​The authors Debatri Chattopadhyay (Swinburne University) and Isobel Romero-Shaw (Monash University)—who are both completing their PhDs in astrophysics with OzGrav—are determined to educate children and young people about the pivotal scientific discoveries and contributions made by women scientists. They also want to encourage more girls, women, and minorities to take up careers in Science, Technology, Engineering, Mathematics and Medicine (STEMM), which is a male-dominated field.

Debatri, who is originally from India, came to Australia pursuing her PhD at Swinburne University in 2017. She was acutely aware of the lack of women in STEMM fields, as both of her parents worked in biological sciences. “My father is a scientist, so I was aware that this was a field I could go into, and he would talk about amazing biologists like Barbara McClintock, but there was almost no representation of female scientists on TV or in newspapers,” she recalls.

“This colouring book will help children learn about the colourful lives and brilliant minds of these amazing women scientists. As a colouring book, it encourages creative minds to think about scientific problems - which is very much needed for problem solving”, says Isobel, who designed the book and illustrated each of the featured scientists. “These women, who made absolutely pioneering discoveries, used their creativity to advance the world as we know it.”

“I did intense research for the biographies of the women featured in the book and at every nook and crevice was amazed at the perseverance they showed. It is for them and countless others, unfortunately undocumented, that we can do what we do today,” says Debatri. “With Christmas approaching, this book is a perfect gift for young children who have a hunger for science. It’s both fun and educational!” she added.

Last year, both Isobel and Debatri were also selected to participate in Homeward Bound, a global program designed to provide cutting-edge leadership training to 1000 women in STEMM over 10 years. To raise awareness of climate change, this journey will also take Isobel and Debatri all the way to Earth’s frozen desert, Antarctica.

The initiative aims to heighten the influence and impact of women with a science background in order to influence policy and decision making as it shapes our planet. In 2017-18, OzGrav Chief Investigator Distinguished Prof Susan Scott was also selected to participate in this program and embark on the journey to Antarctica.

“The saying that ‘you can’t be what you can’t see’ is addressed in this new colouring book,” says Distinguished Prof Scott. “The important women scientists depicted in the book come to life as role models as they are coloured in. Women are underrepresented in physics education and work in Australia. Educating children about women scientists throughout history is an important step in encouraging more girls and women to take up STEMM careers and boost diversity.”

In their “day jobs”, Isobel tries to figure out how the collapsed remains of supergiant stars—black holes and neutron stars—meet up and crash together. She does this by studying the vibrations that these collisions send rippling through space-time—these are called gravitational waves. She also recently published an illustrated book, available on Amazon, called Planetymology: Why Uranus is not called George and other facts about space and words. Planetymology explains the ties between ancient history, astronomy, and language, and introduces the reader to the harsh realities of conditions on other planets.

Debatri is involved in doing simulations in supercomputers of dead stars in binaries or in massive collections of other stellar systems - called globular clusters. Her detailed theoretical calculations help us to understand the astrophysics behind the observations of gravitational waves and radio pulsars, as well as predict what surprising observations might be made in the future. Debatri is also a trained Indian classical dancer and was a voluntary crew member of the Melbourne-based tall ship `Enterprize’.  She has recently submitted her thesis and joined as a postdoctoral fellow at the Gravity Exploration Institute, Cardiff University.

GIVEAWAY: To celebrate the launch of Women in Physics, OzGrav is giving away free copies of the colouring book to three lucky winners. Simply share the book via Twitter (re-tweet) and tag @ARC_OzGrav to be in the draw. Winners will be announced Monday 20 December!

Gravitational waves are cosmic ripples in the fabric of space and time that emanate from catastrophics events in space, like collisions of black holes and neutron stars--the collapsed cores of massive supergiant stars. Extremely sensitive gravitational-wave detectors on Earth, like the Advanced LIGO and Virgo detectors, have successfully observed dozens of gravitational-wave signals, and they’ve also been used to search for dark matter: a hypothetical form of matter thought to account for approximately 85% of all matter in the Universe. Dark matter may be composed of particles that do not absorb, reflect, or emit light, so they cannot be detected by observing electromagnetic radiation. Dark matter is material that cannot be seen directly, but we know that dark matter exists because of the effect it has on objects that we can observe directly.

Ultralight boson particles are a new type of subatomic particle that scientists have put forward as compelling dark matter candidates. However, these ultralight particles are difficult to detect because they have extremely small mass and rarely interact with other matter -- which is one of the key properties that dark matter seems to have.

The detection of gravitational waves provides a new approach to detecting these extremely light boson particles using gravity. Scientists theorise that if there are certain ultralight boson particles near a rapidly spinning black hole, the extreme gravity field causes the particles to be trapped around the black hole, creating a cloud around the black hole. This phenomenon can generate gravitational waves over a very long lifetime. By searching for  these gravitational-wave signals, scientists can finally discover these elusive boson particles, if they do exist, and possibly crack the code of dark matter or rule out the existence of some types of the proposed particles. 

In a recent international study in the LIGO-Virgo-KAGRA collaboration, with OzGrav Associate Investigator Dr Lilli Sun from the Australian National University being one of the leading researchers, a team of scientists carried out the very first all-sky search tailored for these predicted gravitational wave signals from boson clouds around rapidly spinning black holes. 

“Gravitational-wave science opened a completely new window to study fundamental physics. It provides not only direct information about mysterious compact objects in the Universe, like black holes and neutron stars, but also allows us to look for new particles and dark matter,” says Dr Sun.
Although a signal was not detected, the team of researchers were able to draw valuable conclusions about the possible presence of these clouds in our Galaxy. In the analysis, they also took into consideration that the strength of a gravitational wave signal depends on the age of the boson cloud:  the boson cloud shrinks as it loses energy by sending out gravitational waves, so the strength of the gravitational wave signal would decrease as the cloud ages. 

“We learnt that a particular type of boson clouds younger than 1000 years is not likely to exist anywhere in our Galaxy, while such clouds that are up to 10 million years old are not likely to exist within about 3260 light-years from Earth,” says Dr Sun.
​
“Future gravitational wave detectors will certainly open more possibilities. We will be able to reach deeper into the Universe and discover more insights about these particles”.
Also featured on Cosmos Magazine and Sci Tech Daily.

​Many of the heaviest stars in the Universe will end their lives in a bright explosion, known as a supernova, which briefly outshines the rest of its host galaxy, allowing us to view these rare events out to great distances. At the lower end of this mass range, the supernova explosion will squeeze the core of the star into a dense ball of neutrons that is much denser than what can be reproduced in laboratories. So, scientists must rely on theoretical models and astronomical observations to study these objects, known as neutron stars.

At the very low end of this range, the supernova explosions are thought to be weaker and dimmer, but even for state-of-the-art supernova simulations, it’s challenging to test this hypothesis. In our recently published study, we found a new way to test these weaker supernovae: by associating weaker supernova explosions with slowly moving neutron star remnants, neutron star speeds could accurately estimate the weaker supernovae, without the need for expensive simulations.

Neutron stars don’t shine bright like other stars, but instead produce a very narrow beam of radio waves which may (if we’re lucky) point toward the Earth. As the neutron star rotates, the beam of light appears to flash on and off, creating a lighthouse effect. When this effect is observed, , we refer to it as a pulsating star, or pulsar. Recent advances in radio telescopes allow for precise measurements of pulsar velocities. We combined our measurements with simulations of millions of stars and found that the typically high pulsar speeds did not allow for many weak supernovae.

However, there is a caveat: many of the massive stars that produce neutron stars are born in stellar binaries. If a normal supernova occurs in a stellar binary, the neutron star remnant will experience a large recoil kick—like a cannonball rushing away from the exploding gunpowder—and it will likely eject away from its companion star where it may later be observed as a single pulsar. But if the supernova is weak, the neutron star may not have enough energy to escape the gravitational tug of its companion star, and the stellar binary system will remain intact. This is a necessary step in the formation of neutron star binaries, so the existence of these binaries proves that some supernova explosions must be weak.
​
We found that to explain both the existence of neutron star binaries and the absence of slow-moving pulsars, weak supernovae can only occur in very close stellar binaries, not in single, isolated stars. This is useful for modelling supernova simulations and adds to a growing body of research suggesting that weak supernovae may only happen in stellar binaries which have previously interacted with each other. Studies like this, which simulate many stars in relatively low detail, are key to understanding the effects of uncertain physics on stellar populations, which is unfeasible with highly-detailed simulations. 
 
Written by PhD student Reinhold Wilcox, Monash University

​With the growing catalogue of binary black hole mergers, researchers can study the overall spin properties of these systems to uncover how they formed and evolved. Recent work paints a conflicting picture of our understanding of the spin magnitudes and orientations of merging binary black holes, pointing to different formation scenarios. Our recent study, published in the Astrophysical Journal Letters, resolved these conflicts and allowed us to understand the spin distribution of binary black holes.
 
Forming black hole binaries

There are two main pathways to form a binary black hole: the first is via ‘isolated’ evolution, a process which involves the black hole binary being formed from the core collapse of two stars in a binary; the second is ‘dynamical’ evolution where interactions between black holes in dense stellar clusters can lead to a pair of black holes capturing each other to form a binary. These pathways show distinct features in the spin distribution of binary black hole mergers.
 
Binaries formed via isolated evolution tend to have spins that are closely aligned with the orbital angular momentum, whereas dynamically formed systems have spins that are randomly orientated and have a distribution of spin tilts that is isotropic. In the latest population study from LIGO-Virgo, we saw evidence for both of these channels, however a more recent study by Roulet et. al 2021, showed that the population was consistent with the isolated channel alone.
 
This inconsistency raises the question: how can we obtain different conclusions from the same population? The answer is model misspecification: The previous spin models were not designed to capture possible sharp features or sub-populations of spin in the model.

The emerging picture of the spins of black hole binaries

​Using a catalogue of 44 binary black hole mergers, this new study finds evidence for two populations within the spin distribution of black hole binaries: one with negligible spins and the other moderately spinning with preferential alignment with the orbital angular momentum.
 
This result can be fully explained via the isolated formation scenario. The progenitors of most black holes lose their angular momentum when the stellar envelope is removed by the binary companion, forming black hole binaries with negligible spin, while a small fraction of binaries have the second-born black hole spun up via tidal interactions.This study opens a number of interesting avenues to explore, for example, an investigation of the relationship between the mass and spin of these different subpopulations. Investigating such correlations can help improve the accuracy of our models and enable us to better distinguish between different evolution pathways of binary black holes.
 
Written by OzGrav PhD student Shanika Galaudage, Monash University
 
Recently published in ApJL https://iopscience.iop.org/article/10.3847/2041-8213/ac2f3c

​Key points:

• An international team of scientists, including Australian researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), have collaborated on a study released today, presenting the largest number of gravitational wave detections to date - 90 detections!

 

• Gravitational waves are cosmic ripples in space and time that are caused by some of the most violent and energetic processes in the Universe, like supernovas, merging black holes and colliding neutron stars--city-size stellar objects with a mass about 1.4 times that of the Sun.

 

• The newest gravitational wave detections come from the second part of the third observing run which lasted from November 2019 to March 2020. There were 35 new gravitational wave detections in this period: 32 detections were from pairs of merging black holes; 3 were likely to come from the collision of a neutron star and a black hole.

 
The gravitational-wave Universe is teeming with signals produced by merging black holes and neutron stars. In a new paper released today, an international team of scientists, including Australian OzGrav researchers, present 35 new gravitational wave observations, bringing the total number of detections to 90!
 
All of these new observations come from the second part of observing run three, called “O3b”, which was an observing period that lasted from November 2019 to March 2020. There were 35 new gravitational wave detections in this period. Of these, 32 are most likely to come from pairs of merging black holes, 2 are likely to come from a neutron star merging with a black hole, and the final event could be either a pair of merging black holes or a neutron star and a black hole. The mass of the lighter object in this final event crosses the divide between the expected masses of black holes and neutron stars and remains a mystery.
 
Dr Hannah Middleton, postdoctoral researcher at OzGrav, University of Melbourne, and co-author on the study says “Each new observing run brings new discoveries and surprises. The third observing run saw gravitational wave detection becoming an everyday thing, but I still think each detection is exciting!".
 
Highlights
 
Of these 35 new events, here are some notable discoveries (the numbers in the names are the date and time of the observation):

• Two mergers between possible neutron star - black hole pairs. These are called GW191219_163120 and GW200115_042309, the latter of which was previously reported in its own publication. The neutron star in GW191219_163120 is one of the least massive ever observed.
• A merger between a black hole and an object which could either be a light black hole or a heavy neutron star called GW200210_092254
• A massive pair of black holes orbiting each other, with a combined mass 145 times heavier than the Sun (called GW200220_061928)
• A pair of black holes orbiting each other, in which at least one of the pair is spinning upright (called GW191204_171526)
• A pair of black holes orbiting each other which have a combined mass 112 times heavier than the Sun, which seems to be spinning upside-down (called GW191109_010717)
• A ‘light’ pair of black holes that together weigh only 18 times the mass of the Sun (called GW191129_134029)

The different properties of the detected black holes and neutron stars are important clues as to how massive stars live and then die in supernova explosions.
“It’s fascinating that there is such a wide range of properties within this growing collection of black hole and neutron star pairs”, says study co-author and OzGrav PhD student Isobel Romero-Shaw (Monash University). “Properties like the masses and spins of these pairs can tell us how they’re forming, so seeing such a diverse mix raises interesting questions about where they came from.”
 
Not only can scientists look at individual properties of these binary pairs, they can also study these cosmic events as a large collection - or population. “By studying these populations of black holes and neutron stars we can start to understand the overall trends and properties of these extreme objects and uncover how these pairs came to be” says OzGrav PhD student Shanika Galaudage (Monash University) who was a co-author on a companion publication released today: ‘The population of merging compact binaries inferred using gravitational waves through GWTC-3 P2100239’. In this work, scientists analysed the distributions of mass and spin and looked for features which relate to how and where these extreme object pairs form. Shanika adds, “There are features we are seeing in these distributions which we cannot explain yet, opening up exciting research questions to be explored in the future”.
 
 
Detecting gravitational waves: a complicated global effort
 
Detecting and analysing gravitational-wave signals is a complicated task requiring global efforts. Initial public alerts for possible detections are typically released within a few minutes of the observation. Rapid public alerts are an important way of sharing information with the wider astronomy community, so that telescopes and electromagnetic observatories can be used to search for light from merging events - for example, merging neutron stars can produce detectable light. 
 
Says Dr Aaron Jones, co-author and postdoctoral researcher from The University of Western Australia, “It’s exciting to see 18 of those initial public alerts upgraded to confident gravitational wave events, along with 17 new events”.
 
After thorough and careful data analysis, scientists then decipher the shortlist of gravitational-wave detections, delving into the properties of the systems that produced these signals. They use parameter estimation, a statistical technique to learn information about the black holes and neutron stars, such as their masses and spins, their location on the sky and their distance from the Earth.
 
Future detections
 
All of these detections were made possible by the global coordinated efforts from the LIGO (USA), Virgo (Italy) and KAGRA (Japan) gravitational-wave observatories.
 
Between the previous observing runs, the detectors have been continually enhanced in small bursts which improves their overall sensitivity. Says Disha Kapasi, OzGrav student (Australian National University), “Upgrades to the detectors, in particular squeezing and the laser power, have allowed us to detect more binary merger events per year, including the first ever neutron star-black hole binary recorded in the GWTC-3 catalogue. This aids in understanding the dynamics and physics of the immediate universe, and in this exciting era of gravitational wave astronomy, we are constantly testing and prototyping technologies that will help us make the instruments more sensitive.”
 
The LIGO and Virgo observatories are currently offline for improvements before the upcoming fourth observing run (O4), due to begin in August 2022 or later. The KAGRA observatory will also join O4 for the full run. More detectors in the network help scientists to better localise the origin or potential sources of the gravitational waves.
 
“As we continue to observe more gravitational-wave signals, we will learn more and more about the objects that produce them, their properties as a population, and continue to put Einstein’s theory of General Relativity to the test,” says Dr Middleton.
 
There is a lot to look forward to from gravitational-wave astronomy in O4 and beyond. But in the meantime, scientists will continue to analyse and learn from the data, searching for undiscovered types of gravitational waves, including continuous gravitational waves, and of course new surprises! 

​Our Universe shines bright with light across the electromagnetic spectrum. While most of this light comes from stars like our Sun in galaxies like our own, we are often treated with brief and bright flashes that outshine entire galaxies themselves. Some of these brightest flashes are believed to be produced in cataclysmic events, such as the death of massive stars or the collision of two stellar corpses known as neutron stars. Researchers have long studied these bright flashes or ‘transients’ to gain insight into the deaths and afterlives of stars and the evolution of our Universe.
 
Astronomers are sometimes greeted with transients that defy expectations and puzzle theorists who have long predicted how various transients should look. In October 2014, a long-term monitoring programme of the southern sky with the Chandra telescope—NASA’s flagship X-Ray telescope—detected one such enigmatic transient called CDF-S XT1: a bright transient lasting a few thousands of seconds. The amount of energy CDF-S XT1 released in X-rays was comparable to the amount of energy the Sun emits over a billion years. Ever since the original discovery, astrophysicists have come up with many hypotheses to explain this transient; however, none have been conclusive.
 
In a recent study, a team of astrophysicists led by OzGrav postdoctoral fellow Dr Nikhil Sarin (Monash University) found that the observations of CDF-S XT1 match predictions of radiation expected from a a high-speed jet travelling close to the speed of light. Such “outflows” can only be produced in extreme astrophysical conditions, such as the disruption of a star as it gets torn apart by a massive black hole, the collapse of a massive star, or the collision of two neutron stars.
 
Sarin et al’s study found that the outflow from CDF-S XT1 was likely produced by two neutron stars merging together. This insight makes CDF-S XT1 similar to the momentous 2017 discovery called GW170817—the first observation of gravitational-waves, cosmic ripples in the fabric of space and time—although CDF-S XT1 is 450 times further away from Earth. This huge distance means that this merger happened very early in the history of the Universe; it may also be one of the furthest neutron star mergers ever observed.
 
Neutron star collisions are the main places in the Universe where heavy elements such as gold, silver, and plutonium are created. Since CDF-S XT1 occurred early on in the history of the Universe, this discovery advances our understanding of Earth’s chemical abundance and elements.
 
Recent observations of another transient AT2020blt in January 2020—primarily with the Zwicky Transient Facility—have puzzled astronomers. This transient’s light is like the radiation from high-speed outflows launched during the collapse of a massive star. Such outflows typically produce higher energy gamma-rays; however, they were missing from the data – they were not observed. These gamma rays can only be missing due to one of three possible reasons: 1) The gamma-rays were not produced. 2) The gamma rays were directed away from Earth. 3) The gamma-rays were too weak to be seen.
 
In a separate study, led again by OzGrav researcher Dr Sarin, the Monash University astrophysicists teamed up with researchers in Alabama, Louisiana, Portsmouth and Leicester to show that AT2020blt probably did produce gamma-rays pointed towards Earth, they were just really weak and missed by our current instruments.
 
Dr Sarin says: “Together with other similar transient observations, this interpretation means that we are now starting to understand the enigmatic problem of how gamma-rays are produced in cataclysmic explosions throughout the Universe”.
 
The class of bright transients collectively known as gamma-ray bursts, including CDF-S XT1, AT2020blt, and AT2021any, produce enough energy to outshine entire galaxies in just one second.
 
“Despite this, the precise mechanism that produces the high-energy radiation we detect from the other side of the Universe is not known,” explains Dr Sarin. “These two studies have explored some of the most extreme gamma-ray bursts ever detected. With further research, we’ll finally be able to answer the question we’ve pondered for decades: How do gamma ray bursts work?” 

On 17th August 2017, LIGO detected gravitational waves from the merger of two neutron stars. This merger radiated energy across the electromagnetic spectrum, light that we can still observe today. Neutron stars are incredibly dense objects with masses larger than our Sun confined to the size of a small city. These extreme conditions make some consider neutron stars the caviar of astrophysical objects, enabling researchers to study gravity and matter in conditions unlike any other in the Universe.
 
The momentous 2017 discovery connected several pieces of the puzzle on what happens
during and after the merger. However, one piece remains elusive: What remains behind after
the merger?
 
In a recent article published in General Relativity and Gravitation, Nikhil Sarin
and Paul Lasky, two OzGrav researchers from Monash University, review our understanding of the aftermath of binary neutron star mergers. In particular, they examine the different outcomes and their observational signatures.
 
The fate of a remnant is dictated by the mass of the two merging neutron stars and the maximum mass a neutron star can support before it collapses to form a black hole. This mass threshold is currently unknown and depends on how nuclear matter behaves in these extreme conditions. If the remnant's mass is smaller than this mass threshold, then the remnant is a neutron star that will live indefinitely, producing electromagnetic and gravitational-wave radiation. However, if the remnant is more massive than the maximum mass threshold, there are two possibilities: if the remnant mass is up to 20% more than the maximum mass threshold, it survives as a neutron star for hundreds to thousands of seconds before collapsing into a black hole. Heavier remnants will survive less than a second before collapsing to form black holes.
 
Observations of other neutron stars in our Galaxy and several constraints on the behaviour of
nuclear matter suggest that the maximum mass threshold for a neutron star to avoid collapsing into a black hole is likely around 2.3 times the mass of our Sun. If correct, this
threshold implies that many binary neutron star mergers go on to form more massive neutron
star remnants which survive for at least some time. Understanding how these objects behave
and evolve will provide a myriad of insights into the behaviour of nuclear matter and the
afterlives of stars more massive than our Sun.
 
Written by PhD student Nikhil Sarin, University of Adelaide

Binary neutron stars have been detected in the Milky Way as millisecond pulsars and twice outside the galaxy via gravitational-wave emission. Most of them have orbital periods of less than a day—a contrasting difference to their progenitors: massive stellar binaries that have hundreds or thousands of days orbital periods. In the last several decades, there has been much debate about explaining how massive binaries transition to double compact objects. To date, one of the strong contenders to explain this transition is the highly-complex stage of binary stellar evolution known as the common-envelope phase.
 
The common-envelope phase is a particular outcome of a mass transfer episode. It begins with the Roche-lobe overflow of (at least) one of the stars, and it’s prompted by a dynamical instability. In a simple version, the stellar envelope of the mass-transferring star—the donor—bloats and engulfs the whole binary, creating a new system comprised of an inner compact binary, and a shared “common” envelope. The interaction of the inner binary with the common envelope results in drag, and the dissipated gravitational energy is transferred onto the common envelope, which can lead to its ejection. A successful ejection suggests that a compact binary can form. But what does a “successful ejection” mean?
 
To explore the common-envelope phase with three-dimensional hydrodynamical models, we attempted to address the likely outcomes of common-envelope evolution by considering the response of a one-dimensional stellar model to envelope removal. In a recent study, we focussed on the common-envelope phase scenario of a donor star with a neutron star companion. We emulated the common-envelope phase by removing the envelope of the donor star, either partially or completely. After the star was stripped, we followed its radial evolution. The most extreme scenarios resulted as expected: If you remove all the envelope, the stripped star remains compact. Alternatively, if you leave most of the envelope, the stripped star subsequently expands a lot. The question is: what happens in between the extreme cases?
 
Our research shows that when most of the envelope, but not all of it, is removed, the star experiences a short phase of marginal contraction (<100 years), but overall, the star remains compact during the next 1000 years. This suggests that a star doesn’t needs to be stripped all the way to the core to avoid an imminent stellar merger. Moreover, the amount of energy needed to partially strip the envelope is less than the one needed to fully remove it. Finally, it’s reassuring that our results show a strong correlation to variations in donor mass and composition.
 
This research is a step forward in the understanding of the common envelope phase and the formation of double neutron star binaries. Our results imply that a star can be stripped without experiencing Roche lobe overflow immediately after the common envelope, a likely condition for a successful envelope ejection. It also suggests that stripped stars retain a few solar masses of peculiar, hydrogen-poor material in their surface. While this amount of hydrogen is not excessive, it might be observable in the spectra of a star and can play a role at the end of its life when it explodes into a supernova. While the full understanding of the common-envelope phase remains elusive, we are connecting the dots of the evolution and fate of systems that have experienced a common-envelope event.
 
Written by OzGrav research Alejandro Vigna-Gómez from the Niels Bohr Institute (University of Copenhagen)

​​Short gamma-ray bursts are extremely bright bursts of high-energy light that last for a couple of seconds. In many of these bursts, there is a mysterious material left behind: a prolonged ‘afterglow’ of radiation, including X-rays. Despite the efforts of many scientists over many years, we still don’t know where this afterglow comes from.
 
In our recently accepted paper, we investigated a simple model that proposes a rotating neutron star—an extremely dense collapsed core of a massive supergiant star—as the engine behind a type of lengthy X-ray afterglows, known as X-ray plateaux. Using a sample of six short gamma-ray bursts with an X-ray plateau, we worked out the properties of the central neutron star and the mysterious remnant surrounding it.

The model we used was inspired by remnants from young supernova. While remnants from short gamma-ray bursts and supernovae have many differences, the energy driving from a rotating neutron star has the same underlying physics. So, if the remnant of a short gamma-ray burst is a neutron star, it must have a similar energy outflow as a supernova remnant.
 
In our study, we borrowed the basic physics from previous short gamma-ray burst models to predict the luminosity and duration of the X-ray plateau. For each short gamma-ray burst, the results suggested that the remnant neutron star is a millisecond magnetar: a neutron star with an extraordinarily powerful magnetic field. All known magnetars have a very slow rotation frequency; similarly, all observed neutron stars with millisecond spins have weak magnetic fields. This gap in observations isn’t surprising because the magnetic field of the star converts the rotational energy into electromagnetic energy. For a magnetar-strength field, this process happens on a scale from seconds to days – exactly the duration of most X-ray plateaux.

This paper is the first attempt at estimating the source of X-ray afterglows using this kind of model. As the model matures and further data is collected, we’ll be able to make stronger conclusions about the source of X-ray plateaux and, if we’re lucky, discover what these mysterious remnants are.
 
Written by OzGrav PhD student Lucy Strang – The University of Melbourne
 
Direct link to online publications, a journal citation or any other websites
https://arxiv.org/pdf/2107.13787.pdf

Current status of paper (submitted/accepted/published)
Accepted

Acceptance and/or publication date
Accepted 27/07/21, publication TBC

In the 2030’s, gravitational-wave detectors will be thousands of times more sensitive than Advanced LIGO, Virgo, and KAGRA. The network of “third generation” (3G) observatories will almost certainly include Cosmic Explorer (US), Einstein Telescope (EU), and may include a Southern-hemisphere Cosmic-Explorer like observatory. These amazing instruments will see every binary neutron star merger in the Universe, and most binary black holes out to redshifts beyond 10: hundreds of thousands, possibly millions, of resolvable signals per year. Many of these signals will be extremely loud, with signal to noise ratios in the thousands, facilitating breakthroughs in fundamental physics and cosmology. And herein lies a challenge! How do we extract all the information from these signals? On the surface it seems like a straightforward task: just keep on running parameter estimation like we’re already doing! But it turns out that our current parameter estimation methods don’t scale so well when signals are really loud, and very long in band. 
 
To see why, we imagined a binary neutron star merger signal “GW370817”, which originated about 40 Mpc from Earth — roughly the distance of GW170817 (assuming 3G detectors are online in 2037, we’re guaranteed to observe a thousand or so binary neutron star mergers on August 17th, 2037!). A network of 3G detectors would observe GW370817 for 90 minutes, with a staggering signal to noise ratio of 2500. Analysing this signal is around a thousand times more computationally expensive than analysing a signal in today’s detectors — by our back of the envelope estimates, it would take around 1000 years! This prohibitive analysis time is a hurdle to astrophysics with 3G data, and it’s the problem we solve in our paper. To drive down the computation time, we developed “reduced order models” of gravitational-wave signals which allow us to infer binary neutron star properties using heavily compressed data, with almost no loss in accuracy. We reduced the computational cost of inference on 3G data by a factor of 13’000. Together with a pinch of parallel computing, we’re able to perform data analysis in a few hours. This is good news for astrophysics in the 3G era.
 
While the 2030’s and 3G detectors are a few years away, our results and methods are useful for a wide range of theoretical and design studies, which are ramping up in lockstep with the development of the detector technology. For those old enough to remember, the first LISA mock-data challenges began in 2005, which gives a sense of how much exploratory work takes place before a detector is operational. For the time being, there are plenty of interesting astrophysics questions we can start to think about in the context of 3G detectors: how well will we be able to measure the neutron star equation of state and the maximum mass of neutron stars? And what will this tell us about extreme matter? How well can neutron star spins be measured and can this tell us anything about supernova mechanisms? etc…Our results and method will facilitate this kind of theoretical work by enabling us to perform robust inferences on binary neutron star properties in mock 3G data. 
 
Link to research paper: https://arxiv.org/abs/2103.12274 
Written by Rory Smith, Monash University