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A ‘bang’ in LIGO and Virgo detectors signals most massive gravitational-wave source yet

For all its vast emptiness, the universe is humming with activity in the form of gravitational waves. Produced by extreme astrophysical phenomena, these reverberations ripple forth and shake the fabric of space-time, like the clang of a cosmic bell.

Now researchers have detected a signal from what may be the most massive black hole merger yet observed in gravitational waves. The product of the merger is the first clear detection of an “intermediate-mass” black hole, with a mass between 100 and 1,000 times that of the sun.

They detected the signal, which they have labeled GW190521, on May 21, 2019, with the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO), a pair of identical, 4-kilometer-long interferometers in the United States; and Virgo, a 3-kilometer-long detector in Italy.

The signal, resembling about four short wiggles, is extremely brief in duration, lasting less than one-tenth of a second. From what the researchers can tell, GW190521 was generated by a source that is roughly 5 gigaparsecs away, when the universe was about half its age, making it one of the most distant gravitational-wave sources detected so far.

As for what produced this signal, based on a powerful suite of state-of-the-art computational and modeling tools, scientists think that GW190521 was most likely generated by a binary black hole merger with unusual properties.

Almost every confirmed gravitational-wave signal to date has been from a binary merger, either between two black holes or two neutron stars. This newest merger appears to be the most massive yet, involving two inspiraling black holes with masses about 85 and 66 times the mass of the sun.

The LIGO-Virgo team has also measured each black hole’s spin and discovered that as the black holes were circling ever closer together, they could have been spinning about their own axes, at angles that were out of alignment with the axis of their orbit. The black holes’ misaligned spins likely caused their orbits to wobble, or “precess,” as the two Goliaths spiraled toward each other.

The new signal likely represents the instant that the two black holes merged. The merger created an even more massive black hole, of about 142 solar masses, and released an enormous amount of energy, equivalent to around 8 solar masses, spread across the universe in the form of gravitational waves.

“This doesn’t look much like a chirp, which is what we typically detect,” says Virgo member Nelson Christensen, a researcher at the French National Centre for Scientific Research (CNRS), comparing the signal to LIGO’s first detection of gravitational waves in 2015. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”

The international team of scientists, who make up the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration, have reported their findings in two papers published today. One, appearing in Physical Review Letters, details the discovery, and the other, in The Astrophysical Journal Letters, discusses the signal’s physical properties and astrophysical implications.

“LIGO once again surprises us not just with the detection of black holes in sizes that are difficult to explain, but doing it using techniques that were not designed specifically for stellar mergers,” says Pedro Marronetti, program director for gravitational physics at the National Science Foundation. “This is of tremendous importance since it showcases the instrument’s ability to detect signals from completely unforeseen astrophysical events. LIGO shows that it can also observe the unexpected.”

In the mass gap

The uniquely large masses of the two inspiraling black holes, as well as the final black hole, raise a slew of questions regarding their formation.

All of the black holes observed to date fit within either of two categories: stellar-mass black holes, which measure from a few solar masses up to tens of solar masses and are thought to form when massive stars die; or supermassive black holes, such as the one at the center of the Milky Way galaxy, that are from hundreds of thousands, to billions of times that of our sun.

However, the final 142-solar-mass black hole produced by the GW190521 merger lies within an intermediate mass range between stellar-mass and supermassive black holes — the first of its kind ever detected.

The two progenitor black holes that produced the final black hole also seem to be unique in their size. They’re so massive that scientists suspect one or both of them may not have formed from a collapsing star, as most stellar-mass black holes do.

According to the physics of stellar evolution, outward pressure from the photons and gas in a star’s core support it against the force of gravity pushing inward, so that the star is stable, like the sun. After the core of a massive star fuses nuclei as heavy as iron, it can no longer produce enough pressure to support the outer layers. When this outward pressure is less than gravity, the star collapses under its own weight, in an explosion called a core-collapse supernova, that can leave behind a black hole.

This process can explain how stars as massive as 130 solar masses can produce black holes that are up to 65 solar masses. But for heavier stars, a phenomenon known as “pair instability” is thought to kick in. When the core’s photons become extremely energetic, they can morph into an electron and antielectron pair. These pairs generate less pressure than photons, causing the star to become unstable against gravitational collapse, and the resulting explosion is strong enough to leave nothing behind. Even more massive stars, above 200 solar masses, would eventually collapse directly into a black hole of at least 120 solar masses. A collapsing star, then, should not be able to produce a black hole between approximately 65 and 120 solar masses — a range that is known as the “pair instability mass gap.”

But now, the heavier of the two black holes that produced the GW190521 signal, at 85 solar masses, is the first so far detected within the pair instability mass gap.

“The fact that we’re seeing a black hole in this mass gap will make a lot of astrophysicists scratch their heads and try to figure out how these black holes were made,” says Christensen, who is the director of the Artemis Laboratory at the Nice Observatory in France.

One possibility, which the researchers consider in their second paper, is of a hierarchical merger, in which the two progenitor black holes themselves may have formed from the merging of two smaller black holes, before migrating together and eventually merging.

“This event opens more questions than it provides answers,” says LIGO member Alan Weinstein, professor of physics at Caltech. “From the perspective of discovery and physics, it’s a very exciting thing.”

“Something unexpected”

There are many remaining questions regarding GW190521.

As LIGO and Virgo detectors listen for gravitational waves passing through Earth, automated searches comb through the incoming data for interesting signals. These searches can use two different methods: algorithms that pick out specific wave patterns in the data that may have been produced by compact binary systems; and more general “burst” searches, which essentially look for anything out of the ordinary.

LIGO member Salvatore Vitale, assistant professor of physics at MIT, likens compact binary searches to “passing a comb through data, that will catch things in a certain spacing,” in contrast to burst searches that are more of a “catch-all” approach.

In the case of GW190521, it was a burst search that picked up the signal slightly more clearly, opening the very small chance that the gravitational waves arose from something other than a binary merger.

“The bar for asserting we’ve discovered something new is very high,” Weinstein says. “So we typically apply Occam’s razor: The simpler solution is the better one, which in this case is a binary black hole.”

But what if something entirely new produced these gravitational waves? It’s a tantalizing prospect, and in their paper the scientists briefly consider other sources in the universe that might have produced the signal they detected. For instance, perhaps the gravitational waves were emitted by a collapsing star in our galaxy. The signal could also be from a cosmic string produced just after the universe inflated in its earliest moments — although neither of these exotic possibilities matches the data as well as a binary merger.

“Since we first turned on LIGO, everything we’ve observed with confidence has been a collision of black holes or neutron stars,” Weinstein says “This is the one event where our analysis allows the possibility that this event is not such a collision. Although this event is consistent with being from an exceptionally massive binary black hole merger, and alternative explanations are disfavored, it is pushing the boundaries of our confidence. And that potentially makes it extremely exciting. Because we have all been hoping for something new, something unexpected, that could challenge what we’ve learned already. This event has the potential for doing that.”

This research was funded by the U.S. National Science Foundation.

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The quiet Sun is much more active than we thought

Solar activity varies in 11-year cycles. As the activity cycle switches to a new one, the Sun is usually very calm for several years.

For a long time, researchers have believed that there is not much of interest going on in the Sun during the passive period, therefore not worth studying. Now this assumption is showed to be false by Juha Kallunki, Merja Tornikoski and Irene Björklund, researchers at Metsähovi Radio Observatory, in their peer-reviewed research article published in Solar Physics. This is the first time that astronomers are systematically studying the phenomena of the solar minimum.

Not all phenomena could be explained — yet

The researchers reached their conclusion by examining the solar radio maps detected by the Metsähovi Radio Observatory and comparing them with the data collected by a satellite observing the Sun in the ultraviolet range. The solar maps showed active areas, or radio brightenings, which can be observed on the maps as hotter areas than the rest of the solar surface. According to researchers, there are three explanations for radio brightenings.

First, some brightenings were observed in the polar areas on the solar maps that could be identified as coronal holes. Particle flows, or solar winds, ejected by coronal holes can cause auroras when they reach the Earth’s atmosphere. The corona is the outer atmosphere of the Sun.

Second, the researchers observed brightenings from which, based on other observations, ejections of hot material from the surface of the sun could be detected.

Third, radio brightenings were found in areas where, based on satellite observations, strong magnetic fields were detected.

Researchers also found radio brightenings in some areas where no explanatory factor was found on the basis of satellite observations.

‘The other sources used did not explain the cause of the brightening. We don’t know what causes those phenomena. We must continue our research’, Kallunki says.

Additional observations and research are also needed to predict whether the phenomena of the solar minimum indicate something about the next active period, about its onset and intensity, for example. Each one of the last four cycles has been weaker than the previous one. Researchers do not know why the activity curves do not rise as high as during the previous cycles.

‘Solar activity cycles do not always last exactly 11 years, either’, explains Docent Merja Tornikoski.

‘A new activity period will not be identified until it is already ongoing. In any case, these observations of the quiet phase we are now analysing are clearly during a period when activity is at its lowest. Now we are waiting for a new rise in activity.’

Solar storms can cause danger

On the Earth, solar activity can be seen as auroras, for example. Solar activity can even cause major damage, as solar storms caused by solar flares can damage satellites, electricity networks and radio frequency communications. Research helps to prepare for such damage.

‘In solar storms, it takes 2 to 3 days before the particles hit the Earth. They reach satellites higher up in orbit much faster, which would leave us even less time to prepare for damage’, Kallunki points out.

Located in Kirkkonummi, Aalto University Metsähovi is the only astronomical radio observatory and continuously operational astronomical observation station in Finland. Metsähovi is internationally known for its unique, continuous datasets, including a solar monitoring programme spanning over 40 years that has collected data from scientifically very interesting high radio frequencies. This is possible thanks to the exceptionally precise mirror surface of the Metsähovi radio telescope.

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Motions in the Sun reveal inner workings of sunspot cycle

The Sun’s magnetic activity follows an eleven-year cycle. Over the course of a solar cycle, the Sun’s magnetic activity comes and goes. During solar maximum, large sunspots and active regions appear on the Sun’s surface. Spectacular loops of hot plasma stretch throughout the Sun’s atmosphere and eruptions of particles and radiation shoot into interplanetary space. During solar minimum, the Sun calms down considerably. A striking regularity appears in the so-called butterfly diagram, which describes the position of sunspots in a time-latitude plot. At the beginning of a solar cycle, sunspots emerge at mid-latitudes. As the cycle progresses, they emerge closer and closer to the equator. To explain this “butterfly diagram,” solar physicists suspect that the deep magnetic field is carried toward the equator by a large-scale flow.

“Over the course of a solar cycle, the meridional flow acts as a conveyor belt that drags the magnetic field along and sets the period of the solar cycle,” says Prof. Dr. Laurent Gizon, MPS Director and first author of the new study. “Seeing the geometry and the amplitude of motions in the solar interior is essential to understanding the Sun’s magnetic field,” he adds. To this end, Gizon and his team used helioseismology to map the plasma flow below the Sun’s surface.

Helioseismology is to solar physics what seismology is to geophysics. Helioseismologists use sound waves to probe the Sun’s interior, in much the same way geophysicists use earthquakes to probe the interior of the Earth. Solar sound waves have periods near five minutes and are continuously excited by near surface convection. The motions associated with solar sound waves can be measured at the Sun’s surface by telescopes on spacecrafts or on the ground. In this study, Gizon and his team used observations of sound waves at the surface that propagate in the north-south direction through the solar interior. These waves are perturbed by the meridional flow: they travel faster along the flow than against the flow. These very small travel-time perturbations (less than 1 second) were measured very carefully and were interpreted to infer the meridional flow using mathematical modeling and computers.

Because it is small, the meridional flow is extremely difficult to see in the solar interior. “The meridional flow is much slower than other components of motion, such as the Sun’s differential rotation,” Gizon explains. The meridional flow throughout the convection zone is no more than its maximum surface value of 50 kilometers per hour. “To reduce the noise level in the helioseismic measurements, it is necessary to average the measurements over very long periods of time,” says Dr. Zhi-Chao Liang of MPS.

The team of scientists analyzed, for the first time, two independent very long time series of data. One was provided by SOHO, the oldest solar observatory in space which is operated by ESA and NASA. The data taken by SOHO’s Michelson Doppler Imager (MDI) covers the time from 1996 until 2011. A second independent data set was provided by the Global Oscillation Network Group (GONG), which combines six ground-based solar telescopes in the USA, Australia, India, Spain, and Chile to offer nearly continuous observations of the Sun since 1995. “The international solar physics community is to be commended for delivering multiple datasets covering the last two solar cycles,” says Dr. John Leibacher, a former director of the GONG project. “This makes it possible to average over long periods of time and to compare answers, which is absolutely essential to validate inferences,” he adds.

Gizon and his team find the flow is equatorward at the base of the convection zone, with a speed of only 15 kilometers per hour (running speed). The flow at the solar surface is poleward and reaches up to 50 kilometers per hour. The overall picture is that the plasma goes around in one gigantic loop in each hemisphere. Remarkably, the time taken for the plasma to complete the loop is approximately 22 years — and this provides the physical explanation for the Sun’s eleven-year cycle. Furthermore, sunspots emerge closer to the equator as the solar cycle progresses, as is seen in the butterfly diagram. “All in all, our study supports the basic idea that the equatorward drift of the locations where sunspots emerge is due to the underlying meridional flows,” says Dr. Robert Cameron of MPS.

“It remains to be understood why the solar meridional flow looks like it does, and what role the meridional flow plays in controlling magnetic activity on other stars,” adds Laurent Gizon.

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Albirght Silicone introduces 3D printing capabilities; WACKER launches new ACEO silicone 3D printing material

3D printing with silicone is a rather niche area, however, activity does appear to be heating up. The two companies in this news update both offer 3D printing solutions for users who want to access the benefits and material properties of silicones. Albright Silicone, a Massachusetts-based engineering company, has launched a new 3D printing silicone […]

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Tungsten suboxide improves the efficiency of platinum in hydrogen production

Researchers presented a new strategy for enhancing catalytic activity using tungsten suboxide as a single-atom catalyst (SAC). This strategy, which significantly improves hydrogen evolution reaction (HER) in metal platinum (pt) by 16.3 times, sheds light on the development of new electrochemical catalyst technologies.

Hydrogen has been touted as a promising alternative to fossil fuels. However, most of the conventional industrial hydrogen production methods come with environmental issues, releasing significant amounts of carbon dioxide and greenhouse gases.

Electrochemical water splitting is considered a potential approach for clean hydrogen production. Pt is one of the most commonly used catalysts to improve HER performance in electrochemical water splitting, but the high cost and scarcity of Pt remain key obstacles to mass commercial applications.

SACs, where all metal species are individually dispersed on a desired support material, have been identified as one way to reduce the amount of Pt usage, as they offer the maximum number of surface exposed Pt atoms.

Inspired by earlier studies, which mainly focused on SACs supported by carbon-based materials, a KAIST research team led by Professor Jinwoo Lee from the Department of Chemical and Biomolecular Engineering investigated the influence of support materials on the performance of SACs.

Professor Lee and his researchers suggested mesoporous tungsten suboxide as a new support material for atomically dispersed Pt, as this was expected to provide high electronic conductivity and have a synergetic effect with Pt.

They compared the performance of single-atom Pt supported by carbon and tungsten suboxide respectively. The results revealed that the support effect occurred with tungsten suboxide, in which the mass activity of a single-atom Pt supported by tungsten suboxide was 2.1 times greater than that of single-atom Pt supported by carbon, and 16.3 times higher than that of Pt nanoparticles supported by carbon.

The team indicated a change in the electronic structure of Pt via charge transfer from tungsten suboxide to Pt. This phenomenon was reported as a result of strong metal-support interaction between Pt and tungsten suboxide.

HER performance can be improved not only by changing the electronic structure of the supported metal, but also by inducing another support effect, the spillover effect, the research group reported. Hydrogen spillover is a phenomenon where adsorbed hydrogen migrates from one surface to another, and it occurs more easily as the Pt size becomes smaller.

The researchers compared the performance of single-atom Pt and Pt nanoparticles supported by tungsten suboxide. The single-atom Pt supported by tungsten suboxide exhibited a higher degree of hydrogen spillover phenomenon, which enhanced the Pt mass activity for hydrogen evolution up to 10.7 times compared to Pt nanoparticles supported by tungsten suboxide.

Professor Lee said, “Choosing the right support material is important for improving electrocatalysis in hydrogen production. The tungsten suboxide catalyst we used to support Pt in our study implies that interactions between the well-matched metal and support can drastically enhance the efficiency of the process.”

This research was supported by the Ministry of Science and ICT and introduced in the International Edition of the German journal Angewandte Chemie.

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Computer model helps make sense of human memory

Brains are a mazy network of overlapping circuits — some pathways encourage activity while others suppress it. While earlier studies focused more on excitatory circuits, inhibitory circuits are now understood to play an equally important role in brain function. Researchers at the Okinawa Institute of Science and Technology Graduate University (OIST) and the RIKEN Center for Brain Science have created an artificial network to simulate the brain, demonstrating that tinkering with inhibitory circuits leads to extended memory.

Associative memory is the ability to connect unrelated items and store them in memory — to associate co-occurring items as a single episode. In this study, published in Physical Review Letters, the team used sequentially arranged patterns to simulate a memory, and found that a computer is able to remember patterns spanning a longer episode when the model takes inhibitory circuits into account. They go on to explain how this finding could be applied to explain our own brains.

“This simple model of processing shows us how the brain handles the pieces of information given in a serial order,” explains Professor Tomoki Fukai, head of OIST’s Neural Coding and Brain Computing Unit, who led the study with RIKEN collaborator Dr. Tatsuya Haga. “By modelling neurons using computers, we can begin to understand memory processing in our own minds.”

Lower Your Inhibitions

Thinking about the brain in terms of physical, non-biological phenomena is now a widely accepted approach in neuroscience — and many ideas lifted from physics have now been validated in animal studies. One such idea is understanding the brain’s memory system as an attractor network, a group of connected nodes that display patterns of activity and tend towards certain states. This idea of attractor networks formed the basis of this study.

A tenet of neurobiology is that “cells that fire together wire together” — neurons that are active at the same time become synchronized, which partly explains how our brains change over time. In their model, the team created excitatory circuits — patterns of neurons firing together — to replicate the brain. The model included many excitatory circuits spread across a network.

More importantly, the team inserted inhibitory circuits into the model. Different inhibitory circuits act locally on a particular circuit, or globally across the network. The circuits block unwanted signals from interfering with the excitatory circuits, which are then better able to fire and wire together. These inhibitory circuits allowed the excitatory circuits to remember a pattern representing a longer episode.

The finding matches what is currently known about the hippocampus, a brain region involved in associative memory. It is thought that a balance of excitatory and inhibitory activity is what allows new associations to form. Inhibitory activity could be regulated by a chemical called acetylcholine, which is known to play a role in memory within the hippocampus. This model is a digital representation of these processes.

A challenge to the approach, however, is the use of random sampling. The sheer number of possible outputs, or attractor states, in the network, overworks a computer’s memory capacity. The team instead had to rely on a selection of outputs, rather than a systematic review of every possible combination. This allowed them to overcome a technical difficulty without jeopardizing the model’s predictions.

Overall, the study allowed for overarching inferences — inhibitory neurons have an important role in associative memory, and this maps to what we might expect in our own brains. Fukai says that biological studies will need to be completed to determine the exact validity of this computational work. Then, it will be possible to map the components of the simulation to their biological counterparts, building a more complete picture of the hippocampus and associative memory.

The team will next move beyond a simple model toward one with additional parameters that better represents the hippocampus, and look at the relative importance of local and global inhibitory circuits. The current model comprises neurons that are either off or on — zeros and ones. A future model will include dendrites, the branches that connect neurons in a complicated mesh. This more realistic simulation will be even better placed to make conclusions about biological brains.

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