A team of international scientists, led by the Galician Institute of High Energy Physics (IGFAE) and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), has proposed a simple and novel method to bring the accuracy of the Hubble constant measurements down to 2%, using a single observation of a pair of merging neutron stars. The Universe is in continuous expansion. Because of this, distant objects such as galaxies move away from us. In fact, the further away they are, the faster they move. Scientists describe this expansion through a famous number known as the Hubble constant, which tells us how fast objects in the Universe recede from us depending on their distance to us. By measuring the Hubble constant in a precise way, we can also determine some of the most fundamental properties of the Universe, including its age. For decades, scientists have measured Hubble’s constant with increasing accuracy, collecting electromagnetic signals emitted throughout the Universe but arriving at a challenging result: the two current best measurements give inconsistent results. Since 2015, scientists have tried to tackle this challenge with the science of gravitational waves: ripples in the fabric of space-time that travel at the speed of light. Gravitational waves are generated in the most violent cosmic events and provide a new channel of information about the Universe. They’re emitted during the collision of two neutron stars—the dense cores of collapsed stars–and can help scientists dig deeper into the Hubble constant mystery. Unlike black holes, merging neutron stars produce both gravitational and electromagnetic waves, such as x-rays, radio waves and visible light. While gravitational waves can measure the distance between the neutron-star merger and Earth, electromagnetic waves can measure how fast its whole galaxy is moving away from Earth. This creates a new way to measure the Hubble constant. However, even with the help of gravitational waves, it’s still tricky to measure the distance to neutron-star mergers--that’s, in part, why current gravitational-wave based measurements of the Hubble constant have an uncertainty of ~16%, much larger than existing measurements using other traditional techniques. In a recently published article in the prestigious journal The Astrophysical Journal Letters, a team of scientists led by ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and Monash University alumni Prof Juan Calderón Bustillo (now La Caixa Junior Leader and Marie Curie Fellow at the Galician institute of High Energy Physics of the University of Santiago de Compostela, Spain), has proposed a simple and novel method to bring the accuracy of these measurements down to 2% using a single observation of a pair of merging neutron stars. According to Prof Calderón Bustillo, it’s difficult to interpret how far away these mergers occur because ‘currently, we can’t say if the binary is very far away and facing Earth, or if it’s much closer, with the Earth in its orbital plane’. To decide between these two scenarios, the team proposed to study secondary, much weaker components of the gravitational-wave signals emitted by neutron-star mergers, known as higher modes. ‘Just like an orchestra plays different instruments, neutron-star mergers emit gravitational waves through different modes,’ explains Prof Calderón Bustillo. ‘When the merging neutron stars are facing you, you will only hear the loudest instrument. However, if you are close to the merger’s orbital plane, you should also hear the secondary ones. This allows us to determine the inclination of the neutron-star merger, and better measure the distance’. However, the method is not completely new: ‘We know this works well for the case of very massive black hole mergers because our current detectors can record the merger instant when the higher modes are most prominent. But in the case of neutron stars, the pitch of the merger signal is so high that our detectors can’t record it. We can only record the earlier orbits,’ says Prof Calderón Bustillo. Future gravitational-wave detectors, like the proposed Australian project NEMO, will be able to access the actual merger stage of neutron stars. ‘When two neutron stars merge, the nuclear physics governing their matter can cause very rich signals that, if detected, could allow us to know exactly where the Earth sits with respect to the orbital plane of the merger,’ says co-author and OzGrav Chief Investigator Dr Paul Lasky, from Monash University. Dr Lasky is also one of the leads on the NEMO project. ‘A detector like NEMO could detect these rich signals,’ he adds. In their study, the team performed computer simulations of neutron-star mergers that can reveal the effect of the nuclear physics of the stars on the gravitational waves. Studying these simulations, the team determined that a detector like NEMO could measure Hubble’s constant with a precision of 2%. Co-author of the study Prof Tim Dietrich, from the University of Potsdam, says: ‘We found that fine details describing the way neutrons behave inside the star produce subtle signatures in the gravitational waves that can greatly help to determine the expansion rate of the Universe. It is fascinating to see how effects at the tiniest nuclear scale can infer what happens at the largest possible cosmological one’. Samson Leong, undergraduate student at The Chinese University of Hong Kong and co-author of the study points out “one of the most exciting things about our result is that we obtained such a great improvement while considering a rather conservative scenario. While NEMO will indeed be sensitive to the emission of neutron-star mergers, more evolved detectors like Einstein Telescope or Cosmic Explorer will be even more sensitive, therefore allowing us to measure the expansion of the Universe with even better accuracy!”. One of the most outstanding implications of this study is that it could determine if the Universe is expanding uniformly in space as currently hypothesised. 'Previous methods to achieve this level of accuracy rely on combining many observations, assuming that the Hubble constant is the same in all directions and throughout the history of the Universe,’ says Calderón Bustillo. ‘In our case, each individual event would yield a very accurate estimate of “its own Hubble constant”, allowing us to test if this is actually a constant or if it varies throughout space and time.’
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Remember the days before working from home? It's Monday morning, you're running late to beat the traffic, and you can't find your car keys. What do you do? You might try moving from room to room, casting your eye over every flat surface, in the hope of spotting the missing keys. Of course, this assumes that they are somewhere in plain sight; if they're hidden under a newspaper, or fallen behind the sofa, you'll never spot them. Or you might be so convinced that you last saw the keys in the kitchen and search for them there: inside every cupboard, the microwave, dishwasher, back of the fridge, etc. Of course, if you left them on your bedside table, upending the kitchen is doomed to failure. So, which is the best strategy? Scientists face a similar conundrum in the hunt for gravitational waves—ripples in the fabric of space and time—from rapidly spinning neutron stars. These stars are the densest objects in the Universe and, provided they're not perfectly spherical, emit a very faint "hum" of continuous gravitational waves. Hearing this "hum" would allow scientists to peer deep inside a neutron star and discover its secrets, yielding new insights into the most extreme states of matter. However, our very sensitive "ears"—4-kilometre-sized detectors using powerful lasers—haven’t heard anything yet. Part of the challenge is that, like the missing keys, scientists aren’t sure of the best search strategy. Most previous studies have taken the "room-to-room" approach, trying to find continuous gravitational waves in as many different places as possible. But this means you can only spend a limited amount of time listening for the tell-tale "hum" in any one location—in the same way that you can only spend so long staring at your coffee table, trying to discern a key-shaped object. And since the "hum" is very quiet, there's a good chance you won’t even hear it. In a recently published study, a team of scientists, led by postdoctoral researcher Karl Wette from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at the Australian National University, tried the "where else could they be but the kitchen?" approach. Wette explains: “We took an educated guess at a specific location where continuous gravitational waves might be, based in part on what we already know about pulsars—they’re like neutron stars but send out radio waves instead of continuous gravitational waves. We hypothesised that there would be continuous gravitational waves detected near pulsar radio waves.” Just like guessing that your missing keys will probably be close to your handbag or wallet. Using existing observational data, the team spent a lot of time searching in this location (nearly 6000 days of computer time!) listening carefully for that faint "hum". They also used graphic processing units—specialist electronics normally used for computer games—making their algorithms run super-fast. “Our search was significantly more sensitive than any previous search for this location,” says Wette. “Unfortunately, we didn't hear anything, so our guess was wrong this time. It’s back to the drawing board for now, but we'll keep listening.” Scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) have described a way to determine the birth population of double neutron stars--some of the densest objects in the Universe formed in collapsing massive stars. The recently published study observed different life stages of these neutron star systems.
Scientists can observe the merging of double neutron star systems using gravitational waves--ripples in the fabric of space and time. By studying neutron star populations, scientists can learn more about how they formed and evolved. So far, there have been only two double neutron star systems detected by gravitational-wave detectors; however, many of them have been observed in radio astronomy. One of the double neutron stars observed in gravitational wave signals, called GW190425, is far more massive than the ones in typical Galactic populations observed in radio astronomy, with a combined mass of 3.4 times that of our Sun. This raises the question: why is there a lack of these massive double neutron stars in radio astronomy? To find an answer, OzGrav PhD student Shanika Galaudage, from Monash University, investigated how to combine radio and gravitational-wave observations. The birth, mid-life and death of double neutron stars Radio and gravitational-wave astronomy enables scientists to study double neutron stars at different stages of their evolution. Radio observations probe the lives of double neutron stars, while gravitational waves study their final moments of life. To achieve a better understanding of these systems, from formation to merger, scientists need to study the connection between radio and gravitational wave populations: their birth populations. Shanika and her team determined the birth mass distribution of double neutron stars using radio and gravitational-wave observations. “Both populations evolve from the birth populations of these systems, so if we look back in time when considering the radio and gravitational-wave populations we see today, we should be able to extract the birth distribution,” says Shanika Galaudage. The key is to understand the delay-time distribution of double neutron stars: the time between the formation and merger of these systems. The researchers hypothesised that heavier double neutron star systems may be fast-merging systems, meaning that they’re merging too fast to be visible in radio observations and could only be seen in gravitational-waves. GW190425 and the fast-merging channel The study found mild support for a fast-merging channel, indicating that heavy double neutron star systems may not need a fast-merging scenario to explain the lack of observations in radio populations. “We find that GW190425 is not an outlier when compared to the broader population of double neutron stars,” says study co-author Christian Adamcewicz, from Monash University. “So, these systems may be rare, but they‘re not necessarily indicative of a separate fast-merging population.” In future gravitational wave detections, researchers can expect to observe more double neutron star mergers. “If future detections reveal a stronger discrepancy between the radio and gravitational-wave populations, our model provides a natural explanation for why such massive double neutron stars are not common in radio populations,” adds co-author Dr Simon Stevenson, an OzGrav postdoctoral researcher at Swinburne University of Technology. There is a growing interest in introducing quantum physics at an early age in schools because of its applications in emerging technologies, such as quantum computers. To make it accessible to school students, our recently published study presents a novel way of exploring basic quantum mechanical phenomena, such as matter-wave interference, diffraction, and reflection. Our graphical approach, based on Feynman path integrals, offers insights into the quantum world in which observations represent quantum probability density. We combine tactile tools called phasor-wheels with real-life analogies and videos of single-quanta interference and employ elementary mathematics to teach these concepts. Our approach uses practical, hands-on tools for teaching, making it appealing to students from high school (years 9 and 10) and above. The engaging material encouraged active participation and students found it easy to understand these abstract scientific concepts. Written by By Rahul Choudhary – PhD student at UWA General relativity, Einstein’s theory of gravity, is best tested at its most extreme--close to the event horizon of a black hole. This regime is accessible through observations of shadows of supermassive black holes and gravitational waves--ripples in the fabric of our Universe from colliding stellar-mass black holes. For the first time, scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), the Event Horizon Telescope (EHT) and the LIGO Scientific Collaboration, have outlined a consistent approach to exploring deviations from Einstein’s general theory of relativity in these two different observations. This research, published in Physical Review D, confirms that Einstein’s theory accurately describes current observations of black holes, from the smallest to the largest. One of the hallmark predictions from general relativity is the existence of black holes.The theory provides a specific description of a black hole’s effect on the fabric of space-time: a four-dimensional mesh which encodes how objects move through space and time. Known as the Kerr metric, this prediction can be related to the bending of light around a black hole, or the orbital motion of binary black holes. In this study, the deviations from the Kerr metric were linked to features in these black hole observations. In 2019, the Event Horizon Telescope generated silhouette images of the black hole at the centre of the galaxy M87, with a mass several billion times that of our Sun. The angular size of the shadow is related to the mass of the black hole, its distance from Earth and possible deviations from general relativity’s prediction. These deviations can be calculated from the scientific data, including previous measurements of the black hole’s mass and distance. Meanwhile, since 2015 the LIGO and Virgo gravitational-wave observatories have been detecting gravitational waves from merging stellar mass black holes. By measuring the gravitational waves from the colliding black holes, scientists can explore the mysterious nature and metrics of the black holes. This study focussed on deviations from general relativity that appear as slight changes to the pitch and intensity of the gravitational waves, before the two black holes collide and merge. Combining the measurements of the shadow of the supermassive black hole in M87 and gravitational waves from a couple of binary black hole detections, called GW170608 and GW190924, the researchers found no evidence for deviations from general relativity. Co-author of the study and OzGrav research assistant Ethan Payne (Australian National University) explained that the two measurements provided similar, consistent constraints. “Different sizes of black holes may help break the complementary behaviour seen here between EHT and LIGO/Virgo observations,” said Payne. “This study lays the groundwork for future measurements of deviations from the Kerr metric.” Written by OzGrav research assistant Ethan Payne, the Australian National University. A new black hole breaks the record––not for being the smallest or the biggest––but for being right in the middle. The recently discovered ‘Goldilocks’ black hole is part of a missing link between two populations of black holes: small black holes made from stars and supermassive giants in the nucleus of most galaxies. In a joint effort, researchers from the University of Melbourne and Monash University––including OzGrav Chief Investigator Eric Thrane––have uncovered a black hole approximately 55,000 times the mass of the sun, a fabled “intermediate-mass” black hole. The discovery was published today in the paper Evidence for an intermediate mass black hole from a gravitationally lensed gamma-ray burst in the journal Nature Astronomy. Lead author and University of Melbourne PhD student, James Paynter, said the latest discovery sheds new light on how supermassive black holes form. “While we know that these supermassive black holes lurk in the cores of most, if not all galaxies, we don’t understand how these behemoths are able to grow so large within the age of the Universe,” he said. The new black hole was found through the detection of a gravitationally lensed gamma-ray burst. The gamma-ray burst, a half-second flash of high-energy light emitted by a pair of merging stars, was observed to have a tell-tale ‘echo’. This echo is caused by the intervening intermediate-mass black hole, which bends the path of the light on its way to Earth, so that astronomers see the same flash twice. Powerful software developed to detect black holes from gravitational waves was adapted to establish that the two flashes are images of the same object. “This newly discovered black hole could be an ancient relic––a primordial black hole––created in the early Universe before the first stars and galaxies formed,” said study co-author Eric Thrane. “These early black holes may be the seeds of the supermassive black holes that live in the hearts of galaxies today.” The researchers estimate that some 46,000 intermediate mass black holes are in the vicinity of our Milky Way galaxy. This article is an edited version of the original media release produced by Lito Vilisoni Wilson at the University of Melbourne. Also featured in CNet , Cosmos magazine, New Scientist, SciTech Daily, The Independent, Sky News and Space.com There are some stars that just don’t look at all like stars. Rather than being composed nearly entirely of hydrogen and helium like other stars, they consist of matter that has no electromagnetic signature. These stars are hypothetical because we’re still refining the techniques to find them. Boson stars are just one in this family of objects, known as exotic stars, and they’re composed almost entirely of bosons. And what is a boson? It’s one of the two types of fundamental particles, the one that carries forces. The other, the fermion, is what makes up ‘normal’ stars, and all the other matter that we see. There are a variety of boson stars and sometimes they’re categorised to reflect the type of boson that they are made of. For example, Proca stars are vector boson stars, meaning that their constituent bosons have a spin of one. They’re also unique amongst boson stars because the stars themselves can spin without being disrupted. A boson star would most likely be shaped like an enormous donut because of the centrifugal forces acting on the bosonic matter, and, bizarrely, they’d be transparent; any matter absorbed by them would be visible at their centres. If boson stars do exist, they might provide the evidence we need for a long sought-after dark matter particle. That’s because the said particle, the axion, is a boson. And we’ve been searching (unsuccessfully) for axions in numerous experiments on Earth for decades. After the initial excitement of the first-ever observation of an intermediate-mass black hole—gravitational-wave event GW190521—it was quickly realised that the very existence of such an object was not consistent with any of our stellar models. Perhaps it was itself a product of previous, smaller, black hole collisions, or maybe there was something else at play. Thus, the challenge for scientists was to come up with a theory that could explain the presence of the intermediate-mass black hole progenitor of GW190521, while still being consistent with the original signal. And by assuming that it was caused by merging boson stars, rather than black holes, an international team of scientists, led by OzGrav alumnus Dr Juan Calderón Bustillo at the University of Santiago de Compostela and Dr Nicolás Sanchis-Gual at the University of Lisbon, might have been able to do just that. Apart from the problems associated with the pair-instability mass gap, any potential hypothesis needed to explain something a bit unusual about the GW190521 signal. Normally gravitational waves that originate in merging binary systems oscillate at higher and higher frequencies as the two progenitors spiral in towards each other. But for GW190521, the inspiral signal before the merger was barely detectable. An extremely abbreviated inspiral could perhaps be explained if two black holes collided head-on rather than by circling into each other, and so that is the first thing that Dr Bustillo and Dr Sanchis-Gual’s team looked at. What they found didn’t help much. ‘We first tried to fit the data to head-on collisions of black holes, but these happen to produce a final black hole whose spin is too low to reproduce the GW190521 signal. The reason is that the lack of an inspiral diminishes a lot of the spin of the final black hole, and the individual spins of the black holes, which also contribute to the spin of the final one, are bounded by a limit called the Kerr limit,’ says Dr Bustillo. That’s when the team started looking at boson stars, or Proca stars to be exact. They compared the GW190521 signal to computer simulations of Proca star mergers and found that statistically they were a considerably better fit to the data than when it was assumed that the progenitors were black holes. ‘First, we would not be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole,’ explains Dr Bustillo. ‘Second, because boson star mergers are much weaker, we infer a much closer distance than the one estimated by LIGO and Virgo. This leads to a much larger mass for the final black hole, of about 250 solar masses, so the fact that we have witnessed the formation of an intermediate-mass black hole remains true.’ This is an exciting result as the final black hole formed by the merger in this case would have to be about 62% larger than previously thought. And rather than the signal originating from a point that is now some 17-billion light-years from us, it would have been just over 1.8-billion light-years away. ‘Of course, there are potentially many ways in which this event may be explained, as this is an event for which we have very little information about what produced the final black hole we observe. The best we can say right now is that the data tells us that a collision of Proca stars is approximately 8 times more likely than the black hole collision scenario.’ And what of the implications of discovering the first boson stars? ‘That would be dramatic,’ says Dr Bustillo. ‘Boson stars and their building blocks—the ultralight bosons—are one of the most solid candidates for forming what we know as dark matter. If our result is further confirmed by future observations, it would represent the first actual measurement of the particle responsible for dark matter.’ ‘Gravitational-wave astronomy is still very much in its infancy,’ says Dr Rory Smith—an OzGrav researcher from Monash University and one of the collaborators in this research. ‘However, the fact that we’re already starting to draw connections between gravitational-wave observations and fundamental particle physics is a remarkable sign of how powerful this new field is. Even if future observations rule out boson stars as real astronomical objects, we should expect many new and exciting discoveries in the future.’ This is an edited extract from the original article featured in Space Times, written by Dan Lambeth. RESEARCH BRIEF: Simulating the complicated history of Eta Carinae - Ryosuke Hirai, Monash University15/3/2021 ‘Eta Carinae’ is an extraordinary star that has fascinated mankind for decades. It’s surrounded by an expanding ‘Homunculus nebula’, shaped like an hourglass. This nebula was expelled in an energetic explosion called the ‘Great Eruption’ that occurred in the 1840s, when Eta Carinae became the second brightest star in the sky and was visible to the naked eye for over a decade. There are other clusters of bullets outside the Homunculus nebula, that were shot out several centuries before the Great Eruption. Eta Carinae itself is extremely massive with a mass more than 100 times the Sun and is orbited by another star that has a mass about 30 times the Sun on a highly eccentric orbit. With all these and many more peculiar features, scientists have been puzzled for a long time on how the star exploded and created the surrounding messy nebula. Out of many other proposed models, our recently published study focused on one hypothesis that the star system used to be a triple system that eventually became unstable and caused a stellar merger. As more detailed observations are made, this scenario is becoming increasingly popular but has lacked detailed theoretical investigations so far. In this work we performed the first comprehensive set of detailed theoretical calculations for this scenario. We first carried out three-body dynamical simulations to see how a triple system becomes unstable and eventually two of the stars collide. We started with a stable system in which one star is in a wide orbit around the other two stars which are in close orbit. Once the most massive star approaches the end of its life, it expands and starts transferring matter to its companion. This makes the system unstable and causes two of the stars to merge within a few thousand years. We found that before the final merger, the stars can wildly swap their positions and encounter each other at close distances, grazing each other’s surfaces. We carried out additional N-body simulations to see how a star responds to these close encounters. Part of the surface material can be peeled off and sent away as narrow sprays. Combining the orbital dynamics and close-encounter simulations, we found that the multiple grazing encounters—that occur centuries before the merger—can reproduce the messy structure outside the Homunculus nebula. We also carried out hydrodynamical simulations to see how the outflow from the stellar merger is shaped into the hourglass shape we see today. We proposed a new scenario that takes similar ideas for how the triple-ring nebula for the supernova SN1987A was formed. As the stars merge, a huge amount of energy is released inside the star, causing the Great Eruption. But unlike supernovae, a large fraction of the energy and mass remains in the star. This energy slowly leaks out over the following century as strong bipolar winds. The wind sweeps up the inner parts of the explosion ejecta and forms a hollow shell-like structure. Our simulations show that with this scenario, we can closely reproduce the shape and size of the Homunculus nebula. Our combination of simulations successfully reproduces the main features of Eta Carinae’s surrounding nebula and provides strong support to the stellar-merger-in-a-triple scenario. This not only gives us insight into the origin of Eta Carinae, but also many other astronomical objects that can be created through mergers in triple systems. For example, some massive black holes found by LIGO (GW190521) are considered to have been created this way. Using the rich information from Eta Carinae, we can learn much more about the formation of exotica in our Universe. Written by OzGrav researcher Ryosuke Hirai, Monash University An artist’s impression of the Cygnus X-1 system. This system contains the most massive stellar-mass black hole ever detected without the use of gravitational waves, weighing in at 21 times the mass of the Sun. Credit: International Centre for Radio Astronomy Research. Credit: International Centre for Radio Astronomy Research. New observations of the first black hole ever detected have led astronomers to question what they know about the Universe’s most mysterious objects. Published in the journal Science, the research shows the system known as Cygnus X-1 contains the most massive stellar-mass black hole ever detected without the use of gravitational waves. Cygnus X-1 is one of the closest black holes to Earth. It was discovered in 1964 when a pair of Geiger counters were carried on board a sub-orbital rocket launched from New Mexico. The object was the focus of a famous scientific wager between physicists Stephen Hawking and Kip Thorne, with Hawking betting in 1974 that it was not a black hole. Hawking conceded the bet in 1990. In this latest work, an international team of astronomers used the Very Long Baseline Array—a continent-sized radio telescope made up of 10 dishes spread across the United States—together with a clever technique to measure distances in space. OzGrav Chief Investigator and study co-author Prof Ilya Mandel, from Monash University, says the black hole is so massive it’s actually challenging how astronomers thought they formed. ‘Stars lose mass to their surrounding environment through stellar winds that blow away from their surface. But to make a black hole this heavy, we need to dial down the amount of mass that bright stars lose during their lifetimes,’ says Prof Mandel. ‘The black hole in the Cygnus X-1 system began life as a star approximately 60 times the mass of the Sun and collapsed tens of thousands of years ago,’ he says. ‘Incredibly, it’s orbiting its companion star—a supergiant—every five and a half days at just one-fifth of the distance between the Earth and the Sun. These new observations tell us the black hole is more than 20 times the mass of our Sun—a 50 per cent increase on previous estimates.’ Second study author Dr Arash Bahramian from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) says this was an exciting discovery, resulting from a collaboration between astronomers focused on different observational and theoretical aspects of black holes, coming together for a new extensive and rigorous look at a known but previously elusive black hole. ‘It is exciting that we can measure so precisely so many aspects of the system, like its distance from us, its motion and speed through the Galaxy, and the binary motion of the black hole and the star around each other,’ says Dr Bahramian. ‘Our new distance estimate caused an interesting domino effect, leading us to new measurements for the mass and spin of the black hole, which in turn led to fascinating new insights about how stars evolve and how black holes form.’ Lead researcher James Miller-Jones also from ICRAR says over six days the researchers observed a full orbit of the black hole and used observations taken of the same system with the same telescope array in 2011. ‘This method and our new measurements show the system is further away than previously thought, with a black hole that’s significantly more massive,’ says Prof Miller-Jones. In a separate but related development University of Birmingham PhD candidate Coenraad Neijssel, affiliated with OzGrav and Monash, led a companion paper to this work simultaneously published in the Astrophysical Journal. ‘Using the updated measurements of the system properties, we were able to unwind the previous history of the binary as well as predict its future,’ says Coenraad. ‘Precise observations like this are critical for improving our understanding of the evolution of massive stars. This article is an edited of the original media release written by Silvia Dropulich at Monash University Media Office. Also featured in the New York Times and The Daily Mail. A new technology that can improve gravitational-wave detectors, one of the most sensitive instruments used by scientific researchers, has been pioneered by physicists at The University of Western Australia in collaboration with an international team of researchers. The new technology allows the world’s existing gravitational wave detectors to achieve a sensitivity that was previously thought only to be achievable by building much bigger detectors. The paper, published today in Communications Physics, was led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at UWA, in collaboration with the ARC Centre of Excellence for Engineered Quantum Systems, the Niels Bohr Institute in Copenhagen and the California Institute of Technology in Pasadena. Emeritus Professor David Blair, from UWA’s Department of Physics, said the technology merged quantum particles of sound vibration called phonons with photons of laser light, to create a new type of amplification in which the merged particles cycled back and forth billions of times without being lost. “More than a hundred years ago Einstein proved that light comes as little energy packets, which we now call photons,” Emeritus Professor Blair said. One of the most sophisticated applications of photons are gravitational-wave detectors, which allow physicists to observe ripples in space and time caused by cosmic collisions. “Two years after Einstein's prediction of photons, he proposed that heat and sound also come in energy packets, which we now call phonons,” Emeritus Professor Blair said. “Phonons are much trickier to harness individually in their quantum form because they’re usually swamped by vast numbers of random phonons called thermal background.” Emeritus Professor Blair was awarded the prestigious Prime Minister’s Prize for Science in 2020 for his contribution to the first detection of gravitational waves. Lead author Dr Michael Page said the trick was to combine phonons and photons together in such a way that a broad range of gravitational wave frequencies could be amplified simultaneously. “The new breakthrough will let physicists observe the most extreme and concentrated matter in the known universe as it collapses into a black hole, which happens when two neutron stars collide,” Dr Page said. Emeritus Professor David Blair said the waveforms sounded like a brief scream that was pitched too high for current detectors to hear. “Our technology will make those waveforms audible, and will also reveal whether the neutrons in neutron stars get split up into their constituents called quarks when they are in this extreme state” Emeritus Professor Blair said. “The most exciting thing about seeing nuclear matter turn into a black hole is that the process is like the reverse of the Big Bang that created the universe. Observing this happen will be like watching a movie of the Big Bang played backwards.” Emeritus Professor Blair said while the technology did not represent an instant solution to improving gravitational-wave detectors it offers a low-cost route to improvement. As featured on the UWA news website. Astronomers have for the first time used distant galaxies as ‘scintillating pins’ to locate and identify a piece of the Milky Way’s missing matter. For decades, scientists have been puzzled as to why they couldn’t account for all the matter in the universe as predicted by theory. While most of the universe’s mass is thought to be mysterious dark matter and dark energy, 5 percent is ‘normal matter’ that makes up stars, planets, asteroids, peanut butter and butterflies. This is known as baryonic matter. However, direct measurement has only accounted for about half the expected baryonic matter. Yuanming Wang, a doctoral candidate in the School of Physics at the University of Sydney, has developed an ingenious method to help track down the missing matter. She has applied her technique to pinpoint a hitherto undetected stream of cold gas in the Milky Way about 10 light years from Earth. The cloud is about a trillion kilometres long and 10 billion kilometres wide but only weighing about the mass of our Moon. The results, published in the Monthly Notices of the Royal Astronomical Society, offer a promising way for scientists to track down the Milky Way’s missing matter. “We suspect that much of the ‘missing’ baryonic matter is in the form of cold gas clouds either in galaxies or between galaxies,” said Ms Wang, who is pursuing her PhD at the Sydney Institute for Astronomy. “This gas is undetectable using conventional methods, as it emits no visible light of its own and is just too cold for detection via radio astronomy,” she said. What the astronomers did is look for radio sources in the distant background to see how they ‘shimmered’. “We found five twinkling radio sources on a giant line in the sky. Their signals show their light must have passed through the same cold clump of gas,” Ms Wang said. Just as visible light is distorted as it passes through our atmosphere to give stars their twinkle, when radio waves pass through matter, it also affects their brightness. It was this ‘scintillation’ that Ms Wang and her colleagues detected. Dr Artem Tuntsov, a co-author from Manly Astrophysics, said: “We aren’t quite sure what the strange cloud is, but one possibility is that it could be a hydrogen ‘snow cloud’ disrupted by a nearby star to form a long, thin clump of gas.” Hydrogen freezes at about minus 260 degrees and theorists have proposed that some of the universe’s missing baryonic matter could be locked up in these hydrogen ‘snow clouds’. They are almost impossible to detect directly. “However, we have now developed a method to identify such clumps of ‘invisible’ cold gas using background galaxies as pins,” Ms Wang said. Ms Wang’s supervisor, Professor Tara Murphy, said: “This is a brilliant result for a young astronomer. We hope the methods trailblazed by Yuanming will allow us to detect more missing matter.” The data to find the gas cloud was taken using the CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope in Western Australia. Dr Keith Bannister, Principal Research Engineer at CSIRO, said: “It is ASKAP’s wide field of view, seeing tens of thousands of galaxies in a single observation that allowed us to measure the shape of the gas cloud.” Professor Murphy said: “This is the first time that multiple ‘scintillators’ have been detected behind the same cloud of cold gas. In the next few years, we should be able to use similar methods with ASKAP to detect a large number of such gas structures in our galaxy.” The research was done in collaboration with CSIRO, Manly Astrophysics, the University of Wisconsin-Milwaukee and the ARC Centre of Excellence for Gravitational Wave Discovery, OzGrav. Media release written and edited by The University of Sydney media office. Congratulations to OzGrav researchers Adam Deller, Ryan Shannon, Cherie Day, Stefan Oslowski, Chris Flynn, Wael Farah, on receiving the Newcomb Cleveland prize from the AAAS. The award is for the best paper published in Science Magazine in the last year and was awarded for their paper that presented the discovery of the first localised one-off FRB: 'A single fast radio burst localized to a massive galaxy at cosmological distance'.
Full media release by CSIRO here: https://www.csiro.au/en/News/News-releases/2021/In-the-blink-of-an-eye-astronomers-win-prestigious-American-science-prize A recent study by an international team of scientists—led by the Galician Institute of High Energy Physics, the University of Aveiro, and including OzGrav researchers—shows that the “heaviest black hole collision” ever observed might be something even more mysterious—dark matter. Gravitational waves are ripples in the fabric of space-time that travel at the speed of light. Predicted in Einstein’s General Theory of Relativity, they originate in the most violent events of our Universe, carrying information about their sources. Since 2015, humankind can observe and interpret gravitational waves thanks to the two Advanced LIGO detectors (Livingston and Hanford, USA) and the Advanced Virgo detector (Cascina, Italy). To date, these detectors have already observed around 50 gravitational-wave signals which originated in the coalescence and merger of two of the most mysterious entities in the Universe—black holes and neutron stars—deepening our knowledge of the Universe. Gravitational wave astronomy could eventually provide us with evidence for previously unobserved or unexpected objects and shed light on current open issues, like the nature of dark matter—a discovery that may have already happened. In September 2020, the LIGO and Virgo collaborations (LVC) announced the gravitational-wave signal called GW190521. The signal was consistent with the collision of two black holes of 85 and 66 times the mass of the Sun, which produced a final 142 solar mass black hole—this was the first-ever detected intermediate-mass black hole. This discovery was extremely important as intermediate black holes were long considered the missing link between two well-known black-hole families: the stellar-mass black holes, that form from the collapse of stars, and the supermassive black holes, that hide in the centre of almost every galaxy. Despite its significance, the observation of GW190521 posed an enormous challenge to scientists’ understanding of stellar evolution: the life and death of stars is significantly more massive than our Sun. If this is correct, the heaviest of the two colliding black holes shouldn’t have occurred as the end-result of the gravitational collapse of a massive star. In an article recently published in Physical Review Letters, a team of scientists lead by OzGrav alumnus Dr Juan Calderón Bustillo, (now “La Caixa Junior Leader - Marie Curie Fellow”, at the Galician Institute of High Energy Physics) and Dr Nicolás Sanchis-Gual, a postdoctoral researcher at the University of Aveiro and at the Instituto Superior Técnico (University of Lisbon), together with OzGrav researchers from Monash University Dr Rory Smith and Avi Vajpeyi, and collaborators from the University of Valencia and The Chinese University of Hong Kong, has proposed an alternative explanation for the origin of the signal GW190521: the collision of two exotic compact objects known as boson stars. Such hypothetical stars are among the simplest exotic compact objects proposed, and present as well-founded dark matter candidates. Within this interpretation, the team estimated the mass of a new particle constituent of these stars: an ultra-light boson with a mass billionths of times smaller than that of the electron. Dr Nicolás Sanchis-Gual, explains: “Boson stars are objects almost as compact as black holes, but they don’t have a ‘no-return’ surface, or event horizon. When they collide, they form a boson star that can become unstable, eventually collapsing to a black hole, and producing a signal consistent with what LIGO and Virgo observed. Unlike regular stars, which are made of what we commonly know as matter, boson stars are made up of what we know as ultralight bosons. These bosons are one of the most appealing candidates for constituting dark matter, which forms ~27% of the Universe.” The team compared the GW190521 signal to computer simulations of boson-star mergers and found that these explain the data slightly better than the analysis conducted by LIGO and Virgo. The result implies that the source would have different properties than stated earlier. Dr Calderón Bustillo explains: “First, we wouldn’t be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole. Second, because boson star mergers are much weaker, we infer a much closer distance than the one estimated by LIGO and Virgo. This leads to a much larger mass for the final black hole, of about 250 solar masses, so the fact that we have witnessed the formation of an intermediate-mass black hole remains true”. The team found that even though the analysis tends to favour “by design” the merging black-holes hypothesis, a boson star merger is actually preferred by the data, although in a non-conclusive way. Professor José A. Font from the University of Valencia says: “Our results show that the two scenarios are almost indistinguishable given the data, although the exotic boson-star one is slightly preferred. This is very exciting since the computational framework of our current boson-star simulations is still fairly limited and subject to major improvements. A more evolved model might lead to even larger evidence for the boson-star scenario and would also allow us to study similar gravitational-wave observations under the boson-star merger assumption”. This result would not only involve the first observation of boson stars, but also that of their building block, a new particle known as ultra-light boson. Such ultra-light bosons have been proposed as the constituents of what we know as dark matter, which makes up around 27% of the observable Universe. Professor Carlos Herdeiro, from University of Aveiro says that “one of the most fascinating results is that we can actually measure the mass of this putative new dark-matter particle, and that a value of zero is discarded with high confidence. If confirmed by subsequent analysis of this and other gravitational-wave observations, our result would provide the first observational evidence for a long-sought dark matter candidate”. OzGrav researcher Dr Rory Smith adds: “Gravitational-wave astronomy is still very much in its infancy. However, the fact that we are already able to start drawing connections between gravitational-wave observations and fundamental particle physics is a remarkable sign of how powerful this new field is. Even if future observations rule out boson stars as real astronomical objects, we should expect many new and exciting discoveries in the future”. Written by Dr Juan Calderón Bustillo. Also featured on Space Australia. In the moments immediately following the Big Bang, the very first gravitational waves rang out. The product of quantum fluctuations in the new soup of primordial matter, these earliest ripples through the fabric of space-time were quickly amplified by inflationary processes that drove the universe to explosively expand.
Primordial gravitational waves, produced nearly 13.8 billion years ago, still echo through the Universe today. But they are drowned out by the crackle of gravitational waves produced by more recent events, such as colliding black holes and neutron stars. Now a team of international scientists, including reasearchers from the Massachusetts Institute of Technology and OzGrav, has developed a method to tease out the very faint signals of primordial ripples from gravitational-wave data. Their results were published in Physical Review Letters. Gravitational waves are being detected on an almost daily basis by LIGO and other gravitational-wave detectors, but primordial gravitational signals are several orders of magnitude fainter than what these detectors can register. It’s expected that the next generation of detectors will be sensitive enough to pick up these earliest ripples. In the next decade, as more sensitive instruments come online, the new method could be applied to dig up hidden signals of the Universe’s first gravitational waves. The pattern and properties of these primordial waves could then reveal clues about the early universe, such as the conditions that drove inflation. ‘If the strength of the primordial signal is within the range of what next-generation detectors can detect, which it might be, then it would be a matter of more or less just turning the crank on the data, using this method we’ve developed,’ says Sylvia Biscoveanu—MIT graduate student and the study’s lead author. ‘These primordial gravitational waves can then tell us about processes in the early Universe that are otherwise impossible to probe.’ OzGrav researchers Colm Talbot, Eric Thrane and Rory Smith were also co-authors of the study. The hunt for primordial gravitational waves has concentrated mainly on the cosmic microwave background, or CMB, which is thought to be radiation that is leftover from the Big Bang. Scientists believe that when primordial gravitational waves rippled out, they left an imprint on the CMB, in the form of B-modes, a type of subtle polarization pattern.Physicists have looked for signs of B-modes, most famously with the BICEP Array, a series of experiments including BICEP2, which in 2014 scientists believed had detected B-modes; however, the signal turned out to be due to galactic dust. As scientists continue to look for primordial gravitational waves in the CMB, others are hunting the ripples directly in gravitational-wave data. The general idea has been to try and subtract away the ‘astrophysical foreground’—any gravitational-wave signal that arises from an astrophysical source, such as colliding black holes, neutron stars, and exploding supernovae. Only after subtracting this astrophysical foreground can physicists get an estimate of the quieter, nonastrophysical signals that may contain primordial waves. The problem with these methods, Biscoveanu says, is that the astrophysical foreground contains weaker signals that are too faint to discern and difficult to estimate in the final subtraction. In their study, the researchers used a predictive model to describe the more obvious ‘conversations’ of the astrophysical foreground. The team used this more accurate model to create simulated data of gravitational wave patterns and then characterize every astrophysical signal. Once they identified distinct, nonrandom patterns in gravitational-wave data, they were left with more random primordial gravitational-wave signals and instrumental noise specific to each detector. Applying their new methods, the team was able to fit both the foreground and the background at the same time, so the background signal didn’t get contaminated by the residual foreground. Biscoveanu says she hopes that once more sensitive, next-generation detectors come online, the new method can be used to cross-correlate and analyse data from two different detectors, to sift out the primordial signal. Then, scientists may have a useful thread they can trace back to the conditions of the early Universe. This article is an edited extract from the original article featured on MIT’s news website written by Jennifer Chu. Astronomers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and CSIRO have just observed bizarre, never-seen-before behaviour from a ‘radio-loud’ magnetar—a rare type of neutron star and one of the strongest magnets in the Universe. Their new findings, published today in the Monthly Notices of the Royal Astronomical Society (MNRAS), suggest magnetars have more complex magnetic fields than previously thought – which may challenge theories of how they are born and evolve over time. Magnetars are a rare type of rotating neutron star with some of the most powerful magnetic fields in the Universe. Astronomers have detected only thirty of these objects in and around the Milky Way—most of them detected by X-ray telescopes following a high-energy outburst. However, a handful of these magnetars have also been seen to emit radio pulses similar to pulsars—the less magnetic cousins of magnetars that produce beams of radio waves from their magnetic poles. Tracking how the pulses from these ‘radio-loud’ magnetars change over time offers a unique window into their evolution and geometry. In March 2020, a new magnetar named Swift J1818.0-1607 (J1818 for short) was discovered after it emitted a bright X-ray burst. Rapid follow-up observations detected radio pulses originating from the magnetar. Curiously, the appearance of the radio pulses from J1818 were quite different to those seen from other radio-loud magnetars. Most radio pulses from magnetars maintain a consistent brightness across a wide range of observing frequencies. However, the pulses from J1818 were much brighter at low frequencies than high frequencies – similar to what is seen in pulsars, another more common type of radio-emitting neutron star. In order to better understand how J1818 would evolve over time, a team led by scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) observed it eight times using the CSIRO Parkes radio telescope (also known as Murriyang) between May and October 2020. During this time, they found the magnetar underwent a brief identity crisis: in May it was still emitting the unusual pulsar-like pulses that had been detected previously; however, by June it had started flickering between a bright and a weak state. This flickering behaviour reached a peak in July where they saw it flicking back and forth between emitting pulsar-like and magnetar-like radio pulses. “This bizarre behaviour has never been seen before in any other radio-loud magnetar,” explains study lead author and Swinburne University/CSIRO PhD student Marcus Lower. “It appears to have only been a short-lived phenomenon as by our next observation it had settled permanently into this new magnetar-like state.” The scientists also looked for pulse shape and brightness changes at different radio frequencies and compared their observations to a 50-year-old theoretical model. This model predicts the expected geometry of a pulsar, based on the twisting direction of its polarised light. “From our observations, we found that the magnetic axis of J1818 isn’t aligned with its rotation axis,” says Lower. “Instead, the radio-emitting magnetic pole appears to be in its southern hemisphere, located just below the equator. Most other magnetars have magnetic fields that are aligned with their spin axes or are a little ambiguous.” “This is the first time we have definitively seen a magnetar with a misaligned magnetic pole.” Remarkably, this magnetic geometry appears to be stable over most observations. This suggests any changes in the pulse profile are simply due to variations in the height the radio pulses are emitted above the neutron star surface. However, the August 1st 2020 observation stands out as a curious exception. “Our best geometric model for this date suggests that the radio beam briefly flipped over to a completely different magnetic pole located in the northern hemisphere of the magnetar,” says Lower. A distinct lack of any changes in the magnetar’s pulse profile shape indicate the same magnetic field lines that trigger the ‘normal’ radio pulses must also be responsible for the pulses seen from the other magnetic pole. The study suggests this is evidence that the radio pulses from J1818 originate from loops of magnetic field lines connecting two closely spaced poles, like those seen connecting the two poles of a horseshoe magnet or sunspots on the Sun. This is unlike most ordinary neutron stars which are expected to have north and south poles on opposite sides of the star that are connected by a donut-shaped magnetic field. This peculiar magnetic field configuration is also supported by an independent study of the X-rays pulses from J1818 that were detected by the NICER telescope on board the International Space Station. The X-rays appear to come from either a single distorted region of magnetic field lines that emerge from the magnetar surface or two smaller, but closely spaced, regions. These discoveries have potential implications for computer simulations of how magnetars are born and evolve over long periods of time, as more complex magnetic field geometries will change how quickly their magnetic fields are expected to decay over time. Additionally, theories that suggest fast radio bursts can originate from magnetars will have to account for radio pulses potentially originating from multiple active sites within their magnetic fields. Catching a flip between magnetic poles in action could also afford the first opportunity to map the magnetic field of a magnetar. "The Parkes telescope will be watching the magnetar closely over the next year" says scientist and study co-author Simon Johnston, from the CSIRO Astronomy and Space Science. Research brief: Massive Stellar Triples Leading to Sequential Binary Black-Hole Mergers in the Field21/1/2021 The merger of two black holes in a binary system emits energy that can be detected on Earth by gravitational-wave observatories. The LIGO Scientific Collaboration and the VIRGO Collaboration have announced tens of confident detections of such mergers to date. Now, one of the main questions we can try to address concerns the origin of such merging binaries: do they come from isolated binary stars or from dense stellar environments? The answer might not be that simple. A recent study published in the Astrophysical Journal Letters, led by OzGrav Alumni (Monash University) Dr. Alejandro Vigna-Gómez—and current DARK Fellow at the Niels Bohr Institute—shows that some binary black holes can originate from triple stellar systems. A triple stellar system consists of an inner binary and a triple stellar companion orbiting around it. If the inner binary is close enough, it can become a binary black hole which rapidly merges. The product of a binary black hole merger is a single rotating black hole. The merger of the inner binary black hole transforms the initial triple system to a binary, which itself might be able to merge within the age of the Universe. However, the assembly of these triple systems is not as simple as it sounds, as they need to be formed at low metallicities. Time evolution of massive stellar triples: The figure illustrates compact stars in blue, standard evolving stars with red envelopes, black holes in black and merging binary black holes with a surrounding swirl. Black holes formed as sequential mergers are labeled “SM”. Top: the outer tertiary is the most massive star in the system and forms the first black hole in this triple. The inner binary needs to be constituted of compact stars and the tertiary can be either standard evolving or a compact star. Middle: all stars have similar masses. These triples can only lead to sequential mergers if all stars are compact. GW170729 might have experienced this evolution. Bottom: the tertiary is of significantly lower mass than either of the inner binary stars. This configuration does not lead to sequential mergers and is only presented for completion. Credit: T. Rebagliato & A. Vigna-Gomez. Astronomers consider metals to be all elements except hydrogen and helium. Low metallicity environments are those in which hydrogen and helium compose more than approximately 99% of matter. Scientists believe that rare compact stars exist in low metallicity environments. In these environments, rapid rotation and mixing stir the stellar fuel and restrict chemically homogeneously-evolving stars from expanding. Additionally, metallicity increases alongside the age of the Universe, and therefore compact stars are more likely to be formed in the distant past. Vigna-Gómez and collaborators studied the properties of such sequential binary black hole mergers and conclude that GW170729, one of the detected signals of a binary black hole merger, might be of triple stellar origin. The progenitor of GW170729 has at least one rapidly spinning black hole, plausibly from a previous merger. Moreover, the masses are consistent with those of chemically homogeneously evolving stars, and the inferred formation time coincideswith the time it would take a triple system to experience two sequential mergers. Future observations from gravitational-wave observatories will help to further probe this formation channel and, more in general, understand the origin of binary black hole mergers. Written by OzGrav alumnus Dr Alejandro Vigna-Gomez A group of international scientists, including an Australian astrophysicist, has used knowhow from gravitational wave astronomy (used to find black holes in space) to study ancient marine fossils as a predictor of climate change. The research, published in the journal Climate of the Past, is a unique collaboration between palaeontologists, astrophysicists and mathematicians – to improve the accuracy of a palaeo-thermometer, which can use fossil evidence of climate change to predict what is likely to happen to the Earth in coming decades. Professor Ilya Mandel, from the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav), and colleagues, studied biomarkers left behind by tiny single-cell organisms called archaea in the distant past, including the Cretaceous period and the Eocene. Marine archaea in our modern oceans produce compounds called Glycerol Dialkyl Glycerol Tetraethers (GDGTs). The ratios of different types of GDGTs they produce depend on the local sea temperature at the site of formation. When preserved in ancient marine sediments, the measured abundances of GDGTs have the potential to provide a geological record of long-term planetary surface temperatures. To date, scientists have combined GDGT concentrations into a single parameter called TEX86, which can be used to roughly estimate the surface temperature. However, this estimate is not very accurate when the values of TEX86 from recent sediments are compared to modern sea surface temperatures. “After several decades of study, the best available models are only able to measure temperature from GDGT concentrations with an accuracy of around 6 degrees Celsius,” Professor Mandel said. Therefore, this approach cannot be relied on for high-precision measurements of ancient climates. Professor Mandel and his colleagues at the University of Birmingham in the UK have applied modern machine-learning tools -- originally used in the context of gravitational-wave astrophysics to create predictive models of merging black holes and neutron stars -- to improve temperature estimation based on GDGT measurements. This enabled them to take all observations into account for the first time rather than relying on one particular combination, TEX86. This produced a far more accurate palaeo-thermometer. Using these tools, the team extracted temperature from GDGT concentrations with an accuracy of just 3.6 degrees – a significant improvement, nearly twice the accuracy of previous models. According to Professor Mandel, determining how much the Earth will warm in coming decades relies on modelling, “so it is critically important to calibrate those models by utilising literally hundreds of millions of years of climate history to predict what might happen to the Earth in the future,” he said. A discovery that links stellar flares with radio-burst signatures will make it easier for astronomers to detect space weather around nearby stars outside the Solar System. Unfortunately, the first weather reports from our nearest neighbour, Proxima Centauri, are not promising for finding life as we know it. “Astronomers have recently found there are two ‘Earth-like’ rocky planets around Proxima Centauri, one within the ‘habitable zone’ where any water could be in liquid form,” said Andrew Zic from the University of Sydney. Proxima Centauri is just 4.2 light years from Earth. “But given Proxima Centauri is a cool, small red-dwarf star, it means this habitable zone is very close to the star; much closer in than Mercury is to our Sun,” he said. “What our research shows is that this makes the planets very vulnerable to dangerous ionising radiation that could effectively sterilise the planets.” Led by Mr Zic, under the supervision of OzGrav Associate Investigator Professor Tara Murphy, astronomers have for the first time shown a definitive link between optical flares and radio bursts on a star that is not the Sun. The finding, published in The Astrophysical Journal, is an important step to using radio signals from distant stars to effectively produce space weather reports. “Our own Sun regularly emits hot clouds of ionised particles during what we call ‘coronal mass ejections’. But given the Sun is much hotter than Proxima Centauri and other red-dwarf stars, our ‘habitable zone’ is far from the Sun’s surface, meaning the Earth is a relatively long way from these events,” Mr Zic said. “Further, the Earth has a very powerful planetary magnetic field that shields us from these intense blasts of solar plasma.” The research was done in collaboration with CSIRO, the University of Western Australia, University of Wisconsin-Milwaukee, University of Colorado and Curtin University, with contributions from the ARC Centre for Gravitational Wave Discovery (OzGrav) and University of California Berkeley. Zic said: “M-dwarf radio bursts might happen for different reasons than on the Sun, where they are usually associated with coronal mass ejections. But it’s highly likely that there are similar events associated with the stellar flares and radio bursts we have seen in this study.” Coronal mass ejections are hugely energetic expulsions of ionised plasma and radiation leaving the stellar atmosphere. “This is probably bad news on the space weather front. It seems likely that the galaxy’s most common stars – red dwarfs – won’t be great places to find life as we know it,” Mr Zic said. In the past decade, there has been a renaissance in the discovery of planets orbiting stars outside our Solar System. There are now more than 4000 known exoplanets. This has boosted hopes of finding ‘Earth-like’ conditions on exoplanets. Recent research says that about half the Sun-like stars in the Milky Way could be home to such planets. However, Sun-like stars only make up 7 percent of the galaxy’s stellar objects. By contrast, M-type red dwarfs like Proxima Centauri make up about 70 percent of stars in the Milky Way. The findings strongly suggest planets around these stars are likely to be showered with stellar flares and plasma ejections. Methodology The Proxima Centauri observations were taken with the CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) telescope in Western Australia, the Zadko Telescope at the University of Western Australia and a suite of other instruments. OzGrav scientist Dr Bruce Gendre, from the University of Western Australia, said the research helps understand the dramatic effects of space weather on solar systems beyond our own. “Understanding space weather is critical for understanding how our own planet biosphere evolved – but also for what the future is,” Dr Gendre said. Professor Murphy said: “This is an exciting result from ASKAP. The incredible data quality allowed us to view the stellar flare from Proxima Centauri over its full evolution in amazing detail. “Most importantly, we can see polarised light, which is a signature of these events. It’s a bit like looking at the star with sunglasses on. Once ASKAP is operating in full survey mode we should be able to observe many more events on nearby stars.” This will give us much greater insights to the space weather around nearby stars. Other facilities, including NASA’s planet-hunting Transiting Exoplanet Survey Satellite and the Zadko Telescope observed simultaneously with ASKAP providing the crucial link between the radio bursts and powerful optical flares observed. Mr Zic said: “The probability that the observed solar flare and received radio signal from our neighbour were not connected is much less than one chance in 128,000.” The research shows that planets around Proxima Centauri may suffer strong atmospheric erosion, leaving them exposed to very intense X-rays and ultraviolet radiation. But could there be magnetic fields protecting these planets? Mr Zic said: “This remains an open question. How many exoplanets have magnetic fields like ours?” So far there have been no observations of magnetic fields around exoplanets and finding these could prove tricky. Mr Zic said one potential way to identify distant magnetic fields would be to look for aurorae, like those around Earth and also witnessed on Jupiter. “But even if there were magnetic fields, given the stellar proximity of habitable zone planets around M-dwarf stars, this might not be enough to protect them,” Mr Zic said. This is an extract of a media release originally written by the Media Office at the University of Sydney. RESEARCH HIGHLIGHT: MASSIVE OVERCONTACT BINARY STARS OPEN A NEW WINDOW TO STELLAR EVOLUTION8/12/2020 The massive O4.5 V + O5.5 V binary VFTS 352 in the Tarantula Nebula (location indicated by the red cross) is one of the shortest-period and most massive overcontact binaries known. Recent theoretical studies indicate that some of these systems could ultimately lead to the formation of gravitational waves via black hole binary mergers through the chemically homogeneous evolution pathway. Image credit: ESO / M.-R. Cioni / VISTA Magellanic Cloud survey / Cambridge Astronomical Survey Unit. In 2015, the first direct observation of gravitational waves was made by the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) (Abbott et al. 2016). The detected signal, known as GW150914, not only provided the most rigorous test of General Relativity—Einstein’s theory of gravity—it was also the first observation of two black holes merging. This confirmed the existence of black holes, binary stellar-mass black hole systems, and proved that they can merge within the current age of the Universe. Since that first detection, aLIGO has detected many more black hole mergers but scientists are still puzzled: many of the black hole mergers that aLIGO has detected so far, including GW150914, involve systems with the component black holes weighing-in at more than 20 solar masses each—some much more. These are massive black holes—heavier than any previously known black holes from X-ray binary observations—which raises the question: how did they form? Our recent study seeks to answer that question. aLIGO records (part of) the inspiral, merger, and ring-down of the orbiting black holes—as shown in figure 1[JR1] . aLIGO can only record these events if they occur within its operating lifetime—that means that the progenitor stars must collapse to black holes before they merge, and the subsequent orbiting black holes must spiral in towards each other and merge within the age of the Universe. For that to happen, the black holes must be big and close together; however, progenitor stars big enough and close enough to produce a binary black hole (BBH) system that would spiral in and eventually merge within the age of the universe, and that could generate gravitational waves detectable by aLIGO, would be too big and too close; so these progenitor stars merge first and then collapse into black holes. Thus, the aLIGO detections raise the intriguing questions: how did the black holes get so massive? and how did they get so close together? Chemically Homogeneous Evolution (CHE) has been proposed as a possible solution. Rapid spinning stirs a star leading to its interior becoming homogeneous, and possibly fusing entirely rather than just the core. A hot, rapidly spinning, chemically homogeneous star doesn’t expand as it ages in the way a conventionally evolving star does. The chemically homogeneous model begins with a pair of close, massive stars rotating around each other extremely rapidly—so close that they become tidally locked, causing the stars to spin rapidly on their own axes. The component stars of this massive, overcontact binary eventually collapse into massive black holes, which are now close enough to spiral in and merge within the age of the Universe. For the first time, we simultaneously explore conventional isolated binary star evolution and chemically homogeneous evolution under the same set of assumptions. This approach allows us to constrain population properties and make simultaneous predictions about the gravitational-wave detection rates of BBH mergers for the CHE and conventional formation channels. This joint model for the classical and CHE isolated binary evolution channels will enable simultaneous inference on binary evolution model parameters and the metallicity-specific star formation history once the full trove of observations from the aLIGO third gravitational-wave observing run is available. Ultimately, the relatively short delay times of CHE BBHs make them ideal probes of high-redshift star formation history; their high masses make them perfect targets for third-generation gravitational-wave detectors with good low-frequency sensitivity, such as the Einstein Telescope or the Cosmic Explorer. Written by OzGrav PhD student Jeff Riley - Monash University Link: https://arxiv.org/abs/2010.00002 Over the past five years, astronomy has been revolutionised as scientists have used ripples in the fabric of spacetime, called gravitational waves, to reveal the secrets of the previously hidden world of black holes. Gravitational waves are created when two black holes merge in a cataclysmic release of energy, but until now, there were few clues as to how and why black holes merge.
Today, researchers from the LIGO and Virgo Collaborations announced a series of discoveries providing some of the first hints as to the origin of black hole mergers. Researchers from Monash University––members of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)––helped lead the effort. “We are announcing the discovery of 44 confirmed black hole mergers, which is a more than a four-fold increase in the number of previously known gravitational-wave signals,” explains Monash University School of Physics and Astronomy PhD student, Shanika Galaudage, who helped write one of the new LIGO papers. “With so many black holes to study, we can start to answer deep questions about how these systems came to merge," Ms Galaudage says. A key clue comes from the fact that black holes spin. The orientation of the black hole spins affects the gravitational-wave signal. Study author Dr Colm Talbot, also from the School of Physics and Astronomy, says “There are two theories for how two black holes can get together. Sometimes, pairs of stars called binaries make pairs of black holes that merge, creating ripples in spacetime called gravitational waves. Alternatively, two black holes can stumble into each other.” The verdict? “It seems there are multiple ways for two black holes to get together,” says OzGrav Chief Investigator Professor Eric Thrane from Monash University. “Some binary black holes are born from pairs of stars. Others wander the cosmos before finding a partner to merge with. Either way, a tremendous amount of energy is released in gravitational waves.” Tonight four members of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) have been awarded the 2020 Prime Minister’s Prizes for Science for their critical contributions to the first direct detection of gravitational waves―a landmark achievement in human discovery: Chief Investigator Emeritus Professor David Blair (University of Western Australia); Deputy Director Professor David McClelland (Australian National University); Chief Investigator Professor Susan Scott (Australian National University); and Chief Investigator Professor Peter Veitch (University of Adelaide). The Prime Minister’s Prizes for Science are Australia’s most prestigious awards for outstanding achievements in scientific research, research-based innovation and excellence in science teaching. This year’s recipients are four pioneering Australian physicists from OzGrav that contributed to the groundbreaking discovery of gravitational-wave signals from the collision of two black holes 1.3 billion years ago. This was made possible by decades of research and innovation of the team as part of the Laser Interferometer Gravitational-wave Observatory Scientific Collaboration (LSC). Emeritus Prof. David Blair says: “It is wonderful to receive the Prime Minister's Prize for Science. It's a fitting tribute to all of the students and scientists who participated in this amazing quest that was finally rewarded with the detection of gravitational waves. This is a prize for physics in Australia.” Emer. Prof. Blair created a large-scale high-optical power research facility in Gingin, Western Australia, to mimic Advanced LIGO interferometers and investigate the subtle interactions between light, sound and heat that would occur in full-scale detectors. His pioneering work predicted that laser light would scatter from sound in the mirrors, causing parametric instability at power levels far below that needed to obtain detector sensitivity. When this theory was validated during LIGO commissioning, Emeritus Professor Blair sent team members to help implement stabilisation methods that allowed the detectors to achieve sufficient power levels to make the first detection of gravitational waves. In 1916, Albert Einstein first predicted the existence of gravitational waves―minute distortions in the fabric of space-time that are non-electromagnetic in nature and spread from their source at the speed of light; however, he believed they would never be detectable. Prof. Peter Veitch says: “Einstein developed his theory of relativity in 1915, but people questioned whether gravitational waves really existed or whether they were just some sort of mathematical nonsense predicted by the theory. Since then there has been a large advance in our understanding of the Universe, and our research has focused on developing the technologies required to detect Einstein's theorised gravitational waves.” Prof. Veitch’s University of Adelaide team invented and installed critical instrumentation for the Advanced LIGO detectors, namely their Hartman sensors. These sensors provide a solution to a major technological problem – the distortion of the laser beam within the detector – by measuring them simply and with a sensitivity that is 30-times better than any other sensor. The Hartman sensors are used at all stages of the detection process: commissioning, measurement and adaptive correction of the distortions, and optimising the detector sensitivity and stability. “Over the last few decades, there were many scientists who either didn't believe that gravitational waves existed, or felt that they were simply too small to ever be detected,” explains Prof. Susan Scott. “We had enormous technological difficulties to overcome – everything had to be about a thousand times better, including the shapes of the mirrors, the frequency of the lasers, the acoustic wringing of the mirrors and the vibration isolation.” Prof. Scott initiated the Australian effort in gravitational wave data analysis in 1998, and led Australian research in digging gravitational wave signals out of detector noise. Her Australian National University team contributed key components to the LIGO Data Analysis System through which the detection signal was processed in 2015, designing and conducting the first gravitational wave search to be carried out under Australian leadership. In September 2015, recent advances in detector sensitivity led to the first direct detection of gravitational waves; two Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) laser interferometers simultaneously detected a signal characteristic of a pair of black holes – 29 and 36 times the mass of the sun – merging into one. This was followed by a further detection in 2017, from the collision of two neutron stars. The first detection of its kind, this event solved a 50-year-old mystery confirming that these mergers are the source of previously observed high-energy gamma ray bursts, and of heavy metals such as gold, platinum and uranium in the Universe. Prof. David McClelland says: “This achievement (gravitational wave detection) came about only through long term investment in basic R&D by the Australian Research Council, our universities and many similar organisations around the world. This investment has led to impactful science, impactful technologies and inspired a generation of scientists and engineers.” Prof. McClelland led the Australian National University team that played a crucial role in designing, installing and commissioning Advanced LIGO’s lock acquisition system, and in the construction and installation of Australian hardware for precision routing of the laser beam. His pioneering quantum ‘squeezing’ technology (now installed in all detectors) is essential for boosting interferometer sensitivity to the current level where signals are detected weekly when in operation. The impact of the first 2015 detection, the acclaimed ’discovery of the century’, has been immense, opening up previously unknown parts of the Universe, such as hidden black holes; understanding the origin of gamma ray bursts (and with the potential to discover how supernovae explode); and to even peer back to the beginning of time at the Big Bang. The legacy of the team’s combined research ensures that Australia is now ‘front-and-centre’ in exploring this brand-new window into the Universe. “The pioneering work of our team over the last quarter of a century has ensured that Australia played a leading role in the first direct detection of gravitational waves,” says Prof. Susan Scott. “Australia is now in a position to be a powerhouse in the emergent field of gravitational wave astronomy.” For OzGrav Director Professor Matthew Bailes (Swinburne University of Technology), the result is especially pleasing. “This is fantastic recognition of the role Australia has played in opening this new window on Einstein’s Universe. I’m thrilled for not only these four pioneers of the field in Australia, but for the future generations of scientists and engineers that will follow in their footsteps.” The Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO) detectors and the Virgo detector have directly observed transient gravitational waves (GWs) from compact binary coalescences. After a series of instrument upgrades to further improve the sensitivity—like replacing test masses and optics, increasing laser power, and adding squeezed light—the two LIGO detectors started the third observing run (O3), together with Virgo, on April 1st, 2019 and finished the year-long observation on Mar 27th, 2020.
The time series of dimensionless strain, defined by the differential changes in the length of the two orthogonal arms divided by the averaged full arm length (~4 km in the two LIGO detectors), is used to determine the detection of a GW signal and infer the properties of the astrophysical source. (The figure shows a conceptual diagram of the optical configuration of the Advanced LIGO interferometers.) Due to the presence of noise and the desire to maintain the resonance condition of the optical cavities, the detectors do not directly measure the strain. The differential arm displacement is suppressed by the control force allocated to the test masses. The residual differential arm displacement in the control loop is converted into digitized photodetector output signals. Therefore, the strain is reconstructed from the raw digitized electrical output of each detector, with an accurate and precise model of the detector response to the strain. This reconstruction process is referred to as detector ‘calibration.’ The accuracy and precision of the estimated detector response, and hence the reconstructed strain data, are important for detecting GW signals and crucial for estimating their astrophysical parameters. Understanding, accounting, and/or compensating for the complex-valued response of each part of the upgraded detectors improves the overall accuracy of the estimated detector response to GWs. Calibration systematic error is the frequency-dependent and time-dependent deviation of the estimated detector response from the true detector response. We describe our improved understanding and methods used to quantify the response of each detector, with a dedicated effort to define all places where systematic error plays a role. We use the two LIGO detectors as they stand in the first half (six months) of O3 to demonstrate how each identified systematic error impacts the reconstructed strain and constrain the statistical uncertainty therein. We report the accuracy and precision of the strain data by estimating the 68% confidence interval bounds on the systematic error and uncertainty for the response of each detector (in magnitude and phase). In the first half of O3, the overall systematic error and uncertainty of the best calibrated data is within 7% in magnitude and 4 degrees(?) in phase in the most sensitive frequency band 20–2000 Hz. The systematic error alone, in the same band, is estimated to be below 2% in magnitude and 2 degrees in phase. Current detection of GW events and estimation of their astrophysical parameters are not yet limited by such levels of systematic error and uncertainty. However, as the global GW detector network sensitivity increases, detector calibration systematic error and uncertainty plays an increasingly important role. Limitations caused by calibration systematics on estimated GW source parameters, precision astrophysics, population studies, cosmology, and tests of general relativity are possible. Efforts are being carried out to better integrate the work presented in this paper into future GW data analyses. Research and development of new techniques are underway to further reduce calibration systematic error and uncertainty below the 1% level, a key milestone towards minimizing impacts of calibration systematics on astrophysical and cosmological results. Written by Lilli Sun, OzGrav Associate Investigator, ANU New research suggests innovative method to analyse the densest star systems in the Universe13/10/2020 In a recently published study, a team of researchers led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash suggests an innovative method to analyse gravitational waves from neutron star mergers, where two stars are distinguished by type (rather than mass), depending on how fast they’re spinning. Neutron stars are extremely dense stellar objects that form when giant stars explode and die—in the explosion, their cores collapse, and the protons and electrons melt into each other to form a remnant neutron star. In 2017, the merging of two neutron stars, called GW170817, was first observed by the LIGO and Virgo gravitational-wave detectors. This merger is well-known because scientists were also able to see light produced from it: high-energy gamma rays, visible light, and microwaves. Since then, an average of three scientific studies on GW170817 have been published every day. In January this year, the LIGO and Virgo collaborations announced a second neutron star merger event called GW190425. Although no light was detected, this event is particularly intriguing because the two merging neutron stars are significantly heavier than GW170817, as well as previously known double neutron stars in the Milky Way. Scientists use gravitational-wave signals—ripples in the fabric of space and time—to detect pairs of neutron stars and measure their masses. The heavier neutron star of the pair is called the ‘primary’; the lighter one is ‘secondary’. The recycled-slow labelling scheme of a binary neutron star system A binary neutron star system usually starts with two ordinary stars, each around ten to twenty times more massive than the Sun. When these massive stars age and run out of ‘fuel’, their lives end in supernova explosions that leave behind compact remnants, or neutron stars. Each remnant neutron star weighs around 1.4 times the mass of the Sun, but has a diameter of only 25 kilometres. The first-born neutron star usually goes through a ‘recycling’ process: it accumulates matter from its paired star and begins spinning faster. The second-born neutron star doesn’t accumulate matter; its spin speed also slows down rapidly. By the time the two neutron stars merge—millions to billions of years later—it’s predicted that the recycled neutron star may still be spinning rapidly, whereas the other non-recycled neutron star will probably be spinning slowly. Another way a binary neutron star system might form is through continuously changing interactions in dense stellar clusters. In this scenario, two unrelated neutron stars, on their own or in other separate star systems, meet each other, pair up and eventually merge like a happy couple due to their gravitational waves. However, current modelling of stellar clusters suggests that this scenario is ineffective in merging the neutron stars. OzGrav postdoctoral researcher and lead author of the study Xingjiang Zhu says: ‘The motivation for proposing the recycled-slow labelling scheme of a binary neutron star system is two-fold. First, it’s a generic feature expected for neutron star mergers. Second, it might be inadequate to label two neutron stars as primary and secondary because they’re most likely to be of similar masses and it’s hard to tell which one is heavier.” The recent OzGrav study takes a new look at both GW170817 and GW190425 by adopting the recycled-slow scheme. It was found that the recycled neutron star in GW170817 is only mildly or even slowly spinning, whereas that of GW190425 is spinning rapidly, possibly once every 15 milliseconds. It was also found that both merger events are likely to contain two nearly equal-mass neutron stars. Since there is little or no evidence of spin in GW170817, and neutron stars spin down over time, the researchers deduced that the binary probably took billions of years to merge. This agrees well with observations of its host galaxy, called NGC 4993, where little star formation activities are found in the past billions of years. OzGrav associate investigator and collaborator Gregory Ashton says: “Our proposed astrophysical framework will allow us to answer important questions about the Universe, such as are there different supernova explosion mechanisms in the formation of binary neutron stars? And to what degree do interactions inside dense star clusters contribute to forming neutron star mergers?” The LIGO/Virgo detectors finished their joint third observing run (O3) earlier this year and are currently conducting scheduled maintenance and upgrades. When the fourth run (O4) starts in 2021, scientists will be readily anticipating more discoveries of neutron star mergers. The prospect will be even brighter when the Japanese underground detector KAGRA and the LIGO-India detector join the global network over the coming years. ‘We are in a golden era of studying binary neutron stars with highly-sensitive gravitational-wave detectors that will deliver dozens of discoveries in the next few years,’ adds Zhu. The black hole always chirps twice: Scientists find clues to decipher the shape of black holes13/10/2020 A team of gravitational-wave scientists led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) reveal that when two black holes collide and merge, the remnant black hole ‘chirps’ not once, but multiple times, emitting gravitational waves—intense ripples in the fabric space and time—that inform us about its shape. Today the study has been published in Communications Physics (from the prestigious Nature journal). Black holes are one the most fascinating objects in the Universe. At their surface, known as the ‘event horizon’, gravity is so strong that not even light can escape from them. Usually, black holes are quiet, silent creatures that swallow anything getting too close to them; however, when two black holes collide and merge together, they produce one of the most catastrophic events in Universe: in a fraction of a second, a highly-deformed black hole is born and releases tremendous amounts of energy as it settles to its final form. This phenomenon gives astronomers a unique chance to observe rapidly changing black holes and explore gravity in its most extreme form. Fig. 1. a: The stages of a black hole merger. First, both black holes orbit each other, slowly approaching, during the inspiral stage.. Second the two black holes merge, forming a distorted black hole. Finally, the black hole reaches its final form. b: Frequency of the gravitational-wave signals observed from the top of the collision (leftmost) and from various positions on its equator (rest) as a function of time. The first signal shows the typical “chirping” signal, in which the frequency raises as a function of time. The other three show that, after the collision (at t=0) the frequency drops and rises again, producing a second “chirp”. CREDIT: C. Evans, J. Calderón Bustillo Although colliding black holes do not produce light, astronomers can observe the detected gravitational waves—ripples in the fabric of space and time—that bounce off them. Scientists speculate that, after a collision, the behaviour of the remnant black hole is key to understanding gravity and should be encoded in the emitted gravitational waves. In the article published in Communications Physics (Nature), a team of scientists led by OzGrav alumnus Prof. Juan Calderón Bustillo—now ‘La Caixa Junior Leader - Marie Curie Fellow’ at the Galician Institute for High Energy Physics (Santiago de Compostela, Spain)—has revealed how gravitational waves encode the shape of merging black holes as they settle to their final form. Graduate student and co-author Christopher Evans from the Georgia Institute of Technology (USA) says: ‘We performed simulations of black-hole collisions using supercomputers and then compared the rapidly changing shape of the remnant black hole to the gravitational waves it emits. We discovered that these signals are far more rich and complex than commonly thought, allowing us to learn more about the vastly changing shape of the final black hole’. The gravitational waves from colliding black holes are very simple signals known as ‘chirps’. As the two black holes approach each other, they emit a signal of increasing frequency and amplitude that indicates the speed and radius of the orbit. According to Prof. Calderón Bustillo, ‘the pitch and amplitude of the signal increases as the two black holes approach faster and faster. After the collision, the final remnant black hole emits a signal with a constant pitch and decaying amplitude—like the sound of a bell being struck’. This principle is consistent with all gravitational-wave observations so far, when studying the collision from the top. Fig. 2. Detail of the shape of the remnant black hole after a black hole collision, with a ‘chestnut shape’. Regions of strong gravitational-wave emission (in yellow) cluster near its cusp. This black hole spins making the cusp point to all observers around it. CREDIT: C. Evans, J. Calderón Bustillo However, the study found something completely different happens if the collision is observed from the ‘equator’ of the final black hole. 'When we observed black holes from their equator, we found that the final black hole emits a more complex signal, with a pitch that goes up and down a few times before it dies,’ explains Prof. Calderón Bustillo. ‘In other words, the black hole actually chirps several times.’ The team discovered that this is related to the shape of the final black hole, which acts like a kind of gravitational-wave lighthouse: ‘When the two original, ‘parent’ black holes are of different sizes, the final black hole initially looks like a chestnut, with a cusp on one side and a wider, smoother back on the other,’ says Bustillo. ‘It turns out that the black hole emits more intense gravitational waves through its most curved regions, which are those surrounding its cusp. This is because the remnant black hole is also spinning and its cusp and back repeatedly point to all observers, producing multiple chirps.’ Co-author Prof. Pablo Laguna, former chair of the School of Physics at Georgia Tech and now Professor at University of Texas at Austin, pointed out ‘while a relation between the gravitational waves and the behaviour of the final black hole has been long conjectured, our study provides the first explicit example of this kind of relation’. Rapidly rotating, asymmetric neutron stars that undergo free precession can produce both modulated pulse signals and continuous gravitational radiation with characteristic features, and thus are potential interesting multi-messenger astrophysical sources. Studies have been carried out to characterise the electromagnetic and gravitational-wave signals from freely-precessing neutron stars, mostly focused on biaxial stars; however, in the most generic cases, triaxially-deformed neutron stars demonstrate more complex features as a result of free precession. In this study, co-authored by OzGrav Associate Investigator Lilli Sun from Australian National University (who was working with Caltech at the time of this research), scientists extend previous work and derive the dynamical evolution of a generic, triaxially-deformed, freely-precessing neutron star with both analytical and numerical approaches. If the neutron star is observed as a pulsar via radio and/or X-ray telescopes, the free precession could introduce observable characteristic modulations in both the timing and width of the pulse signals, depending on the wobble angle and other source properties. Moreover, free precession of a triaxially-deformed neutron star could manifest as additional lines in the spectra of continuous gravitational waves, detectable by the ground-based gravitational-wave detectors like Advanced LIGO, Virgo, and KAGRA. The researchers introduce a numerical method to integrate the equations of motion in generic cases where analytical solutions are difficult to derive. The timing residuals, pulse-width modulations, as well as the gravitational-wave spectra of a precessing triaxial star, are presented with concrete examples. The results in this work provide guidance for future multi-messenger studies of triaxially-deformed, freely-precessing neutron stars. Multi-messenger observation of precessing neutron stars will become promising with future high-precision electromagnetic observations (e.g., NICER X-ray timing) and next-generation gravitational-wave detectors (e.g., Einstein Telescope and Cosmic Explorer). Combining characteristic features in radio/X-ray signals and continuous gravitational waves of precessing neutron stars allows scientists to obtain valuable information about the source properties, e.g., the wobble angle, the non-axisymmetry and oblateness of the star. These measurements could shed light on the long-standing questions about the neutron star internal structure and the supranuclear matter equation of state. Link to study: https://doi.org/10.1093/mnras/staa2476 |
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