Massive stars end their lives with energetic explosions known as supernova explosions. ‘Stripped-envelope supernovae’ show weak or no traces of hydrogen in its ejecta, meaning that the star lost most or all of its hydrogen-rich outer layers before it exploded. Scientists hypothesise that these stars mostly originate in binary star systems, where one of the stars rips off the outer layers of the other star with its gravitational pull–many searches have been made to discover the remaining companion star following the stripped-envelope supernovae. In some searches, the companion star was successfully detected, but there are also numerous cases where the companion couldn’t be found, posing a serious problem for the binary hypothesis. The most famous case is called Cassiopeia A (Cas A): a stripped-envelope supernova remnant that is predicted to have a stellar companion, but nothing could be found in its explosive aftermath. In a recently published study led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), researchers propose a new scenario for creating these ‘lonely’ stripped-envelope stars. OzGrav researcher and lead author of the study Dr Ryosuke Hirai explains: ‘In our scenario, the stripped-envelope star used to have a binary companion with a mass very similar to itself. Because the masses are similar, they have very similar lifetimes, meaning that the explosion of the first star will occur when the second star is close to death too’. In the last million years of their lives, massive stars are known to become red supergiants where their outer layers are very puffed up and unstable. So, if the first supernova of the binary star system hits the other massive star—while it‘s this puffy red supergiant—it can easily strip off the outer layers, making it a stripped-envelope star. The stars disrupt after the supernova, so the secondary star becomes a lonely stellar widow and will appear to be single by the time it explodes itself, a million years later. The OzGrav scientists performed hydrodynamical simulations of a supernova colliding with a red supergiant to investigate how much mass can be stripped off through this process. They found that if the two stars are close enough, the supernova can strip nearly 90% of the ‘envelope’—the outer layer—off the companion star. ‘This is enough for the second supernova of the binary system to become a stripped-envelope supernova, confirming that our proposed scenario is plausible,’ says Hirai. ‘Even if it’s not sufficiently close, it can still remove a large fraction of the outer layers which makes the already unstable envelope even more unstable, which can lead to other interesting phenomena like pulsations or eruptions.’ If OzGrav’s scenario occurs, the stripped-off envelope should be floating as a one-sided shell at about 30- 300 light years away from the second supernova site. Recent observations revealed that there is indeed a shell of material located at around 30-50 light years away from the famous Cas A. Hirai adds: ‘This may be indirect evidence that Cas A was originally created through our scenario, which explains why it does not have a binary companion star. Our simulations prove that our new scenario could be one of the most promising ways to explain the origin of one of the most famous supernova remnants, Cas A’. The OzGrav scientists also predict that this scenario has a much wider range of possible outcomes—for example, it can produce a similar number of ‘partially-stripped’ stars. In the future, it will be interesting to explore what happens to these partially-stripped stars and how they could be observed.
0 Comments
A new study has developed an innovative method to detect colliding supermassive black holes in our Universe. The study has just been published in the Astrophysical Journal and was led by postdoctoral researcher Xingjiang Zhu from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), at Monash University. At the centre of every galaxy in our Universe lives a supermassive black hole—a black hole that’s millions to billions times the mass of our Sun. Big galaxies are assembled from smaller galaxies merging together, so collisions of supermassive black holes are expected to be common in the cosmos. But merging supermassive black holes remain elusive: no conclusive evidence of their existence has been found so far. One way to look for these mergers is through their emission of gravitational waves—ripples in the fabric of space and time. A distant merging pair of supermassive black holes emit gravitational waves as they spiral in around each other. Since the black holes are so large, each wave takes many years to pass by Earth. Astronomers use a technique known as pulsar timing array to catch gravitational waves from supermassive binary black holes—so far to no avail. In parallel, astronomers have been looking for the collision of supermassive black holes with light. A number of candidate sources have been identified by looking for regular fluctuations in the brightness of distant galaxies called “quasars”. Quasars are extremely bright, believed to be powered by the accumulation of gas clouds onto supermassive black holes. If the centre of a quasar contains two black holes orbiting around each other (instead of a single black hole), the orbital motion might change the gas cloud accumulation and lead to periodic variation in its brightness. Hundreds of candidates have been identified through such searches, but astronomers are yet to find the smoking-gun signal. ‘If we can find a pair of merging supermassive black holes, it will not only tell us how galaxies evolved, but also reveal the expected gravitational-wave signal strength for pulsar watchers,’ says Zhu. The OzGrav study seeks to settle the debate, determining if any of the identified quasars are likely to be powered by colliding black holes. The verdict? Probably not. “We’ve developed a new method allowing us to search for a periodic signal and measure quasar noise properties at the same time,” says Zhu. “Therefore, it should produce a reliable estimate of the detected signal’s statistical significance.” Applying this method to one of the most prominent candidate sources, called PG1302-102, the researchers found strong evidence for periodic variability; however, they argued that the signal is likely to be more complicated than current models. “The commonly assumed model for quasar noise is wrong,” adds Zhu. “The data reveal additional features in the random fluctuations of gas accumulation onto supermassive black holes.” “Our results are showing that quasars are complicated,” says collaborator and OzGrav Chief Investigator Eric Thrane. “We’ll need to improve our models if we are going to use them to identify supermassive binary black holes.” Scientists develop a new tool ‘METISSE’, offering new insights into the lives of massive stars8/9/2020 This artist's impression of different mass stars; from the smallest “red dwarfs”, weighing in at about 0.1 solar masses, to massive “blue” stars weighing around 10 to 100 solar masses. While red dwarfs are the most abundant stars in the Universe, it’s the massive blue stars that contribute the most to the evolution of stars clusters and galaxies. Credit: ESO/M. Kornmesser Massive stars are larger than about 10 times the mass of the Sun and are born far less often than their low mass counterparts. However, they contribute the most to the evolution of stars clusters and galaxies. From enriching their surroundings in supernova explosions, to altering the dynamics of their systems, massive stars are the precursors of many vivid and energetic phenomena in the Universe. The best tool to study massive stars are ‘detailed stellar evolution codes': computer programs which can calculate both the interior structure and the evolution of these stars. Unfortunately, detailed codes are computationally expensive and time-consuming—it can take several hours to compute the evolution of just a single star. For this reason, it’s impractical to use these codes for modelling stars in complex systems, such as globular star clusters, which can contain millions of interacting stars. To address this problem, a team of scientists led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) developed a stellar evolution code called METhod of Interpolation for Single Star Evolution (METISSE). Interpolation is a method for estimating a quantity based on nearby values, such as estimating the size of a star based on the sizes of stars with similar masses. METISSE uses interpolation to quickly calculate the properties of a star at any instant by using selected stellar models computed with detailed stellar evolution codes. Lightning fast, METISSE can evolve 10,000 stars in less than 3 minutes. Most importantly, it can use different sets of stellar models to predict the properties of stars—this is extremely important for massive stars. Massive stars are rare, and their complex and short lives make it difficult to accurately determine their properties. Consequently, detailed stellar evolution codes often have to make various assumptions while computing the evolution of these stars. The differences in the assumptions used by the different stellar evolution codes can significantly impact their predictions about the lives and the properties of the massive stars. In their recently published study, the OzGrav researchers used METISSE with two different sets of state-of-the-art stellar models: one computed by the Modules for Experiments in Stellar Astrophysics (MESA), and the other by the Bonn Evolutionary Code (BEC). Poojan Agrawal—OzGrav researcher and the study’s lead author—explains: ‘We interpolated stars that were between 9 and 100 times the mass of the Sun and compared the predictions for the final fates of these stars. For most massive stars in our set, we found that the masses of the stellar remnants (neutron stars or black holes) can vary by up to 20 times the mass of our Sun’. When the stellar remnants merge, they create gravitational waves—ripples in space and time—that scientists can detect. Therefore, this study’s results will have a huge impact on future predictions in gravitational-wave astronomy. Agrawal adds: ‘METISSE is just the first step in uncovering the part massive stars play in stellar systems such as star clusters, and already the results are very exciting.’ Black holes are massive, right? The first pair of black holes detected were each about 30 times more massive than the Sun. When they merged, the resulting ‘remnant’ was a black hole that was a whopping 60 times more massive than the Sun.
Today astronomers from the LIGO and Virgo Scientific Collaboration (LVC) have reported the first ever direct observation of the most massive black hole merger to date. Two monster black holes collided to form an even more massive object—an intermediate-mass black hole, about 150 times as heavy as the Sun. Researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) contributed to the detection and used the computing resources of the new Gravitational-Wave Data Centre to infer the masses of the merging black holes. Juan Calderón Bustillo—co-author and OzGrav postdoctoral researcher at Monash University—reports: ‘This is the first time we’ve observed an intermediate-mass black hole, almost twice as heavy as any other black hole ever observed with gravitational-waves. For this reason, the detected signal is much shorter than those previously observed. In fact, it’s so short that we can barely observe the black hole collision, we can only see its result’. The online detection team at the University of Western Australia detected the event, GW190521, seconds after the gravitational-wave data were available, and helped generate public alerts for the LIGO Scientific Collaboration. OzGrav PhD student and co-author Manoj Kovalam: ‘We were among the fastest detection programs to report GW190521. Such a heavy system has never been observed before. It’s exciting to be among the first few to identify it in real-time’. These ‘impossible’ black holes have ‘forbidden’ masses according to what we currently understand about the lives of massive stars. OzGrav postdoctoral researcher Vaishali Adya from Australian National University explains: ‘Stars that are massive enough to make black holes this heavy should blow themselves apart in a dramatic ‘pair instability supernova’. Events like this are now in range due to the improved sensitivity of the instruments compared to the first-generation detectors.’ The rare event has prompted researchers to question how the black hole formed, its origins and how the two black holes found each other in the first place. OzGrav PhD student and co-author Isobel Romero-Shaw, from Monash University, comments on the perplexing masses: ‘Black holes form when massive stars die, both exploding in a supernova and imploding at the same time. But, when the star has a core mass in a specific range—between approximately 65 and 135 times the mass of the Sun—it usually just blows itself apart, so there’s no leftover black hole. Because of this, we don’t expect to see black holes in this solar mass range, unless some other mechanism is producing them.’ Since gravitational waves directly measure the masses of the colliding black holes, this measurement should be much more robust than the similar mass black hole previously reported by Liu et al. (published in the journal Nature last year). That measurement was based on an interpretation of the spectrum of light from the Galactic star system LB-1, which has since been refuted. Based on this current study’s mass measurements, researchers found that this kind of black hole couldn’t have formed from a collapsing star—instead, it may have formed from a previous black hole collision. OzGrav postdoctoral researcher and LVC member Simon Stevenson, from Swinburne University of Technology, says: ‘These ‘impossibly’ massive black holes may be made of two smaller black holes which previously merged. If true, we have a big black hole made of smaller black holes, with even smaller black holes inside them—like Russian Dolls.’ We are witnessing the birth of an intermediate mass black hole: a black hole more than 100 times as heavy as the Sun, almost twice as heavy as any black hole previously observed with gravitational-waves. These intermediate mass black holes could be the seeds that grow into the supermassive black holes that reside in the centres of galaxies. Meg Millhouse, OzGrav Postdoctoral researcher and LVC member from the University of Melbourne, was involved in the discovery paper’s analysis. Millhouse says: ‘We had to use extremely precise and complex models to analyse these heavier black holes compared to previous models used by LIGO for gravitational waves’. OzGrav Chief Investigator and co-author David Ottaway, from University of Adelaide, says: ‘This is a huge step towards understanding the link between the smaller black holes that have been seen by gravitational-wave detectors and the massive black holes that are found in the centre of galaxies’. These gravitational waves came from over 15 billion light years away! But isn't the Universe only around 14 billion years old, you ask? It turns out that the Universe was actually around 7 billion years old when these two black holes collided. As the gravitational waves rippled out through the Universe, the Universe was expanding. Consequently, the measured distance to this collision is now further than the product of the speed of light and the time travelled—mind (and space) bending stuff! This shows gravitational waves are able to probe the ancient history of the Universe, when galaxies were forming stars at a rate around 10 times higher than the present day. Astronomers have just made the most accurate distance measurements yet to the ultra-magnetised star XTE J1810-197 – and at a distance of about 8,000 light-years, the rare magnetar is one of the closest to us, and is quite a bit closer than we previously thought. The research was led by OzGrav astronomers and made use of the OzSTAR supercomputer at Swinburne University of Technology. In addition to getting perhaps the most precise distance to a magnetar to date, astronomers were also able to hypothesise about the magnetar’s genesis. A supernova remnant, located rather too close by to be coincidental, may be the remains of a former companion star of XTE J1810-197. Magnetars are a special and particularly terrifying variety of neutron star; stars composed nearly entirely of neutrons that spin at insane rates, up to hundreds of times per second. Their magnetic fields are stronger than anything else known. The magnetar XTE J1810-197 was discovered in 2003 by the Rossi X-Ray Timing Explorer (RXTE) as it was observing another magnetar, a soft gamma repeater, known as SGR 1806-20. At the time it was discovered, XTE J1810-197 was furiously emitting X-rays, but gradually faded away until 2018 when it became active again. Magnetars are rather rare, with less than 30 known examples in our galaxy. The magnetar XTE J1810-197 is rarer still, a special class of neutron star that has the properties of both magnetars and pulsars. Recent evidence suggests that neutron stars may go through different stages of evolution, first as a pulsar then as a magnetar (or maybe the other way around), but there is still a lot to learn about stars like XTE J1810-197. Knowing with some accuracy how far away they are is a good start. Led by OzGrav PhD student Hao Ding from Swinburne University, a collaboration between researchers in Australia, the USA and South Africa have taken measurements of the parallax of XTE J1810-197 over a period of a little more than a year. Previously thought to be 10,000 light-years away, they found it to be substantially closer at about 8,000 light-years. ‘In this work, we measure the positions of the magnetar with respect to two quasars that are quasi-linear to the magnetar. The technique we used can also be used to measure the parallaxes of radio-bright stars within about 10 kpc [kilo-parsecs, a commonly used unit of measurement amongst astronomers] distance. Beyond the distance limit, a parallax would be too small to detect.’ According to Hao Ding, having an accurate distance for XTE J1810-197, as well as an understanding of its motion across the sky (known as its proper motion), will benefit researchers trying to understand the properties of these fascinating stars. ‘Precise proper motion and parallax measurements would benefit long-term pulsar timing of magnetars, and, in particular, lead to a more reliable characteristic age’. Collaborator and OzGrav PhD student Marcus Lower is also from Swinburne and has an affiliation with the CSIRO. ‘Having an accurate distance measurement to the magnetar is extremely useful. For instance, we can now accurately measure the temperature of its surface based on how bright it appears in X-rays. It also allows us to measure the distance to blobs of hot gas between us and the magnetar based on the twinkling of its radio pulses.’ There’s also the question of whether there is any link between the nearby supernova remnant (SNR) – the remains of stars blown apart in supernovae explosions – and XTE J1810-197. While the two are separated by some distance, ‘our precise proper motion points back to the central region of the SNR called G11.0-0.0 at about 70,000 years ago,’ says Hao Ding. The magnetar XTE J1810-197 was the first one observed emitting radio pulses, and over 15 years later it is still giving up its secrets in the biggest science lab there is – the universe. Extracted from the feature article on Space Australia written by Dan Lambeth Gamma-ray bursts (GRBs) are extremely energetic explosions that have been observed in distant galaxies; they are the most luminous explosions in the Universe. A team of scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) from the University of Western Australia recently studied a high redshift long GRB (a more common explosion lasting between 2 seconds to several minutes) called GRB160203A. After four hours, the afterglow of this specific GRB begins rebrightening, spiking in luminosity at different times. Using data from every telescope that observed the cosmic event with the ‘fireball’ simulation model, the OzGrav team concluded that these bright features are best explained by the jet of intense light, electrons, and swept-up debris (known as the fireball) crashing into irregularities of the environment, like a fast car hitting a speed bump. The afterglow of GRB160203A has two main parts—the ‘well-behaved’ period, lasting four hours, and the ‘unusual’ period afterwards. The fireball model suggested that, in the well-behaved period, the fireball was in the interstellar medium—the space between the stars, which consists of gas with at least ten trillion times fewer particles than air in the same volume. In the unusual period, the fireball model failed to make any predictions about the environment of the burst due to the rebrightening events. This was our clue to investigate popular modifications to the model and explain the odd behaviour. The two most popular modifications to the fireball model are the magnetar collapse model and the termination shock model. The magnetar collapse model predicts that a magnetic neutron star (a small celestial object densely packed with neutrons) collapses, injecting energy into the fireball with the quickly changing magnetic field which is then converted to light. The termination shock model proposes that the fireball is passing through the boundary between the stellar wind and the interstellar medium. As the front of the fireball crashes into the boundary and slows down, the back of the fireball catches up and smashes into the front. This ‘reverse shock’ releases energy in the form of light; however, neither model can explain why there are at least two rebrightening events. The magnetar cannot collapse twice and the fireball cannot cross the stellar/interstellar medium more than once. The other models considered involved a non-uniform medium for the fireball—a turbulence model. As the fireball enters a region of higher density, it causes a reverse shock and releases light. The source of this spike in density could come from the wind surrounding a rare Wolf-Rayet star, or the natural turbulence of the interstellar medium. Wolf-Rayet stars constantly shed material in shells around themselves due to their unstable nature. The interstellar medium, like all turbulent fluids, has pockets of high and low density scattered throughout, like the chips in a chocolate chip biscuit. OzGrav researcher and lead of the study Hayden Crisp says: ‘We created a model of the brightness of the GRB as if it was well-behaved throughout the observations. By comparing the modelled brightness to the actual brightness, the relationship between brightness and medium density showed the fireball medium’s density over time. We saw that the rebrightening events correspond to a 5 to 50x increase in the density of the fireball medium’. Crisp adds: ‘Of the two plausible models, we prefer the turbulence model as the fireball model implies the well-behaved period is in the interstellar medium. Our main conclusion from this research is that the assumption of a uniform fireball medium is inappropriate in this case. A non-uniform environment may provide a new lens to examine the growing number of unusual bursts and provides a competitive model for explaining their features’. Every ten seconds or so, a pair of black holes or neutron stars collide somewhere in the Universe. These collisions generate gravitational waves—ripples in the fabric of space and time—which are observed on Earth using hyper-sensitive laser interferometers, based in the US and Italy. However, the detectors are not sensitive enough to see every collision, only those that are sufficiently close by. The data from the detectors is publicly available, allowing scientists from around the world to check the findings of the LIGO and Virgo collaborations who operate the detectors. Open data also allows outside groups to try new ways to find signals, providing healthy competition for LIGO and Virgo! Last year, a group from Princeton did just this—nalysing the open data, they found a new binary black hole candidate called GW151216. One way to ascertain if a candidate gravitational-wave event is real is to look for consistent signals in two or more observatories. Using this principle, a team of researchers from the ARC Centre of Excellence of Gravitational Wave Discovery (OzGrav), at Monash University, developed a new method to determine if candidate gravitational-wave events are real. OzGrav researcher Dr Greg Ashton likens the detection of gravitational-wave signals to listening to sounds in the night: “Imagine that you wake in the middle of the night to a strange noise. You turn to your partner to ask if they heard it too. If you both describe the same sound, then you are unlikely to have imagined it. We use the same idea. We compare the signal between the detectors and against known terrestrial noise”. In their study, Ashton and his collaborator, Prof Eric Thrane, applied their method to the candidate GW151216 and found that there’s only a 3% chance that it’s real. “We would’ve liked to conclude that it is real event,” says Ashton. “As the number of gravitational-wave detections grows, it will become increasingly important to assess the provenance of candidate events to ensure we draw conclusions from bona fide gravitational-wave signals.” Ashton and Thrane are now looking to further develop their method and apply it the many other candidates in the data. A new study makes a compelling case for the development of "NEMO"—a new observatory in Australia that could deliver on some of the most exciting gravitational-wave science next-generation detectors have to offer, but at a fraction of the cost. The study, co-authored by the ARC Center of Excellence for Gravitational Wave Discovery (OzGrav), coincides with an Astronomy Decadal Plan mid-term review by Australian Academy of Sciences where "NEMO" is identified as a priority goal. "Gravitational-wave astronomy is reshaping our understanding of the Universe," said one of the study's lead authors OzGrav Chief Investigator Paul Lasky, from Monash University. "Neutron stars are an end state of stellar evolution," he said. "They consist of the densest observable matter in the Universe, and are believed to consist of a superfluid, superconducting core of matter at supranuclear densities. Such conditions are impossible to produce in the laboratory, and theoretical modeling of the matter requires extrapolation by many orders of magnitude beyond the point where nuclear physics is well understood." The study presents the design concept and science case for a Neutron Star Extreme Matter Observatory (NEMO): a gravitational-wave interferometer optimized to study nuclear physics with merging neutron stars. The concept uses high circulating laser power, quantum squeezing and a detector topology specially designed to achieve the high frequency sensitivity necessary to probe nuclear matter using gravitational waves. The study acknowledges that third-generation observatories require substantial, global financial investment and significant technological development over many years. According to Monash Ph.D. candidate Francisco Hernandez Vivanco, who also worked on the study, the recent transformational discoveries were only the tip of the iceberg of what the new field of gravitational-wave astronomy could potentially achieve. "To reach its full potential, new detectors with greater sensitivity are required," Francisco said. "The global community of gravitational-wave scientists is currently designing the so called 'third-generation gravitational-wave detectors (we are currently in the second generation of detectors; the first generation were the prototypes that got us where we are today)." Third-generation detectors will increase the sensitivity achieved by a factor of 10, detecting every black hole merger throughout the Universe, and most of the neutron star collisions. But they have a hefty price tag. At about $1B, they require truly global investment, and are not anticipated to start detecting ripples of gravity until 2035 at the earliest. In contrast, NEMO would require a budget only under $100M, a considerably shorter timescale for development, and it would provide a test-bed facility for technology development for third-generation instruments. The paper concludes that further design studies are required detailing specifics of the instrument, as well as a possible scoping study to find an appropriate location for the observatory, a project known as "Finding NEMO." As featured in The Age, Phys.org and Space Australia Gravitational waves are ripples in space-time that come in many forms. So far, short-duration gravitational wave signals have been observed from colliding black holes and colliding neutron stars, but scientists expect to find other kinds of gravitational waves. Recently published research led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)studied continuous waves: long-lasting gravitational waves, in this particular case, waves from neutron stars--old dead stars--in specific star systems called low-mass X-ray binaries. Gravitational-wave detectors LIGO (Laser Interferometer Gravitational-wave Observatory) and Virgo provide excellent data to search for continuous waves as their signals are likely to be present in the detector data all the time (compared to gravitational waves from colliding black holes, which last only a second or so). Neutron stars, which are typically about one and half times the mass of our Sun, are very compact at only 20km across. Some neutron stars are alone, while others are in “binary systems”--the neutron star and a companion star orbit around each other. The OzGrav team focused on looking for continuous waves from spinning neutron stars in "low mass X-ray binaries" (LMXBs). Low mass describes the neutron star's companion which typically has a lower mass than our Sun;they are called X-ray binaries because scientists have observed X-rays from them using X-ray telescopes. In the study, the team searched for continuous waves from spinning neutron stars by directly targeting five LMXBs, which is a first for these five LMXBs. All the targeted LMXBs have X-ray observations which indicate how fast the neutron star is spinning: its rotation frequency. This is extremely useful information when searching for continuous waves as it’s expected that the frequency of the continuous wave is related to the rotation frequency of the neutron star. This allowed the team to search for each LMXB within a specific frequency range. Lead author and OzGrav researcher from the University of Melbourne Hannah Middleton says: “We used a search method, developed by researchers at the University of Melbourne,which was previously used to search for another LMXB called Scorpius X-1. Scorpius X-1 is a promising continuous wave source, because its X-rays are very bright, but the X-ray observations were unable to measure Scorpius X-1's rotation frequency. This means that a wide range of frequencies need to be looked at. By taking advantage of the X-ray measurements of rotation frequency for our five LMXBs, we can reduce the computational cost of the search, sometimes by as much as 99 per cent.” But knowing the rotation frequency is not quite enough:the continuous wave frequency may not equate to the rotation frequency, so the team searched for small frequency ranges around the measured values. “The continuous wave frequency might even be slowly changing over time, so we need to be able to track it over many months of data,” adds Middleton. “The search uses a technique called a hidden Markov model which is widely used in applications from speech recognition to communication technologies. The resulting search can keep track of a signal even if the frequency changes unpredictably during an observation.” So, what did the scientists find? After analysing data from the second observing run (over 200 days between November 2016 to August 2017), unfortunately they did not find strong evidence for continuous wave signals from these five LMXBs. But the search continues! LIGO and Virgo's third observation run (from April 2019 to March 2020) has just completed, so the OzGrav scientists have plenty of data analysis and star searching to sink their teeth into. EARTH-SHAKING SCIENCE IN THE FREEZER: NEXT GENERATION VIBRATION SENSORS AT CRYOGENIC TEMPERATURES13/7/2020 A cutting-edge vibration sensor may improve the next generation of gravitational-wave detectors to find the tiniest cosmic waves from the background hum of Earth’s motion. Vibration sensor. Credit: Joris van Heijningen During his PhD, postdoctoral researcher Joris van Heijningen from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), developed the world’s most sensitive inertial vibration sensor. Now, he proposes a similar design, but 50 times more sensitive, at frequencies below 10 Hz, using cryogenic temperatures. This new sensor measures vibrations as small as a few femtometre (a millionth of a billionth of a meter) with a 10 to 100 millisecond period (10 Hz to 100 Hz). The paper recently published in IOP’s Journal of Instrumentation reveals a prototype of the next generation of seismic isolation systems with sensitivity down to 1Hz, using cryogenic temperatures—lower than 9.2 degrees and above the absolute zero. Even though we can’t feel it, our planet is always vibrating a tiny bit due to many different events, both cosmic and earthly; for example, from gravitational waves (miniscule ripples in spacetime); ocean waves crashing on the shore; or human activity. According to Dr van Heijningen, some places vibrate more than others and, if you plot these vibrations, they lie between two lines called the Peterson Low and High Noise Models (LNM/HNM). ‘The best commercial vibration sensors have been developed to have a sensitivity that lies below the LNM. They are sufficiently sensitive to measure all places on Earth with a decent signal-to-noise-ratio,’ says van Heijningen. To date, the Laser Interferometer Gravitational-Wave Observatory (LIGO), with its four-kilometre long arms, uses seismic isolation systems to prevent earthly vibrations affecting scientific measurements; however, future gravitational wave-detectors demand more advanced and precise vibration sensors. Scientists are already working on a third generation of detectors that will have the power to detect hundreds of black-hole mergers each year, measuring their masses and spins—even more than LIGO, or its European equivalent, Virgo, can measure. In the US, there will be the Cosmic Explorer: a 40-kilometre observatory that will be able to detect hundreds of thousands of black-hole mergers each year. Equally as impressive will be the Einstein Telescope in Europe, with its 10-kilometre armed, triangular configuration built underground. Future detectors will be able to measure gravitational waves at frequencies lower than the current cut-off ~10 Hz, ‘because that’s where the signals from collisions of black holes are lurking,’ van Heijningen explains. But one of the main issues of these huge detectors is that they need to be extremely stable—the smallest vibration can hamper detections. ‘Essentially getting the system close to zero degrees Kelvin (which is 270 degrees below zero celsius) drastically reduces the so-called thermal noise, which is dominant at low frequencies. Temperature is a vibration of atoms in some sense, and this minuscule vibration causes noise in our sensors and detectors,’ says van Heijningen. Future detectors will need to cool down to cryogenic temperatures, but it is no easy feat. Once scientists achieve that, exploiting the cryogenic environment will improve sensor performance following this proposal design. At his new position as a research scientist at UCLouvain in Belgium, van Heijningen plans to prototype this sensor design and test its performance for The Einstein Telescope. Mysterious spinning neutron star detected in the Milky Way proves to be an extremely rare discovery7/7/2020 On March 12th 2020 a space telescope called Swift, detected a burst of radiation from half-way across the Milky Way. Within a week, the newly discovered X-ray source, named Swift J1818.0–1607, was found to be a magnetar: a rare type of slowly rotating neutron star with one of the most powerful magnetic fields in the Universe. Spinning once every 1.4 seconds, it’s the fastest spinning magnetar known, and possibly one of the youngest neutron stars in the Milky Way. It also emits radio pulses like those seen from pulsars--another type of rotating neutron star in our galaxy. At the time of this detection, only four other radio-pulse-emitting magnetars were known, making Swift J1818.0–1607 an extremely rare discovery. In a recently published study led by a team of scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), it was found that the pulses from the magnetar become significantly fainter when going from low to high radio frequencies: it has a ‘steep’ radio spectrum. Its radio emission is not only steeper than the four other radio magnetars, but also steeper than ~90% of all pulsars! Additionally, they found the magnetar had become over 10 times brighter in only two weeks. Comparatively, the other four radio magnetars have almost constant brightness at different radio frequencies. These observations were made using the Ultra Wideband-Low (UWL) receiver system installed on the Parkes radio telescope, also known as ‘The Dish’. Whereas most telescopes are limited to observing radio waves across very narrow frequency strips, the Parkes UWL receiver can detect radio waves across an extremely wide range of frequencies all at the same time. After further analysis, the OzGrav team found interesting similarities to a highly energetic radio pulsar called PSR J1119–6127. This pulsar underwent a magnetar-like outburst back in 2016, where it too experienced a rapid increase in brightness and developed a steep radio spectrum. If the outburst of this pulsar and Swift J1818.0–1607 share the same power source, then slowly over time, the magnetar’s spectrum should begin to look like other observed radio magnetars. The age of the young magnetar (between 240-320 years), was measured from both its rotation period and how quickly it slows down over time; however, this is unlikely to be accurate. The spin-down rates of magnetars are highly variable on year-long timescales, particularly after outbursts, and can lead to incorrect age estimates. This is also backed up by the lack of any supernova remnant—remnants of luminous stellar explosions—at the magnetars position. Lead author of the study Marcus Lower proposed a theory to explain of the magnetar’s mysterious properties: ‘Swift J1818.0–1607 may have started out life as a more ordinary radio pulsar that obtained the rotational properties of a magnetar over time. This can happen if the magnetic and rotational poles of a neutron star rapidly become aligned, or if supernova material fell back onto the neutron star and buried its magnetic field’. The buried magnetic field would then slowly emerge back to the surface over thousands of years. Continued observations of Swift J1818.0–1607, over many months to years, are needed to test these theories. A recent study on ‘pre-supernova’ neutrinos—tiny cosmic particles that are extremely hard to detect—has brought scientists one step closer to understanding what happens to stars before they explode and die. The study, co-authored by postdoctoral researcher Ryosuke Hirai, from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University, investigated stellar evolution models to test uncertain predictions. When a star dies, a huge number of neutrinos are emitted which are thought to drive the resulting supernova explosion. The neutrinos flow freely through and out of the star before the explosion reaches the surface of the star. Scientists can then detect neutrinos before the supernova occurs, in fact, a few dozen neutrinos were detected from a supernova that exploded in 1987, several hours before the explosion was seen in light. The next generation of neutrino detectors are expected to detect about 50,000 neutrinos from a similar kind of supernova. The technology has become so powerful that scientists predict they will detect the weak neutrino signals that come out days before the explosion; just like a supernova forecast, it will give astronomers a heads up to catch the first light of a supernova. It’s also one of the only ways to directly extract information from a star’s core—similar to an X-ray image of your body, except it’s for stars. But an X-ray image is meaningless unless you know what you’re looking at. Although there is a general understanding of how a massive star evolves and explodes, scientists are still uncertain about the lead up to the supernova explosion. Many physicists have attempted to model these final phases, but the outcomes appear random; there is no way to confirm if they’re correct. Since pre-supernova neutrino detections allow scientists to better assess these models. a team of OzGrav scientists investigated the late stages of stellar evolution models and how that might affect pre-supernova neutrino estimates. OzGrav researcher and co-author Ryosuke Hirai says: ‘This will help us make the most of the information from future pre-supernova neutrino detections’. In this first study, we explored the uncertainty on a single star that is 15 times the mass of the Sun. The neutrino emission calculated from these stellar models differed greatly in the neutrino luminosity. This means that pre-supernova neutrino estimates are very sensitive to these small details of the stellar model.’ The study revealed the significant uncertainty of pre-supernova neutrino predictions, as well as the relationship between the neutrino features and the star’s properties. ‘The next supernova in our galaxy can happen any day, and scientists are looking forward to detecting pre-supernova neutrinos, but we still don't know what we can learn from it. This study lays out the first steps of how to interpret the data. Eventually, we’ll be able to use pre-supernova neutrinos to understand crucial parts of massive star evolution and the supernova explosion mechanism.’ Last year, the Advanced LIGO-VIRGO gravitational-wave detector network recorded data from 35 merging black holes and neutron stars. A great result – but what did they miss? According to Dr Rory Smith from the ARC Centre of Excellence in Gravitational Wave Discovery at Monash University in Australia – it’s likely there are another 2 million gravitational wave events from merging black holes, “a pair of merging black holes every 200 seconds and a pair of merging neutron stars every 15 seconds” that scientists are not picking up. Dr Smith and his colleagues, also at Monash University, have developed a method to detect the presence of these weak or “background” events that to date have gone unnoticed, without having to detect each one individually. The method – which is currently being test driven by the LIGO community – “means that we may be able to look more than 8 billion light years further than we are currently observing,” Dr Smith said. “This will give us a snapshot of what the early universe looked like while providing insights into the evolution of the universe.” The paper, recently published in the Royal Astronomical Society journal, details how researchers will measure the properties of a background of gravitational waves from the millions of unresolved black hole mergers. Binary black hole mergers release huge amounts of energy in the form of gravitational waves and are now routinely being detected by the Advanced LIGO-Virgo detector network. According to co-author, Eric Thrane from OzGrav-Monash, these gravitational waves generated by individual binary mergers “carry information about spacetime and nuclear matter in the most extreme environments in the Universe. Individual observations of gravitational waves trace the evolution of stars, star clusters, and galaxies,” he said. “By piecing together information from many merger events, we can begin to understand the environments in which stars live and evolve, and what causes their eventual fate as black holes. The further away we see the gravitational waves from these mergers, the younger the Universe was when they formed. We can trace the evolution of stars and galaxies throughout cosmic time, back to when the Universe was a fraction of its current age.” The researchers measure population properties of binary black hole mergers, such as the distribution of black hole masses. The vast majority of compact binary mergers produce gravitational waves that are too weak to yield unambiguous detections – so vast amounts of information is currently missed by our observatories. “Moreover, inferences made about the black hole population may be susceptible to a ‘selection bias’ due to the fact that we only see a handful of the loudest, most nearby systems. Selection bias means we might only be getting a snapshot of black holes, rather than the full picture,” Dr Smith warned. The analysis developed by Smith and Thrane is being tested using real world observations from the LIGO-VIRGO detectors with the program expected to be fully operational within a few years, according to Dr Smith. Gravitational wave detectors are extremely complex instruments of precision measurement. They use interference as the physical mechanism to measure passing gravitational waves (GWs)—ripples in space-time—from different astronomical sources and events, like two neutron stars merging. The passing wave signal gets encoded into a wave of light and is read-out after exiting the interferometer. The issue is that the signal is so weak that any movement from the optical components will degrade the signal strength. For example, the random motion of particles that make up the material called ‘thermal noise’. In the design of GW detectors, ‘optomechanical’ cavities are used to enhance the signal from GW detectors. These cavities, or ‘resonators’, typically have two, moving-end mirrors which trap and amplify light. There is one problem however: the mirrors can move too much due to thermal noise! If we can minimise the thermal noise of these resonators, it will improve the GW sensitivity. The Double-End-Mirror-Sloshing (DEMS) cavity—shown in figure 1—is a special type of optomechanical cavity which consists of four mirrors, a transmitting sloshing mirror and a resonator which reflects light from both sides (double-end-mirror). Using the DEMS cavity, the resonator exhibits very low levels of thermal noise through a process called ‘optical dilution’, which works by trapping the resonator in a potential well using radiation pressure. This keeps the resonator tightly bound, so it’s not easily disturbed from the random thermal fluctuations. In a study led by the OzGrav, researchers explain that, although the optical spring is not unique to the DEMS cavity, the troublesome impact of radiation pressure noise and anti-damping effects are circumvented in the DEMS cavity, but are unavoidable in a two-mirror cavity. First author and OzGrav research assistant Parris Trahanas explains: ‘The key mechanism that allows the DEMS cavity these qualities is the transmissive sloshing mirror component—it turns the DEMS cavity into a coupled optical resonator, which is the optical equivalent of connecting two spring mass systems together with a third spring.’ The results in Figure 2 show the avoided crossing of the optical resonances which is characteristic of a coupled oscillator—this will be an extremely useful tool for GW detectors, but could be more broadly applied to any field requiring low thermal noise in mechanical resonators. Dancing stars and black holes in a cosmic cloud of gas: New research of the ‘common envelope phase’16/6/2020 Most massive stars are born in binaries (and sometimes triples, quadruples, and so on—being single isn’t common for such rock stars!). As stars age, they grow larger in size, and not just a little thickening of the waistline, but a hundred-fold or even thousand-fold expansion! When stars in binaries expand, part of them get close to the other star in the binary, whose gravity can then pull off the outer portions of the expanding star. The result is mass transfer from one star to the other. Usually mass is transferred gradually. But sometimes, the more mass is transferred, the more mass gets pulled off, in a runaway process. The outer layers of one star completely surround the other in a phase known as the common envelope. During this phase, the dense cores of the two stars orbit each other inside the cloud, or envelope, of gas. The gas drags on the stellar cores, causing them to spiral in; this heats up the common envelope, which may get expelled. The cores may end up more than one hundred times closer than they started. This common envelope phase is thought to play a crucial role in forming ultra-compact object binaries, including sources of gravitational waves; however, it is also very poorly understood. In a paper recently accepted to the Astrophysical Journal, Soumi De and collaborators from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) explored the common envelope phase through detailed computer simulations. They used ‘wind-tunnel models’, in which a stellar core, a neutron star or a black hole is buffeted by the ‘wind’ of gas, representing its orbit through the envelope. While this is a simplification of the full three-dimensional physics of the common envelope, the hope is that this approach makes it possible to understand the key features of the problem. You can watch an animation of one of the models here: https://sde101.expressions.syr.edu/common-envelope-hydrodynamic-simulations/ . Co-author and OzGrav CI Ilya Mandel explains that ‘the results revealed the drag forces and the rate of accretion onto the black hole. Together, these allow us to predict how much the black hole will grow during the common envelope phase’. ‘While a naive estimate suggests that black holes should gain a lot of mass during this phase, we find that’s not the case, and the black holes do not become much heavier,’ says Mandel. ‘And this has important consequences for understanding the merger rates and mass distributions of gravitational-wave sources.’ A team of scientists, including Chief Investigator Ilya Mandel from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University, recently submitted a paper investigating what happens to rotating massive stars when they reach the end of their lives. Stars produce energy by fusing lighter elements into heavier ones in their core: hydrogen into helium, then helium into carbon, oxygen, and so on, up to iron. The energy produced by this nuclear fusion also provides pressure support inside the star, which balances the force of gravity and allows the star to remain in equilibrium. This process stops at iron. Beyond iron, energy is required for fusion rather than being released by fusion. A heavy iron star core contracts under gravity, creating a neutron star or, if it is heavy enough, a black hole. Meanwhile, the outer layers of the star explode in a brilliant flash, observable as a supernova. However, some massive stars seem to completely disappear without any explosion. Theories suggest that these massive stars completely collapse into black holes, but is that possible? A team led by Ariadna Murguia-Berthier, a PhD candidate at the University of California Santa Cruz, and involving OzGrav Chief Investigator Ilya Mandel, set out to answer this question. They were particularly interested in understanding whether a rotating star could quietly collapse into a black hole. In their paper submitted to Astrophysical Journal Letters, they describe a set of simulations investigating the collapse of a rotating gas cloud into a black hole. It was found that if the gas is rotating too quickly at the beginning, it cannot efficiently collapse; instead, the gas stalls in a donut-like shape around the equator of the black hole. The team hypothesised that the heat generated from falling gas slamming into this spinning gas donut will unbind the outer layers of the star and create a supernova-like explosion. A small percentage of all stars were also found to rotate slowly enough—below the threshold for this gas stalling to occur—and could indeed collapse into black holes quietly. “It’s very exciting to bring together general relativity, sophisticated computational techniques, stellar models, and the latest observations to explore the formation of black holes from massive stars!” says Mandel. Earlier this year, an international team of scientists announced the second detection of a gravitational-wave signal from the collision of two neutron stars. The event, called GW190425, is puzzling: the combined mass of the two neutron stars is greater than any other observed binary neutron star system. The combined mass is 3.4 times the mass of our Sun. A neutron star binary this massive has never been seen in our Galaxy, and scientists have been mystified by how it could have formed—until now. A team of astrophysicists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) think they might have the answer. Binary neutron stars emit gravitational waves—ripples in space-time— as they orbit each other, and scientists can detect these waves when the neutron stars merge. The gravitational waves contain information about the neutron stars, including their masses. The gravitational waves from cosmic event GW190425 tell of a neutron star binary more massive than any neutron star binary previously observed, either through radio-wave or gravitational-wave astronomy. A recent study led by OzGrav PhD student Isobel Romero-Shaw from Monash University proposes a formation channel that explains both the high mass of this binary and the fact that similar systems aren’t observed with traditional radio astronomy techniques. Romero-Shaw explains: ‘We propose that GW190425 formed through a process called ‘unstable case BB mass transfer’, a procedure that was originally defined in 1981. It starts with a neutron star which has a stellar partner: a helium (He) star with a carbon-oxygen (CO) core. If the helium part of the star expands far enough to engulf the neutron star, this helium cloud ends up pushing the binary closer together before it dissipates. The carbon-oxygen core of the star then explodes in a supernova and collapses to a neutron star’. Binary neutron stars that form in this way can be significantly more massive than those observed through radio waves. They also merge very fast following the supernova explosion, making them unlikely to be captured in radio astronomy surveys. ‘Our study points out that the process of unstable case BB mass transfer could be how the massive star system formed,’ says Romero-Shaw. The OzGrav researchers also used a recently-developed technique to measure the eccentricity of the binary—how much the star system’s orbital shape deviates from a circle. Their findings are consistent with unstable case BB mass transfer. Current ground-based gravitational-wave detectors aren’t sensitive enough to precisely measure the eccentricity; however, future detectors—like space-based detector LISA, due for launch in 2034—will allow scientists to make more accurate conclusions. Scientists reveal new insights of exploding massive stars and future gravitational-wave detectors11/5/2020 In a study recently published in the Monthly Notices of the Royal Astronomical Society, researchers Dr Jade Powell and Dr Bernhard Mueller from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) simulated three core-collapse supernovae using supercomputers from across Australia, including the OzSTAR supercomputer at Swinburne University of Technology. The simulation models—which are 39 times, 20 times and 18 times more massive than our Sun— revealed new insights into exploding massive stars and the next generation of gravitational-wave detectors. Core-collapse supernovae are the explosive deaths of massive stars at the end of their lifetime. They are some of the most luminous objects in the Universe and are the birthplace of black holes and neutron stars. The gravitational waves—ripples in space and time—detected from these supernovae, help scientists better understand the astrophysics of black holes and neutron stars. Future advanced gravitational-wave detectors, engineered to be more sensitive, could possibly detect a supernova—a core-collapse supernova could be the first object to be observed simultaneously in electromagnetic light, neutrinos and gravitational waves. To detect a core-collapse supernova in gravitational waves, scientists need to predict what the gravitational wave signal will look like. Supercomputers are used to simulate these cosmic explosions to understand their complicated physics. This allows scientists to predict what the detectors will see when a star explodes and its observable properties. In the study, the simulations of three exploding massive stars follow the operation of the supernova engine over a long duration—this is important for accurate predictions of the neutron star masses and observable explosion energy. OzGrav postdoctoral researcher Jade Powell says: ‘Our models are 39 times, 20 times and 18 times more massive than our Sun. The 39-solar mass model is important because it’s rotating very rapidly, and most previous long duration core-collapse supernova simulations do not include the effects of rotation’. The two most massive models produce energetic explosions powered by the neutrinos, but the smallest model did not explode. Stars that do not explode emit lower amplitude gravitational waves, but the frequency of their gravitational waves lies in the most sensitive range of gravitational wave detectors. ‘For the first time, we showed that rotation changes the relationship between the gravitational-wave frequency and the properties of the newly-forming neutron star,’ explains Powell. The rapidly rotating model showed large gravitational-wave amplitudes that would make the exploding star detectable almost 6.5 million light years away by the next generation of gravitational-wave detectors, like the Einstein Telescope. A team of researchers from the ARC Centre of Excellence for Gravitational Wave discovery (OzGrav) recently published a study revealing something unexpected about black holes: that intermediate-mass black holes with precessing orbits should be easier to detect than standard ones. Black holes are regions of space-time from which nothing can escape, not even light. They are the corpses of dead stars that collapsed under their own weight, after running out of fuel. As archaeology helps us to understand how dinosaurs lived, the study of black holes helps us to understand how stars formed, evolved and died. When two blackholes collide, they release incredible amounts of energy in the form of gravitational-waves (ripples in the fabric of space-time), producing the most powerful space-time storms. By observing these waves, scientists can explore the most fundamental properties of gravity. Black holes can be classified according to their mass—two different kinds have been identified: black holes with a mass several times bigger than the Sun; and supermassive black holes that lie in the centre of most galaxies (the largest type of black hole), containing a mass billions of times the mass of the Sun. Intermediate-mass black holes are the elusive missing link; despite the indirect evidence for their existence, scientists have not yet confirmed a conclusive observation of these black holes. Finding them would help to explain the mystery of how stellar mass black holes can evolve into supermassive ones. The LIGO and Virgo collaboration searched for intermediate-mass black hole collisions during their first and second observing runs, from 2015 to 2018, but were not successful. On the bright side, the lack of a detection allows scientists to confirm how many of these collisions happen in the Universe. To achieve this in the study researchers including OzGrav Alumnus Juan Calderon Bustillo, first determined the observable distance of these collisions using supercomputer simulations. The gravitational-wave signals generated from the collision were recorded and injected into the data to assess their recovery rate in the search algorithms. When doing similar studies, scientists have always assumed that two colliding black holes approach each other with a constant orbital plane, like the orbit of the Earth and the Sun. However, there is another possible situation in which the black holes move up and down, describing what is known as a precessing orbit. The researchers found that this kind of collision can be observed from a further distance, allowing them to better constrain the number of black hole collisions out there. This also means that these black holes may be easier to detect if they follow precessing orbits rather than standard ones, making them better candidates for a first detection of intermediate-mass black holes. Scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) reveal an alternative explanation of the recently announce black hole merger. The paper was just accepted by Astrophysical Journal Letters. On the 12th of April 2019, the LIGO and Virgo observatories detected gravitational waves—ripples in space and time—from an unusual cosmic event of two black holes merging. Unlike the ten previously reported black hole mergers, in which the two black holes may have had equal or nearly-equal masses, this event, called GW190412, definitely had two very unequal black holes, with the heavier one possibly three or four times more massive than the lighter one. In addition, the discovery paper (released in Australia on 19 April 2020) reported that at least one of the merging black holes had to be spinning: rotating around its axis. However, gravitational waves do not allow accurate measurement of individual spins. Only a specific spin combination can be measured. Therefore, to infer individual spins, assumptions must be made based on scientific models. The LIGO and Virgo collaborations assumed that the heavier, first-born black hole could be spinning, and reported that it had a moderate spin in the gravitational-wave discovery paper. Within 24 hours of the discovery’s announcement, OzGrav Chief Investigator Ilya Mandel, from Monash University, and collaborator Tassos Fragos, from the University of Geneva, wrote a follow-up paper which has just been accepted by Astrophysical Journal Letters. Motivated by the best current models of the evolution of massive stars in binaries, Mandel and Fragos argued that the more massive, or ‘heavier’, black hole in the event is very slowly spinning; whereas the ‘lighter’ black hole is spinning very fast, in the same direction as the orbital motion. Mandel and Fragos state that if isolated pairs of stars orbiting around each other give birth to merging black holes, they naturally make first-born, heavier black holes that spin very slowly. Before a star forms a black hole, it evolves into a giant with a gaseous envelope. When it does so, it slows down, like a spinning figure skater extending her arms. When this envelope is stripped off by extreme tidal forces exerted by the other star in the binary, a slowly rotating central core is left behind, which ultimately collapses into a slowly spinning black hole. The same process should typically apply to the second-born, lighter star, which eventually collapses into the lighter black hole. However, when the second star loses its gaseous envelope, the binary separation can be sufficiently small enough to allow the naked star core to spin up through ‘tidal locking’. Mandel explains: ‘Tidal locking occurs when tides from an orbiting companion forces an object’s period of rotation—the time it takes it to spin around its axis—to equal the time it takes for a full orbit of the binary system. For example, tidal locking of the Moon to the Earth sets the Moon to rotate the same 28 days equal to its orbital period around the Earth. This explains why we never get to see the dark side of the Moon—except when listening to Pink Floyd.’ So, sometimes the second black hole can spin up and rapidly rotate. Mandel and Fragos find this to be the case in the GW190412 event. Such systems should also merge soon after their formation, since tidal locking will only happen in very tight binaries. Although it’s difficult to confirm this interpretation, future detections of black hole mergers will allow for more accurate testing of this model. Gravitational-wave astronomy provides a unique new way to study the expansion history of the Universe. On 17 August 2017, the LIGO and Virgo collaborations first detected gravitational waves from a pair of neutron stairs merging. The gravitational wave signal was accompanied by a range of counterparts identified with electromagnetic telescopes. This multi-messenger discovery allowed astronomers to directly measure the Hubble constant, which tells us how fast the Universe is expanding. A recent study by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) led by researchers Zhiqiang You and Xingjiang Zhu, studied an alternative way to do cosmology with gravitational-wave observations.
In comparison to neutron star mergers, black hole mergers are much more abundant sources of gravitational waves. Whereas there have been only two neutron star mergers detected so far, LIGO and Virgo collaborations have published 10 binary black hole merger events and dozens more candidates have been reported. Unfortunately, no electromagnetic emission is expected from black hole mergers. Theoretical modeling of supernovae—powerful and luminous stellar explosions—suggests that there is a gap in the masses of black holes around 45-60 times the mass of our Sun. Some inconclusive evidence that supports this mass gap was found in observations made in the first two observing runs of LIGO and Virgo. The new OzGrav research shows that this unique feature in the black hole mass spectrum can help determine the expansion history of our Universe using gravitational-wave data alone. OzGrav PhD student and first author Zhiqiang You says: ‘Our work studied the prospect with third-generation gravitational-wave detectors, which will allow us to see every binary black hole merger in the Universe’. Apart from the Hubble constant—a unit that describes how fast the Universe is expanding—there are other factors that can affect how black hole masses are distributed. For example, scientists are still uncertain about the exact location of the black hole mass gap and how the number of black hole mergers evolves over the cosmic history. The new study demonstrates that it is possible to simultaneously measure black hole masses along with the Hubble constant. It was found that a third-generation detector like the Einstein Telescope or the Cosmic Explorer should measure the Hubble constant to better than one percent within one-year’s operation. Moreover, with merely one-week observation, the study revealed it is possible to distinguish the standard dark energy-dark matter cosmology with its simple alternatives. Pulsars are dead stars that spin remarkably steadily – they are some of the most regularly ticking clocks in the Universe! However, every few years some pulsars ‘glitch’, and speed up a tiny amount almost instantaneously. Understanding what causes these glitches may unveil what’s really happening inside these super-dense dead stars.
Detailed theoretical and computer models are hard to connect to real observations, so instead PhD student Julian Carlin and Chief Investigator Andrew Melatos, from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), built a ‘meta-model’ in a paper recently published in the Monthly Notices of the Royal Astronomical Society. The meta-model relies on the idea that ‘stress’ builds up inside the pulsar until it reaches a threshold, and then some of this stress is released as a glitch. The interesting thing about this meta-model is that the stress increases by taking a ‘random walk’ upwards: like an intoxicated person returning home from the pub who might take two steps forward, one step back, then three steps forward. The randomness in how the stress builds is supported by some theoretical models, as well as a recent study of a glitch-in-action led by OzGrav researchers Greg Ashton, Paul Lasky, and others. Meta-models make predictions about what we should see in the long term from glitching pulsars. ‘This meta-model predicts that there should always be a correlation between big glitches and the time until the next glitch: if a lot of stress is released, it takes longer on average for the pulsar to build up enough stress for another glitch,’ explains Carlin. Using this prediction, Carlin and Melatos tried to falsify the meta-model, asking the question: ‘Are there long-term observations that can’t be explained?’. The answer depends on the pulsar. Some are well-explained by the meta-model, while others don’t quite match the predictions. ‘We need to see more glitches before this question can be answered for certain, but this work shows a way to answer it for many theoretical models, all at the same time,’ says Carlin. Pulsars are dead stars that spin remarkably steadily – they are some of the most regularly ticking clocks in the Universe! However, every few years some pulsars ‘glitch’, and speed up a tiny amount almost instantaneously. Understanding what causes these glitches may unveil what’s really happening inside these super-dense dead stars. Detailed theoretical and computer models are hard to connect to real observations, so instead PhD student Julian Carlin and Chief Investigator Andrew Melatos, from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), built a ‘meta-model’ in a paper recently published in the Monthly Notices of the Royal Astronomical Society. The meta-model relies on the idea that ‘stress’ builds up inside the pulsar until it reaches a threshold, and then some of this stress is released as a glitch. The interesting thing about this meta-model is that the stress increases by taking a ‘random walk’ upwards: like an intoxicated person returning home from the pub who might take two steps forward, one step back, then three steps forward. The randomness in how the stress builds is supported by some theoretical models, as well as a recent study of a glitch-in-action led by OzGrav researchers Greg Ashton, Paul Lasky, and others. Meta-models make predictions about what we should see in the long term from glitching pulsars. ‘This meta-model predicts that there should always be a correlation between big glitches and the time until the next glitch: if a lot of stress is released, it takes longer on average for the pulsar to build up enough stress for another glitch,’ explains Carlin. Using this prediction, Carlin and Melatos tried to falsify the meta-model, asking the question: ‘Are there long-term observations that can’t be explained?’. The answer depends on the pulsar. Some are well-explained by the meta-model, while others don’t quite match the predictions. ‘We need to see more glitches before this question can be answered for certain, but this work shows a way to answer it for many theoretical models, all at the same time,’ says Carlin. On the 17th of August 2017, the world of astronomy bore witness to an unprecedented event: the LIGO/Virgo collaboration recorded the vibrations of a merging pair of neutron stars. The event was observed in electromagnetic waves across the spectrum, from gamma rays, optical, ultraviolet, infrared, through to radio. The Universe was being seen and heard simultaneously, for the very first time. Astronomers have discovered binary neutron stars before, as a part of pulsar surveys—pulsars are rapidly rotating neutron stars. They’re the size of a city (~10km radius); have extremely strong magnetic fields; rotate up to 1000 times per second; and emit beams of radiation along their poles. As they spin, these beams may point towards the Earth like a lighthouse, allowing them to be observed by radio telescopes. Due to their extremely periodic motion, pulsars can serve as accurate clocks to test Einstein’s theory of General Relativity. All pulsars are neutron stars but not vice versa. Radio telescopes have detected pulsars in binary systems with other neutron stars (and in one case, another pulsar). Given the dynamic and mysterious nature of binary neutron stars, a team of researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) decided to study their formation, evolution and merger, examining the underlying uncertainties. In trying to investigate these mechanisms, astronomers’ observations are clues to solve the puzzle. In the study, led by OzGrav PhD student Debatri Chattopadhyay, the team used COMPAS—a population synthesis code (co-developed by OzGrav) that generates and evolves a group of isolated binary stars to create millions of isolated binaries in the supercomputer OzSTAR. It is assumed every neutron star is born as a pulsar in a supernova (a powerful and luminous stellar explosion), but magnetised pulsars, with their misaligned rotation and radio emission, slow down over time. Losing its rapid rotational motion, slow pulsars become less magnetised and eventually die; however, they can be revived with the help of their partner star. ‘Under specific conditions, matter from the still-evolving companion star may fall onto the pulsar and refuel its rotation. These “zombie pulsars” start to emit radio waves again, spinning fast, but with much lower surface magnetisation,’ explains Chattopadhyay. Implementing the physics of pulsar evolution in COMPAS, the researchers modelled different binary neutron star systems and found a 'best-fit’ model, analysed in both radio and gravitational waves. ‘We underline how the “radio alive”’ pulsar-neutron star binaries may have a different signature than double neutron stars, decipherable from only gravitational wave measurable parameters,’ says Chattopadhyay. These findings provide a better understanding of binary pulsar evolution and their formation channels. The COMPAS code can also be used to study other exotic binary systems, like pulsar-black holes or millisecond pulsar-white dwarfs. Space scientists from The University of Western Australia have worked with the European Space Agency to provide continuous imaging of a space probe passing Earth while on a journey to Mercury.
The scientists from UWA’s Centre of Excellence for Gravitational Wave Discovery (OzGrav) used the powerful robotic Zadko Telescope in Gingin to capture imagery of the space probe, named BepiColumbo. The probe was launched in 2018 and has since completed one and a half orbits around the Sun, travelling a distance of roughly 1.4 billion kilometres. OzGrav scientist Dr Bruce Gendre said BepiColombo would study Mercury’s magnetic field and its interaction with the solar wind, offering insight into how the Earth and solar system formed. “In order to keep the space probe on track to reach Mercury in 2025, BepiColumbo performed a ‘fly-by’ past Earth on 10 April 2020, utilising a gravity assist manoeuvre, which reduces the amount of propellant and thrust needed to complete the mission,” Dr Gendre said. "Space navigation is a complex task and requires large quantities of fuel. To reduce fuel consumption and the resulting cost of the mission, space agencies often use gravitational assistance from planets.” Due to the regional travel restrictions imposed by the WA Government following the COVID-19 outbreak, Dr Gendre was unable to control the Zadko telescope on-site in Gingin and instead operated the telescope remotely from his home in Claremont. “This important contribution to space research helps inspire the engineers and scientists of tomorrow, continuing the legacy of UWA philanthropist James Zadko, who passed away in early 2020,” Dr Gendre said. Associate Professor David Coward, Chief Investigator at OzGrav and Zadko Telescope Director, said tracking the space probe represented a small part of a greater project. “Providing ongoing assistance is part of a broader partnership with the European Space Agency to monitor the space around Earth for potential hazards, including near-earth asteroids,” Associate Professor Coward said. Detlef Koschny, a European Space Agency scientist said the Zadko Telescope was an important part of ensuring the success of the mission. “ESA's Planetary Defence Office is using this flyby as a test for its capabilities to coordinate the observation of possibly dangerous asteroids,” Dr Koschny said. “In the southern hemisphere, there are not many telescopes available for this purpose.” The Zadko Telescope is partially supported by the UWA Faculty of Engineering and Mathematical Sciences and the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav). As featured on UWA News |
|