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Cascades with carbon dioxide: Making substances out of CO2

Carbon dioxide (CO2) is not just an undesirable greenhouse gas, it is also an interesting source of raw materials that are valuable and can be recycled sustainably. In the journal Angewandte Chemie, Spanish researchers have now introduced a novel catalytic process for converting CO2 into valuable chemical intermediates in the form of cyclic carbonates.

Getting CO2 to react is unfortunately not easy. Currently, most research is focused on the conversion of CO2 into methanol, which can be used as an alternative fuel as well as a feedstock for the chemical industry. Innovative catalytic processes could allow CO2 to be converted into valuable chemical compounds without taking a detour through methanol, perhaps for the production of biodegradable plastics or pharmaceutical intermediates.

One highly promising approach is the conversion of CO2 into organic carbonates, which are compounds that contain a building block derived from carbonic acid, comprising carbon atom attached to three oxygen atoms. Researchers working with Arjan W. Kleij at the Barcelona Institute of Science and Technology (Barcelona), the Institute of Chemical Research of Catalonia (Tarragona), and the Catalan Institute of Research and Advanced Studies (Barcelona), have developed a conceptually new process to produce carbonates in the form of six-membered rings, starting from CO2 and basic, easily accessible building blocks. These cyclic carbonates have great potential for the creation of new CO2-based polycarbonates.

The starting materials are compounds with a carbon-carbon double bond and an alcohol group (-OH) on a neighboring carbon atom (homoallylic alcohols). In the first step of the reaction, the double bond is converted into an epoxide, a three-membered ring with one oxygen and two carbon atoms. The epoxide is able to react with CO2 in the presence of a specific catalyst. The product is a cyclic carbonate in the form of a five-membered ring with three carbon and two oxygen atoms. The carbon atom at the “tip” of the five-membered ring is attached to an additional oxygen atom. In the next step, an organic catalyst (N-heterocyclic base) activates the OH group and causes the five-membered ring to rearrange into a six-membered ring. The oxygen atom from the OH group is integrated into the new ring, while one of the oxygen atoms from the original five-membered ring forms a new OH group. However, the reverse reaction also takes place because the original five-membered ring is significantly more energetically favorable, and only a vanishingly small amount of the six-membered ring is present at equilibrium. The trick is to trap the six-membered ring. The new OH group binds to a reagent (acylation) because its different position makes it considerably more reactive than the original OH group.

This newly developed process gives access to a broad palette of novel, six-membered carbonate rings in excellent yields, with high selectivity and under mild reaction conditions. This widens the repertoire of CO2-based heterocycles and polymers, which are difficult to produce by conventional methods.

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LamasaTech Launches APIs for its Temperature Kiosk Solutions

Source: ATM Marketplace

LamasaTech has unveiled the release of its APIs which allow integration with its Zentron temperature kiosk range. This allows organizations to integrate temperature scanning stations into their existing visitor and personnel management processes as they adapt to operating post COVID-19 lockdowns.

Organizations can build a small application to write the information into their database or integrate with a full application, such as an HR system, for seamless two-way sync. Unique to the market, the APIs allow both cloud and on-premise applications to integrate with the temperature check devices.

Customers are required to create an API with five endpoints from their side to send and receive the information from LamasaTech’s kiosks. All scan logs including name and temperature reading can be transmitted. From the other party, new users can be registered or users can be deleted with this information fed directly into LamasaTech’s application. The API calls can run as frequently as every one minute to keep the applications running in sync.

LamasaTech has developed its APIs to eliminate any heavy lifting for its global customer base. Simplicity and speed of set-up are crucial, with the APIs designed to enable quick set-up.

LamasaTech introduced its Zentron kiosks in March 2020 in response to the global pandemic. These non-contact kiosks automatically read an individual’s temperature in less than a second. Since launch, LamasaTech has released several software updates including instant email alerts with custom SMTP settings, an automatic update feature and ID badge printing.

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Researchers introduce new theory to calculate emissions liability

A comparison of the results for conventional point source pollution and bottleneck carbon emissions sources shows that oil and natural gas pipelines are far more important than simple point-source emissions calculations would indicate. It also shifts the emissions liability towards the East Coast from the Midwest. Most surprisingly, the study found that seven out of eight oil pipelines in the U.S. responsible for facilitating the largest amount of carbon emissions are not American.

Fossil fuels (coal, oil and natural gas) emit carbon dioxide when burned, which scientists say is the greenhouse gas primarily responsible for global warming and climate change. Climate change causes numerous problems that economists call “externalities,” because they are external to the market. In a new study published in Energies, Alexis Pascaris, graduate student in environmental and energy policy, and Joshua Pearce, the Witte Professor of Engineering, both of Michigan Technological University, explain how current U.S. law does not account for these costs and explore how litigation could be used to address this flaw in the market. The study also investigates which companies would be at most risk.

Pearce explained their past work found that “as climate science moves closer to being able to identify which emitters are responsible for climate costs and disasters, emissions liability is becoming a profound business risk for some companies.”

Most work in carbon emissions liability focuses on who did the wrong and what the costs are. Pascaris and Pearce’s “bottleneck” theory places the focus on who enables emissions.

Focusing Efforts

The U.S. Environmental Protection Agency defines point source pollution as “any single identifiable source of pollution from which pollutants are discharged.” For example, pipelines themselves create very little point source pollution, yet an enormous amount of effort has been focused on stopping the Keystone XL Pipeline because of the presumed emissions it enables.

The Michigan Tech study asked: Would the magnitude of the emissions enabled by a pipeline warrant the effort, or should lawsuits be focused elsewhere if minimizing climate change is the goal?

In order to answer this question quantitatively, the study presented an open and transparent methodology for prioritizing climate lawsuits based on an individual facility’s ability to act as a bottleneck for carbon emissions.

“Just like a bottleneck that limits the flow of water, what our emissions bottleneck theory does is identify what carbon emissions would be cut off if a facility was eliminated rather than only provide what emissions come directly from it as a point source,” Pearce said. “This study found that point source pollution in the context of carbon emissions can be quite misleading.”

The results showed that the prominent carbon emission bottlenecks in the U.S. are for transportation of oil and natural gas. While the extraction of oil is geographically concentrated in both North Dakota and Texas, the pipeline network is extensive and transcends both interstate and national boundaries, further complicating legal issues.

Overall, seven of eight oil pipelines in the U.S. are foreign owned and accountable for contributing 74% of the entire oil industry’s carbon emissions. They are a likely prioritization for climate-related lawsuits and thus warrant higher climate liability insurance premiums.

As a whole, fossil-fuel related companies identified in the study have increased risks due to legal liability, future regulations meant to curb climate destabilization and as targets for eco-terrorism.

“All of these business risks would tend to increase insurance costs, but significant future work is needed to quantify what climate liability insurance costs should be for companies that enable major carbon emissions,” concluded Pearce.

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Efficient low-cost system for producing power at night

Researchers have designed an off-grid, low-cost modular energy source that can efficiently produce power at night. The system uses commercially available technology and could eventually help meet the need for nighttime lighting in urban areas or provide lighting in developing countries.

Although solar power brings many benefits, its use depends heavily on the distribution of sunlight, which can be limited in many locations and is completely unavailable at night. Systems that store energy produced during the day are typically expensive, thus driving up the cost of using solar power.

To find a less-expensive alternative, researchers led by Shanhui Fan from Stanford University looked to radiative cooling. This approach uses the temperature difference resulting from heat absorbed from the surrounding air and the radiant cooling effect of cold space to generate electricity.

In The Optical Society (OSA) journal Optics Express, the researchers theoretically demonstrate an optimized radiative cooling approach that can generate 2.2 Watts per square meter with a rooftop device that doesn’t require a battery or any external energy. This is about 120 times the amount of energy that has been experimentally demonstrated and enough to power modular sensors such as ones used in security or environmental applications.

“We are working to develop high-performance, sustainable lighting generation that can provide everyone — including those in developing and rural areas — access to reliable and sustainable low cost lighting energy sources,” said Lingling Fan, first author of the paper. “A modular energy source could also power off-grid sensors used in a variety of applications and be used to convert waste heat from automobiles into usable power.”

Maximizing power generation

One of the most efficient ways to generate electricity using radiative cooling is to use a thermoelectric power generator. These devices use thermoelectric materials to generate power by converting the temperature differences between a heat source and the device’s cool side, or radiative cooler, into an electric voltage.

In the new work, the researchers optimized each step of thermoelectric power generation to maximize nighttime power generation from a device that would be used on a rooftop. They improved the energy harvesting so that more heat flows into the system from the surrounding air and incorporate new commercially available thermoelectric materials that enhance how well that energy is used by the device. They also calculated that a thermoelectric power generator covering one square meter of a rooftop could achieve the best trade-off between heat loss and thermoelectric conversion.

“One of the most important innovations was designing a selective emitter that is attached to the cool side of the device,” said Wei Li, a member of the research team. “This optimizes the radiative cooling process so that the power generator can more efficiently get rid of excessive heat.”

The researchers demonstrated the new approach by using computer modeling to simulate a system with realistic physical parameters. The models reproduced previous experimental results faithfully and revealed that the optimized system designed by the researchers could come close to what has been calculated as the maximum efficiency using thermoelectric conversion.

In addition to carrying out experiments, the researchers are also examining optimal designs for operating the system during the day, in addition to nighttime, which could expand the practical applications of the system.

This work is supported by the U.S. Department of Energy under Grant No. DE-FG02-07ER46426.

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White dwarfs reveal new insights into the origin of carbon in the universe

A new analysis of white dwarf stars supports their role as a key source of carbon, an element crucial to all life, in the Milky Way and other galaxies.

Approximately 90 percent of all stars end their lives as white dwarfs, very dense stellar remnants that gradually cool and dim over billions of years. With their final few breaths before they collapse, however, these stars leave an important legacy, spreading their ashes into the surrounding space through stellar winds enriched with chemical elements, including carbon, newly synthesized in the star’s deep interior during the last stages before its death.

Every carbon atom in the universe was created by stars, through the fusion of three helium nuclei. But astrophysicists still debate which types of stars are the primary source of the carbon in our own galaxy, the Milky Way. Some studies favor low-mass stars that blew off their envelopes in stellar winds and became white dwarfs, while others favor massive stars that eventually exploded as supernovae.

In the new study, published July 6 in Nature Astronomy, an international team of astronomers discovered and analyzed white dwarfs in open star clusters in the Milky Way, and their findings help shed light on the origin of the carbon in our galaxy. Open star clusters are groups of up to a few thousand stars, formed from the same giant molecular cloud and roughly the same age, and held together by mutual gravitational attraction. The study was based on astronomical observations conducted in 2018 at the W. M. Keck Observatory in Hawaii and led by coauthor Enrico Ramirez-Ruiz, professor of astronomy and astrophysics at UC Santa Cruz.

“From the analysis of the observed Keck spectra, it was possible to measure the masses of the white dwarfs. Using the theory of stellar evolution, we were able to trace back to the progenitor stars and derive their masses at birth,” Ramirez-Ruiz explained.

The relationship between the initial masses of stars and their final masses as white dwarfs is known as the initial-final mass relation, a fundamental diagnostic in astrophysics that integrates information from the entire life cycles of stars, linking birth to death. In general, the more massive the star at birth, the more massive the white dwarf left at its death, and this trend has been supported on both observational and theoretical grounds.

But analysis of the newly discovered white dwarfs in old open clusters gave a surprising result: the masses of these white dwarfs were notably larger than expected, putting a “kink” in the initial-final mass relation for stars with initial masses in a certain range.

“Our study interprets this kink in the initial-final mass relationship as the signature of the synthesis of carbon made by low-mass stars in the Milky Way,” said lead author Paola Marigo at the University of Padua in Italy.

In the last phases of their lives, stars twice as massive as our Sun produced new carbon atoms in their hot interiors, transported them to the surface, and finally spread them into the interstellar medium through gentle stellar winds. The team’s detailed stellar models indicate that the stripping of the carbon-rich outer mantle occurred slowly enough to allow the central cores of these stars, the future white dwarfs, to grow appreciably in mass.

Analyzing the initial-final mass relation around the kink, the researchers concluded that stars bigger than 2 solar masses also contributed to the galactic enrichment of carbon, while stars of less than 1.5 solar masses did not. In other words, 1.5 solar masses represents the minimum mass for a star to spread carbon-enriched ashes upon its death.

These findings place stringent constraints on how and when carbon, the element essential to life on Earth, was produced by the stars of our galaxy, eventually ending up trapped in the raw material from which the Sun and its planetary system were formed 4.6 billion years ago.

“Now we know that the carbon came from stars with a birth mass of not less than roughly 1.5 solar masses,” said Marigo.

Coauthor Pier-Emmanuel Tremblay at University of Warwick said, “One of most exciting aspects of this research is that it impacts the age of known white dwarfs, which are essential cosmic probes to understand the formation history of the Milky Way. The initial-to-final mass relation is also what sets the lower mass limit for supernovae, the gigantic explosions seen at large distances and that are really important to understand the nature of the universe.”

By combining the theories of cosmology and stellar evolution, the researchers concluded that bright carbon-rich stars close to their death, quite similar to the progenitors of the white dwarfs analyzed in this study, are presently contributing to a vast amount of the light emitted by very distant galaxies. This light, carrying the signature of newly produced carbon, is routinely collected by large telescopes to probe the evolution of cosmic structures. A reliable interpretation of this light depends on understanding the synthesis of carbon in stars.

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A binary star as a cosmic particle accelerator

Scientists have identified the binary star Eta Carinae as a new kind of source for very high-energy (VHE) cosmic gamma-radiation. Eta Carinae is located 7500 lightyears away in the constellation Carina in the Southern Sky and, based on the data collected, emits gamma rays with energies up to 400 gigaelectronvolts (GeV), some 100 billion times more than the energy of visible light.

With a specialised telescope in Namibia a DESY-led team of researchers has proven a certain type of binary star as a new kind of source for very high-energy cosmic gamma-radiation. Eta Carinae is located 7500 lightyears away in the constellation Carina (the ship’s keel) in the Southern Sky and, based on the data collected, emits gamma rays with energies all the way up to 400 gigaelectronvolts (GeV), some 100 billion times more than the energy of visible light. The team headed by DESY’s Stefan Ohm, Eva Leser and Matthias Füßling is presenting its findings, made at the gamma-ray observatory High Energy Stereoscopic System (H.E.S.S.), in the journal Astronomy & Astrophysics. An accompanying multimedia animation explains the phenomenon. “With such visualisations we want to make the fascination of research tangible,” emphasises DESY’s Director of Astroparticle Physics, Christian Stegmann.

Eta Carinae is a binary system of superlatives, consisting of two blue giants, one about 100 times, the other about 30 times the mass of our sun. The two stars orbit each other every 5.5 years in very eccentric elliptical orbits, their separation varying approximately between the distance from our Sun to Mars and from the Sun to Uranus. Both these gigantic stars fling dense, supersonic stellar winds of charged particles out into space. In the process, the larger of the two loses a mass equivalent to our entire Sun in just 5000 years or so. The smaller one produces a fast stellar wind travelling at speeds around eleven million kilometres per hour (about one percent of the speed of light).

A huge shock front is formed in the region where these two stellar winds collide, heating up the material in the wind to extremely high temperatures. At around 50 million degrees Celsius, this matter radiates brightly in the X-ray range. The particles in the stellar wind are not hot enough to emit gamma radiation, though. “However, shock regions like this are typically sites where subatomic particles are accelerated by strong prevailing electromagnetic fields,” explains Ohm, who is the head of the H.E.S.S. group at DESY. When particles are accelerated this rapidly, they can also emit gamma radiation. In fact, the satellites “Fermi,” operated by the US space agency NASA, and AGILE, belonging to the Italian space agency ASI, already detected energetic gamma rays of up to about 10 GeV coming from Eta Carinae in 2009.

“Different models have been proposed to explain how this gamma radiation is produced,” Füßling reports. “It could be generated by accelerated electrons or by high-energy atomic nuclei.” Determining which of these two scenarios is correct is crucial: very energetic atomic nuclei account for the bulk of the so-called Cosmic Rays, a subatomic cosmic hailstorm striking Earth constantly from all directions. Despite intense research for more than 100 years, the sources of the Cosmic Rays are still not exhaustively known. Since the electrically charged atomic nuclei are deflected by cosmic magnetic fields as they travel through the universe, the direction from which they arrive at Earth no longer points back to their origin. Cosmic gamma rays, on the other hand, are not deflected. So, if the gamma rays emitted by a specific source can be shown to originate from high-energy atomic nuclei, one of the long-sought accelerators of cosmic particle radiation will have been identified.

“In the case of Eta Carinae, electrons have a particularly hard time getting accelerated to high energyies, because they are constantly being deflected by magnetic fields during their acceleration, which makes them lose energy again,” says Leser. “Very high-energy gamma radiation begins above the 100 GeV range, which is rather difficult to explain in Eta Carinae to stem from electron acceleration.” The satellite data already indicated that Eta Carinae also emits gamma radiation beyond 100 GeV, and H.E.S.S. has now succeeded in detecting such radiation up to energies of 400 GeV around the time of the close encounter of the two blue giants in 2014 and 2015. This makes the binary star the first known example of a source in which very high-energy gamma radiation is generated by colliding stellar winds.

“The analysis of the gamma radiation measurements taken by H.E.S.S. and the satellites shows that the radiation can best be interpreted as the product of rapidly accelerated atomic nuclei,” says DESY’s PhD student Ruslan Konno, who has published a companion study, together with scientists from the Max Planck Institute for Nuclear Physics in Heidelberg. “This would make the shock regions of colliding stellar winds a new type of natural particle accelerator for cosmic rays.” With H.E.S.S., which is named after the discoverer of Cosmic Rays, Victor Franz Hess, and the upcoming Cherenkov Telescope Array (CTA), the next-generation gamma-ray observatory currently being built in the Chilean highlands, the scientists hope to investigate this phenomenon in greater detail and discover more sources of this kind.

“I find science and scientific research extremely important,” says Nicolai, who sees close parallels in the creative work of artists and scientists. For him, the appeal of this work also lay in the artistic mediation of scientific research results: “particularly the fact that it is not a film soundtrack, but has a genuine reference to reality,” emphasizes the musician and artist. Together with the exclusively composed sound, this unique collaboration of scientists, animation artists and musician has resulted in a multimedia work that takes viewers on an extraordinary journey to a superlative double star some 7500 light years away.

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Etching the road to a hydrogen economy using plasma jets

Hydrogen is a clean energy source that can be produced by splitting water molecules with light. However, it is currently impossible to achieve this on a large scale. In a recent breakthrough, scientists developed a novel method that uses plasma discharge in solution to improve the performance of the photocatalyst in the water-splitting reaction. This opens doors to exploring a number of photocatalysts that can help scale-up this reaction.

The ever-worsening global environmental crisis, coupled with the depletion of fossil fuels, has motivated scientists to look for clean energy sources. Hydrogen (H2) can serve as an eco-friendly fuel, and hydrogen generation has become a hot research topic. While no one has yet found an energy-efficient and affordable way to produce hydrogen on a large scale, progress in this field is steady and various techniques have been proposed.

One such technique involves using light and catalysts (materials that speed up reactions) to split water (H2O) into hydrogen and oxygen. The catalysts have crystalline structures and the ability to separate charges at the interfaces between some of their sides. When light hits the crystal at certain angles, the energy from the light is absorbed into the crystal, causing certain electrons to become free from their original orbits around atoms in the material. As an electron leaves its original place in the crystal, a positively charged vacancy, known as a hole, is created in the structure. Generally, these “excited” states do not last long, and free electrons and holes eventually recombine.

This is the case with bismuth vanadate (BiVO4) crystal catalysts as well. BiVO4 has been recently explored for water-splitting reactions, given its promise as a material in which charge-separation can occur upon excitation with visible light. The quick recombination of pairs of charged entities (“carriers”) is a disadvantage because carriers must separately partake in reactions that break up water.

In a recent study published in Chemical Engineering Journal, scientists from the Photocatalysis International Research Center at Tokyo University of Science, Japan, together with scientists from Northeast Normal University in China, developed a novel method to improve the charge-separation characteristics of decahedral (ten-sided) BiVO4 crystal catalysts. Prof Terashima, lead scientist in the study, explains, “Recent studies have shown that carriers can be generated and separated at the interfaces between the different faces of certain crystals. In the case of BiVO4, however, the forces that separate carriers are too weak for electron-hole pairs that are generated slightly away from the interfaces. Therefore, carrier separation in BiVO4 decahedrons called for further improvements, which motivated us to carry out this study.”

In the technique they propose, BiVO4 nanocrystals are exposed to what is called “solution plasma discharge,” a highly charged jet of energetic matter that is produced by applying high voltages between two terminals submerged in water. The plasma discharge removes some vanadium (V) atoms from the surface of specific faces of the crystals, leaving vanadium vacancies. These vacancies act as “electron traps” that facilitate the increased separation of carriers. Because these vacancies are in greater number on the eight side faces of the decahedron, electrons are trapped on these faces while holes accumulate on the top and bottom faces. This increased charge separation results in better catalytic performance of the BiVO4 nanocrystals, thereby improving its water splitting performance.

This study represents a novel use of solution plasma discharge to enhance the properties of crystals. Prof Akira Fujishima, co-author of the paper, says, “Our work has inspired us to reconsider other crystals that are apparently ineffective for water splitting. It provides a promising strategy using solution plasma to ‘activate’ them.” The use of solution-plasma discharge has many advantages over using conventional gaseous plasma that make it far more attractive from both technical and economic standpoints. Prof Xintong Zhang from Northeast Normal University, China, remarks, “Unlike gaseous plasma, which has to be generated in closed chambers, solution plasma can be generated in an open reactor at room temperature and in a normal air atmosphere. In addition, by working with crystal powders in a solution, it becomes more convenient to change the parameters of the process, and it is also easier to scale up.”

This study hopefully takes us one step closer to an efficient way of producing hydrogen so that we can finally do without fossil fuels and other energy sources that are harmful to our planet. Further commenting on the promise of this study, Prof Terashima says, “If efficient hydrogen energy can be produced using sunlight and water, two of the most abundant resources on earth, a dream clean society could be realized.”

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Ultra-bright X-ray source awakens near a galaxy not so far away

A new ultra-bright source of X-rays has awakened in between our galactic neighbours the Magellanic Clouds, after a 26-year slumber. This is the second-closest such object known to date, with a brightness greater than a million Suns. The discovery is published in the journal Monthly Notices of the Royal Astronomical Society.

The object, known as RX J0209.6-7427, was first detected during a 6-month long outburst in 1993. Though it was initially identified as a Be-type X-ray binary, its true nature remained a mystery as it lingered in a dormant state for the next 26 years, only flaring up again in November last year.

Now, a team of Indian scientists have used AstroSat, India’s first dedicated space observatory, to reveal the extreme nature of the source, and have detected broad-energy X-ray pulsations in the object for the first time. This classifies it as a type of object known as an ultra-luminous X-ray pulsar (ULXP).

The pulsar is located in the Magellanic Bridge, a stream of gas and stars linking the Magellanic Clouds. These are two of our nearest galactic companions, and some of the most distant objects visible to the naked eye. The new X-ray source is the second-closest ULXP known to date, after a 2018 discovery in our own Milky Way galaxy, and is only the eighth such object ever discovered.

Ultra-luminous X-ray sources are observable as single points in the sky, but with brightnesses comparable to entire galaxies. “The conventional theory is that in order to shine so brightly, ULXPs must be glowing accretion discs around black holes,” said Amar Deo Chandra, lead author on the new study. “However, recent discoveries of pulsations in these objects suggest that they may in fact have neutron stars at their heart.”

A neutron star is the remnant of a dead star which contains as much matter as our Sun, but is compressed into a tiny radius of as little as 10km — the size of a small city. The neutron star in this object is thought to be spinning as rapidly as 100 times per second, and emits pulses of energetic X-rays from its magnetic poles, leading to the new ‘X-ray pulsar’ classification.

The group of astronomers, from IISER Kolkata, IUCAA Pune and the Center for Excellence in Basic Sciences (UM-DAE CEBS) Mumbai, have also found that the pulsar may even be speeding up, setting off bright X-ray ‘fireworks’. This is thought to happen when the neutron star captures material from a companion star, injecting energy into the system and speeding up the rotation.

The scarcity of similar sources makes detecting and studying new ULXPs essential for X-ray astronomers seeking to understand the Universe.

“This is only the eighth ULXP detected so far, and the first one near the Magellanic Clouds,” Chandra adds. “It raises the interesting possibility that a significant fraction of ultra-luminous X-ray sources may really be neutron stars accreting at super Eddington rates, rather than black holes as previously thought.”

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X-ray vision through the water window

ETH physicists have developed the first high-​repetition-rate laser source that produces coherent soft x-​rays spanning the entire ‘water window’. That technological breakthrough should enable a broad range of studies in the biological, chemical and material sciences as well as in physics.

The ability to generate light pulses of sub-​femtosecond duration, first demonstrated some 20 years ago, has given rise to an entirely new field: attosecond science and technology. Table-​top laser systems have emerged that enable studies that for decades were but a distant dream — to follow, image and characterise electronic processes in atoms, molecules and solids on their natural, attosecond timescales. The laser systems that make such studies possible typically operate in the extreme ultraviolet spectral band. There has long been a push to achieve higher photon energies though. Of particular interest is the ‘water window’, occupied by soft x-​ray radiation with wavelengths between 2.2 and 4.4 nm. That spectral window owes its name, and importance, to the fact that at those frequencies, photons are not absorbed by oxygen (and hence by water), but they are by carbon. This is ideal for studying organic molecules and biological specimens in their natural aqueous environment. Today, a handful of attosecond sources spanning this frequency range exist, but their applicability is limited by relatively low repetition rates of 1 kHz or below, which in turn means low count rates and poor signal-​to-noise ratios. Writing in Optica, Justinas Pupeikis and colleagues in the Ultrafast Laser Physics group of Prof. Ursula Keller at the Institute for Quantum Electronics report now an essential leap to overcome the limitations of the prior sources. They present the first soft-​x-ray source that spans the full water window at 100 kHz repetition rate — a hundredfold improvement compared to the state-​of-the-art sources.

A boost in technological capability

The bottleneck in producing soft x-​rays at high repetition rates has been the lack of suitable laser systems to drive the key process underlying attosecond-​pulse generation in table-​top systems. That process is known as high-​harmonic generation, and it involves an intense femtosecond laser pulse interacting with a target, typically an atomic gas. The nonlinear electronic response of the target then causes the emission of attosecond pulses at an odd-​order multiple of the frequency of the driving laser field. To ensure that that response contains x-​ray photons spanning the water-​window range, the femtosecond source has to operate in the mid-​infrared range. Also, it has to deliver high-​peak-power pulses. And all of that at high repetition rates. Such a source did not exist so far.

Pupeikis et al. took up the challenge and systematically improved a layout they had already explored in earlier work, based on optical parametric chirped pulse amplification (or OPCPA for short). They had established before that the approach is promising with a view to realizing high-​power mid-​infrared sources, but substantial improvements were still needed to reach the performance required for the high-​harmonic generation of x-​ray photons in the water window. In particular, they pushed the peak power from previously 6.3 GW to 14.2 GW, and they reached an average power of 25 W for pulses just a bit longer than two oscillations of the underlying optical field (16.5 fs). The peak power demonstrated is comfortably the highest reported to date for any high-​repetition-rate system with a wavelength above 2 μm.

Ready for the x-​ray room

With this level of performance at their disposal, the team was ready for the next stage, frequency upconversion through high-​harmonic generation. For that, the output beam of the OPCPA was routed via a periscope system to another laboratory more than 15 m away, to accommodate for local lab-​space constraints. There, the beam met a helium target, kept at a pressure of 45 bar. Such high pressure was necessary for phase-​matching between the infrared and the x-​ray radiation, and thus optimal energy-​conversion efficiency.

All pieces carefully put in place, the system indeed delivered. It generated coherent soft x-​ray radiation extending to an energy of 620 eV (2 nm wavelength), covering the full water window — a stand-​out achievement relative to other high-​repetition-rate sources in this frequency range. A window of opportunity

This demonstration opens up a vast spectrum of fresh opportunities. Coherent imaging in the water-​window spectral region, highly relevant for chemistry and biology, should be possible with a compact setup. At the same time, the high repetition rate available helps, for instance, addressing the limitations due to space-​charge formation which plague photoemission experiments with pulsed sources. Moreover, the ‘water window’ comprises not only the K-​edges of carbon, nitrogen and oxygen, but also the L- and M-​edges of a range of metals, which can now be studied with higher sensitivity or specificity.

With such bright prospects, the realization of the source now presented heralds the beginning of the next generation of attosecond technology, one where experimentalists for the first time can make combined use of high repetition rates and high photon energies. An attosecond beamline designed to exploit these new capabilities is currently under construction in the Keller lab.

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Open source Spaghetti Detective AI software detects failed prints through webcam

With the majority of the world home-bound due to the COVID-19 outbreak, the open source community seems to be alive and kicking with its latest gift to 3D printing: an AI software that automatically pauses failed prints. The Spaghetti Detective (TSD) utilizes the webcam of a printer or home computer to detect when a print […]

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