of a supernova yields its distance (fainter is farther). Measuring distances tracks the effect of dark energy on the expansion of the Universe.
The best way of measuring dark energy just got better, thanks to a new study of Type Ia supernovae led by Ryan Foley of the Harvard-Smithsonian Center for Astrophysics. He has found a way to correct for small variations in the appearance of these supernovae, so that they become even better standard candles. The key is to sort the supernovae based on their color.
"Dark energy is the biggest mystery in physics and astronomy today. Now, we have a better way to tackle it," said Foley, who is a Clay Fellow at the Center. He presented his findings in a press conference at the 217th meeting of the American Astronomical Society.
The new tool also will help astronomers to firm up the cosmic distance scale by providing more accurate distances to faraway galaxies.
Type Ia supernovae are used as standard candles, meaning they have a known intrinsic brightness. However, they're not all equally bright. Astronomers have to correct for certain variations. In particular, there is a known correlation between how quickly the supernova brightens and dims (its light curve) and the intrinsic peak brightness.
Even when astronomers correct for this effect, their measurements still show some scatter, which leads to inaccuracies when calculating distances and therefore the effects of dark energy. Studies looking for ways to make more accurate corrections have had limited success until now.
"We've been looking for this sort of 'second-order effect' for nearly two decades," said Foley.
Foley discovered that after correcting for how quickly Type Ia supernovae faded, they show a distinct relationship between the speed of their ejected material and their color: the faster ones are slightly redder and the slower ones are bluer.
Previously, astronomers assumed that redder explosions only appeared that way because of intervening dust, which would also dim the explosion and make it appear farther than it was. Trying to correct for this, they would incorrectly calculate that the explosion was closer than it appeared. Foley's work shows that some of the color difference is intrinsic to the supernova itself.
The new study succeeded for two reasons. First, it used a large sample of more than 100 supernovae. More importantly, it went back to "first principles" and reexamined the assumption that Type Ia supernovae are one average color.
The discovery provides a better physical understanding of Type Ia supernovae and their intrinsic differences. It also will allow cosmologists to improve their data analysis and make better measurements of dark energy -- an important step on the road to learning what this mysterious force truly is, and what it means for the future of the cosmos.

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The above story is based on materials provided by Harvard-Smithsonian Center for Astrophysics. Note: Materials may be edited for content and length.

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