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?”
Schematic representation of binary neutron star merger outcomes. Panels A and B: Two neutron stars merge as the emission of gravitational waves drives them towards one another. C: If the remnant mass is above a certain mass, it immediately forms a black hole. D: Alternatively, it forms a quasistable ‘hypermassive’ neutron star. E: As the hypermassive star spins down and cools it can not support itself against gravitational collapse and collapses into a black hole. F, G: If the remnant’s mass is sufficiently low, it will survive for longer, as a ‘supramassive’ neutron star, supported against collapse through additional support against gravity through rotation, collapsing into a black hole once it loses this support. H: If the remnant is born with small enough mass, it will survive indefinitely as a neutron star. Schematic from Sarin & Lasky 2021. Image credit: Carl Knox (Swinburne University).
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
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
Current status of paper (submitted/accepted/published)
Acceptance and/or publication date
Accepted 27/07/21, publication TBC
RESEARCH HIGHLIGHT: Bayesian inference for gravitational waves from binary neutron star mergers in third-generation observatories
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
Galaxies host supermassive black holes, which weigh millions to billions times more than our Sun. When galaxies collide, pairs of supermassive black holes at their centres also lie on the collision course. It may take millions of years before two black holes slam into each other. When the distance between them is small enough, the black hole binary starts to produce ripples in space-time, which are called gravitational waves.
Gravitational waves were first observed in 2015, but they were detected from much smaller black holes, which weigh like tens times our Sun. Gravitational waves from supermassive black holes are still a mystery to scientists. Their discovery would be invaluable to figuring out how galaxies and stars form and evolve, and finding the origin of dark matter.
A recent study led by Dr Boris Goncharov and Prof Ryan Shannon—both researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)—has tried to solve this puzzle. Using the most recent data from the Australian experiment known as the Parkes Pulsar Timing Array, the team of scientists searched for these mystery gravitational waves from supermassive black holes.
The experiment observed radio pulsars: extremely dense collapsed cores of massive supergiant stars (called neutron stars) that pulse out radio waves, like a lighthouse beam. The timing of these pulses is extremely precise, whereas the background of gravitational waves advances and delays pulse arrival times in a predicted pattern across the sky, by around the same amount in all pulsars. The researchers now found that arrival times of these radio waves do show deviations with similar properties as we expect from gravitational waves However, more data is needed to conclude whether radio wave arrival times are correlated in all pulsars across the sky, which is considered the “smoking gun”. Similar results have also been obtained by collaborations based in North America and Europe. These collaborations, along with groups based in India, China, and South Africa, are actively combining datasets under the International Pulsar Timing Array, to improve the sky coverage.
This discovery is considered a precursor to the detection of gravitational waves from supermassive blackholes. However, Dr Goncharov and colleagues pointed out that the observed variations in the radio wave arrival times might also be due to ipulsar-intrinsic noise. Dr Goncharov said: “To find out if the observed "common" drift has a gravitational wave origin, or if the gravitational-wave signal is deeper in the noise, we must continue working with new data from a growing number of pulsar timing arrays across the world”.
Making waves at the museum: The interactive science exhibit based on a real-life gravitational-wave detector
modeled of a real-life gravitational-wave detector to explain gravitational-wave science. It was developed by an international team, which includes researchers now at the OzGrav ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).
The recently published research paper is now featured in the American Journal of Physics and the exhibit, which is called a Michelson interferometer, is on long-term display at the Thinktank Birmingham Science Museum in the UK. The project has a lasting international impact with online instructions and parts lists available for others to construct their own versions of the exhibit.
Observations of gravitational waves -- ripples in the fabric of space and time -- have sparked increased public interest in this area of research. The effect of gravitational waves is a stretching and squashing of distances between objects. Real-life observatories are large complex devices based on the Michelson interferometer that use laser light to search for passing gravitational waves.
In a Michelson interferometer, laser light is split into two perpendicular beams by a beam-splitter; the beams of laser light travelling down the detector arms reflect off mirrors back to the beam-splitter where they recombine and produce an interference pattern. If the relative length of the arms changes, the interference pattern will change. The exhibit model cannot detect gravitational waves, but it’s extremely sensitive to vibrations in the room!
The Michelson interferometer exhibit has an attractive high-shine design, using lab-grade optics and custom-made components, drawing people in to take a closer look. A list of all the parts used in the intricate design is available on the official website -- the creators are continuing to investigate low-cost designs using laser pointers and building blocks.
At science fairs, experts are normally present to explain the items on display; however, this is not the case in a museum. ‘Exhibits need to be easily accessible with self-guided learning,’ explains OzGrav postdoc Dr Hannah Middleton, one of the project leads from the University of Melbourne.
‘We’ve developed custom interactive software for the exhibit through which a user can access explanatory videos, animations, images, text, and a quiz. Users can also directly interact with the interferometer by pressing buttons to input a simulated gravitational wave, and produce a visible change in the interference pattern.’
Research highlight: Optical observations of the BepiColombo spacecraft as a proxy for a potential threatening asteroid
BepiColombo is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) designed to study the planet Mercury. Launched in late 2018, its complex trajectory involved a fly-by past Earth on April 10, 2020. We took advantage of the event to organise a coordinated observing campaign. The main goal was to compute and compare the observed fly-by orbit properties with the values available from the Mission Control. The method we designed could then be improved for future observation campaigns targeting natural objects that may collide our planet.
The incoming trajectory of the probe limited the ground-based observability to only a few hours, around the time when it was closest to Earth. The network of telescopes we used has been developed by ESA’s NEO Coordination Centre (NEOCC) with capabilities to quickly observe imminent impactors, thus presenting similar orbits. Our team successfully acquired the target with various instruments such as the 6ROADS Chilean telescope, the 1.0 m Zadko telescope in Australia, the ISON network of telescopes, and the 1.2 m Kryoneri telescope in Corinthia, Greece.
The observations were difficult due to the object’s extremely fast angular motion in the sky. At one point, the telescopes saw the probe covering twice the size of the moon in the sky each minute. This challenged the tracking capabilities and timing accuracy of the telescopes. Each telescope was moving at the predicted instantaneous speed of the target while taking images, "tracking" the spacecraft. Field stars appeared as trails, while BepiColombo itself was a point source, but only if the observation started exactly at the right moment. Because the probe was moving so fast, any date errors of the telescope images translate into position errors of the probe. To reach a precise measurement of 0.1 metres, the date of the images needed to have a precision of 100 milliseconds.
The final results were condensed into two measurable quantities that could be directly compared with the Mission Control ones, the perigee distance, and the time of the probe’s closest approach to Earth. Both numbers were perfectly matched, proving our method a success: it calculated a more accurate prediction of BepiColombo’s orbit; it also provided valuable insights for future observations of objects colliding with Earth:
• A purely optical observing campaign can provide trajectory information during a fly-by at sub-kilometre and sub-second levels of precision.
• A similar campaign would lead to a sub-kilometre and sub-second precision for the time and location of the atmospheric entry of any colliding object.
• Timing accuracy below 100 milliseconds is crucial for the closest observations.
• It’s possible to organise astrometric campaigns with coverage from nearly every continent.
Link to research paper: https://doi.org/10.1016/j.actaastro.2021.04.022
Written by OzGrav researcher Dr Bruce Gendre, University of Western Australia.
From gravitational wave science to global technology company: Liquid Instruments is a Canberra start-up bringing NASA technology to the world.
Liquid Instruments (LI) Pty Ltd, a spin-off company from the Australian National University (ANU), is revolutionising the $17b test and measurement market. Test and Measurement devices are used by scientists and engineers to measure, generate and process the electronic signals that are fundamental to the photonics, semiconductor, aerospace and automotive industries. The LI team has raised more than $25M USD in Venture Capital investment, and now has more than 1000 users in 30 countries.
LI was founded by researchers from the gravitational wave group at ANU to commercialise advanced instrumentation technology derived from both ground and space-based gravity detectors. OzGrav Chief Investigator Daniel Shaddock (ANU), CEO of Liquid Instruments, began as an engineer at NASA’s Jet Propulsion Laboratory in 2002, working on the Laser Interferometer Space Antenna (LISA), a joint project between NASA and the European Space Agency. The work on LISA’s phasemeter was the genesis for forming Liquid Instruments.
LI’s software-enabled hardware employs advanced digital signal processing to replace multiple pieces of conventional equipment at a fraction of the cost and with a drastically improved user experience. Their first product Moku:Lab provides the functionality of 12 instruments in one simple integrated unit.
On 23 June, the company launched two new hardware devices, the Moku:Go—an engineering lab in a backpack for education, and the Moku:Pro – a multi-GHz device for professional scientists and engineers. Lik the Moku:Lab, this revolutionary new hardware includes a suite of instruments with robust hardware features giving a breakthrough combination of performance and versatility.
Daniel Shaddock says: “Moku:Pro takes software defined instrumentation to the next level with more than 10x improvements in many dimensions. Moku:Pro is a new weapon for scientists. Moku:Go takes all the great features of Moku:Lab but reduces the cost by 10x to make it more accessible than ever before. We hope it will help train the next generation of scientists and engineers in universities around the world.
‘Type Ia’ supernovae involve an exploding white dwarf close to its Chandrasekhar mass. For this reason, type Ia supernova explosions have almost universal properties and are an excellent tool to estimate the distance to the explosion, like a cosmic distance ladder. Collapsing massive stars will form a different kind of supernova (type II) with more variable properties, but with comparable peak luminosities.
To date, the most luminous events occur in core-collapse supernovae in a gaseous environment, when the circumstellar medium near the explosion transforms the kinetic energy into radiation and thus increases the luminosity. The origin of the circumstellar material is usually the stellar wind from the massive star’s outer layers as they’re expelled prior to the explosion.
A natural question is how will type Ia supernovae look like in a dense gaseous environment? And what is the origin of the circumstellar medium in this case? Will they also be more luminous than their standard siblings? To address this question, OzGrav researchers Evgeni Grishin, Ryosuke Hirai, and Ilya Mandel, together with an international team of scientists, studied explosions in dense accretion discs around the central regions of active galactic nuclei. They constructed an analytical model which yields the peak luminosity and lightcurve for various initial conditions, such as the accretion disc properties, the mass of the supermassive black hole, and the location and internal properties of the explosion (e.g. initial energy, ejecta mass). The model also used suites of state-of-the-art radiation hydrodynamical simulations.
The explosion generates a shock wave within the circumstellar medium, which gradually propagates outward. Eventually, the shock wave reaches a shell that is optically thin enough, such that the photons can ‘breakout’. The location of this breakout shell and the duration of the photon diffusion determine the lightcurve properties.
If the amount of the circumstellar medium is much smaller than the ejecta mass, the lightcurves look very similar to type Ia supernovae. Conversely, a very massive circumstellar mass can choke the explosion and it will not be seen. The sweet spot lies somewhere in between, where the ejecta mass is roughly comparable to the amount of circumstellar material. In the latter case, the peak luminosity 100 times bigger than the standard type Ia Supernovae, which makes it one of the brightest supernova events to date.
The research paper describing this work (Grishin et al., “Supernova explosions in active galactic nuclear discs”) was recently published in Monthly Notices of the Royal Astronomical Society. The luminous explosions may be observed in accretion discs of accretion rate, or in galaxies with smaller supermassive black hole masses where background active galactic nucleus activity will not hinder observations with advanced instruments.
The underlying physical processes of photon diffusion and shock breakout can be creatively explained with poetry:
All of a sudden, the heat is intense.
We must cool down, but the path is opaque.
Every direction around is so dense,
Which one should the photons take?
They have to break out, for God's sake...
At first, they are stuck, no matter the way,
They sway side to side, they randomly walk.
The leader in front leads them astray,
How hogtied is this radiant flock...
But wait, do you also gaze at the shock?
The ominous furnace is starting to snap,
Its violent grip getting frail.
The path is now clear, the direction is "up!"
We're sitting on the shock front's tail.
We're seizing the shock, we'll prevail!
The shock front behind us, but we're still out of place,
We propel with incredible might.
We keep on ascending, increasing the pace,
Any particle is now out of sight,
In this vacuum, we're free from inside,
And can travel as fast
as the light.
Written by OzGrav researcher Evgeni Grishin, Monash University