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. 

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).

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.