An artist's impression of gravitational waves generated by binary neutron stars.
Credits: R. Hurt/Caltech-JPL
Young single pulsars are observed to move in the sky at speeds of many hundreds of kilometres per second. These high speeds are imparted by asymmetries in the supernova explosions that give birth to the neutron stars. Measuring the distribution of these birth kicks is important for understanding supernova explosions. It is also necessary to explain how neutron stars are retained in clusters with escape velocities of only a few tens of kilometres per second, and for predicting how often neutron star birth kicks will disrupt binaries, flinging out a newly born neutron star. The latter is particularly relevant for the formation of neutron star binaries that can be observed in radio waves, X-rays, or, if merging with another neutron star or black hole, as gravitational waves.
We can generally measure only two components of a pulsar’s motion: the projection onto the plane of the sky. This is done by multiplying the proper motion by the distance to the pulsar. The third component of the motion, along the radial direction connecting the Earth and the pulsar, cannot be measured directly.
The total speed is generally inferred by assuming that the radial component is not special: that, on average, its magnitude samples the same distribution as the two observed velocity components. However, in a paper published in the Astrophysical Journal in 2023 (ApJ 944, 153), OzGrav CI Ilya Mandel (Monash) and collaborator Andrei Igoshev (Leeds) argued that this is not the case, and the radial motion direction can indeed be special.
This paper, entitled "The impact of spin-kick alignment on the inferred velocity distribution of isolated pulsars”, points out that if pulsar kick direction is preferentially aligned with the pulsar rotational (spin) axis, then the very detectability of the pulsar — which requires that the beam of the pulsar sometimes, but not always, sweeps past our radio telescopes on Earth — creates a special direction.
Consider, for example, a pulsar that is emitting two narrow beams of radiation at 90 degrees to its spin axis. This pulsar could only be detected by an observer located in the pulsar’s equatorial plane. Suppose that the pulsar's rotation axis is perfectly aligned with the spin axis. In that case, the pulsar has no radial velocity component: the projected 2-dimensional velocity on the plane of the sky represents the full pulsar speed. Alternatively, if we imagined that the 2-dimensional velocity we see was a random projection of the full velocity, we would systematically over-estimate the pulsar’s speed by a factor of sqrt(3/2).
The exact level of such a bias depends on the degree of misalignment between the pulsar spin and its radio beams, the size of these beams, and the level of kick-spin alignment. While some of these quantities are uncertain, Mandel & Igoshev conclude that pulsar velocities may be over-estimated by up to ~15% by methods that don’t account for this systematic bias.
Written by OzGrav Chief Investigator Ilya Mandel and Andrei P. Igoshev, University of Leeds.
Congratulations to Distinguished Professor Susan Scott (ANU) on being awarded the Thomas Ranken Lyle Medal in 2023. Professor Scott is an internationally recognised mathematical physicist who has made fundamental advances in our understanding of the fabric of space-time in general relativity, and in gravitational wave science.
The Thomas Ranken Lyle Medal is a career award to recognise outstanding achievement by a scientist in Australia for research in mathematics or physics. It is awarded by the Australian Academy of Science every 2 years. This is the second time running an OzGrav Professor has won this award for their work on gravitational waves, with Distinguished Professor David McClelland (ANU) being awarded in 2021 for his contributions to quantum squeezing and gravitational wave discovery.
Read about more 2023 winners - Decoding dragons and devils, what triggers volcanoes, and more from Australia's stars of science.
Photo credits: Tracey Nearmy (ANU) and Australian Academy of Science.
Black hole carnivals may produce the signals seen by gravitational-wave detectors
Since 2015, the LIGO-Virgo-KAGRA Collaboration have detected about 85 pairs of black holes crashing into each other. We now know that Einstein was right: gravitational waves are generated by these systems as they inspiral around each other, distorting space-time with their colossal masses as they go. We also know that these cosmic crashes happen frequently: as detector sensitivity improves, we are expecting to sense these events on a near-daily basis in the next observing run, starting in 2023. What we do not know — yet — is what causes these collisions to happen.
Black holes form when massive stars die. Typically, this death is violent, an extreme burst of energy that would either destroy or push away nearby objects. It is therefore difficult to form two black holes that are close enough together to merge within the age of the Universe. One way to get them to merge is to push them together within densely populated environments, like the centres of star clusters.
In star clusters, black holes that start out very far apart can be pushed together via two mechanisms. Firstly, there’s mass segregation, which leads the most massive objects to sink towards the middle of the gravitational potential well. This means that any black holes dispersed throughout the cluster should wind up in the middle, forming an invisible “dark core”. Secondly, there are dynamical interactions. If two black holes pair up in the cluster, their interactions can be influenced by the gravitational influence of nearby objects. These influences can remove orbital energy from the binary and push it closer together.
The mass segregation and dynamical interactions that can take place in star clusters can leave their fingerprints on the properties of merging binaries. One key property is the shape of the binary’s orbit just before it merged. Since mergers in star clusters can happen very quickly, the orbital shapes can be quite elongated — less like the calm, sedate circle that the Earth traces around the Sun, and more like the squished ellipse that Halley’s Comet races along in its visits in and out of the Solar System. When two black holes are in such an elongated orbit, their gravitational wave signal has characteristic modulations, and can be studied for clues to where the two objects met.
A team of OzGrav researchers and alumni are working together to study the orbital shapes of black hole binaries. The group, led by Dr Isobel Romero-Shaw (formerly of Monash University, now based at the University of Cambridge) together with Professors Paul Lasky and Eric Thrane of Monash University, have found that some of the binaries observed by the LIGO-Virgo-KAGRA collaboration are indeed likely to have elongated orbits, indicating that they may have collided in a densely populated star cluster. Their findings indicate that a large chunk of the observed binary black hole collisions — at least 35% — could have been forged in star clusters.
“I like to think of black hole binaries like dance partners”, explains Dr Romero-Shaw. “When a pair of black holes evolve together in isolation, they’re like a couple performing a slow waltz alone in the ballroom. It’s very controlled and careful; beautiful, but nothing unexpected. Contrasting to that is the carnival-style atmosphere inside a star cluster, where you might get lots of different dances happening simultaneously; big and small dance groups, freestyle, and lots of surprises!” While the results of the study cannot tell us — yet — exactly where the observed black hole binaries are merging, they do suggest that black hole carnivals in the centres of star clusters could be an important contribution.
Astronomers at Swinburne University of Technology have played an important role in the discovery of a rare luminous jet of matter travelling close to the speed of light, created by a supermassive black hole violently tearing apart a star. Published in Nature, the research brings astronomers one step closer to understanding the physics of supermassive black holes, which sit at the centre of galaxies billions of light years away.
Swinburne Professor Jeff Cooke, who is also a Chief Investigator for the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), was a key member of the research team.
“Stars that are literally torn apart by the gravitational tidal forces of black holes help us better understand what exists in the Universe,” says Professor Cooke. “These observations help us explore extreme physics and energies that cannot be created on Earth.”
Supermassive, super rare and super far awayWhen a star gets too close to a supermassive black hole, the star is violently ripped apart by tidal forces, with pieces drawn into orbit around the black hole and eventually completely consumed by it. In extremely rare instances – only about one per cent of the time – these so-called tidal disruption events (TDEs) also launch luminous jets of material moving almost at the speed of light.
The co-lead authors of the work, Dr Igor Andreoni from the University of Maryland and Assistant Professor Michael Coughlin from the University of Minnesota, along with an international team, observed one of the brightest ever TDEs. They measured it to be more than 8.5 billion light years away, or more than halfway across the observable Universe.
The event, officially named “AT2022cmc”, is believed to be at the centre of a galaxy that is not yet visible because the intense light from the flash still outshines it. Future space observations may unveil the galaxy when AT2022cmc eventually fades away.
It is still a mystery why some TDEs launch jets while others do not appear to. From their observations, the researchers concluded that the black holes associated with AT2022cmc and other similarly jetted TDEs are likely spinning rapidly. This suggests that a rapid black hole spin may be one necessary ingredient for jet launching—an idea that brings researchers closer to understanding these mysterious objects at the outer reaches of the universe.
Working together on new discoveriesMore than 20 telescopes operating at all wavelengths were a part of this research. These include the Zwicky Transient Facility in California that made the initial discovery, X-ray telescopes in space and on the International Space Station, radio/mm telescopes in Australia, the US, India and the French Alps, and optical/infrared telescopes in Chile, the Canary Islands and the US, including the W. M. Keck Observatory in Hawaii.
Swinburne postdoctoral researcher Jielai Zhang, a co-author on the research, says that international collaboration was essential to this discovery.
“Although the night sky may appear tranquil, telescopes reveal that the Universe is full of mysterious, explosive and fleeting events waiting to be discovered. Through OzGrav and Swinburne international research collaborations, we are proud to be making meaningful discoveries such as this one,” she said.
The paper, “A very luminous jet from the disruption of a star by a massive black hole,” was published in Nature on November 30, 2022
The Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Swinburne has been awarded a further $35 million in funding to continue their ground-breaking discoveries at the cutting edge of human understanding.
The new funding will support OzGrav’s work investigating the fundamental nature of relativistic gravity, ultra-dense matter and the universe, generating critical discoveries to cement Australia’s leadership role in the growing field of gravitational wave science.
Centre Director Professor Matthew Bailes says the funding will not only allow OzGrav make to landmark discoveries about the nature of our universe, but also lay the foundations for the Australian mega-science instruments that could transform physics in the 2030s and 2040s.
“When OzGrav launched in 2017, we contributed to the birth of a new era of astrophysics. This reinvestment will put us at the forefront of transformational scientific discoveries well into the next decade,” Professor Bailes says.
"The opportunity to attract and work with the talented young scientists and engineers this Centre will attract is incredibly energising.
“By improving our advanced gravitational wave detectors, we will be able to understand more about our universe, probing neutron stars and black holes and mapping the cosmic evolution of the universe.”
Turning Einstein’s imagination into reality
Gravitational waves, first predicted by Albert Einstein in 1915 in his theory of general relativity, went undetected for one hundred years before scientific advancements enabled their detection for the first time in 2015.
Since then, OzGrav researchers have been at the forefront of gravitational wave discovery, making significant discoveries to help understand the extreme physics of black holes and warped spacetime.
“As a technology-focused university with deep expertise in astronomy, physics and space research, Swinburne is proud to continue to be the home of this global collaboration,” says Deputy Vice-Chancellor, Research Professor Karen Hapgood.
“Under the directorship of Professor Matthew Bailes, OzGrav has made a number of field-defining contributions to our understanding of the universe.
“By building closer relationships with industry and through our leading space education programs, we look forward to expanding this impact and inspiring the next generation of graduates in Australia’s high-tech workforce.”
The new funding from the Australian Research Council will enable OzGrav to maximise the sensitivity and yield of gravitational wave detectors, supressing quantum noise and reducing coating losses. This is expected to increase detection rates by over an order of magnitude. This will enable:
Headquartered at Swinburne University of Technology, OzGrav is a collaboration between a number of Australian universities, including the University of Queensland, The Australian National University, The University of Sydney, Monash University, The University of Adelaide, The University of Western Australia and The University of Melbourne, and CSIRO.
Swinburne University of Technology - Matthew Bailes, Chris Blake, Adam Deller, Jarrod Hurley and Ryan Shannon;
Australian National University - David McClelland, Christopher Lidman, Kirk McKenzie, Susan Scott, Bram Slagmolen and Ling (Lilli) Sun;
Monash University - Eric Thrane, Paul Lasky and Ilya Mandel;
University of Adelaide - David Ottoway and Peter Veitch;
University of Melbourne - Katie Auchettl and Andrew Melatos;
University of Queensland - Tamara Davis;
University of Sydney - Tara Murphy and Elaine Sadler;
University of Western Australia - JU Li and Chunnong Zhao.
Other international partners include the NASA Goddard Space Flight Centre, Massachusetts Institute of Technology (MIT) and the Laser Interferometer Gravitational-Wave Observatory in the United States, as well as institutions in the US, the Netherlands, Germany, Italy and the UK.
Excited to announce that three of our OzGrav members (1 x Postdoc and 2 x AIs) were awarded DECRAs today! Congratulations to Dr Katie Auchettl (University of Melbourne), Dr Dan Brown (University of Adelaide) and Dr Anais Möller (Swinburne University of Technology).
DECRAs (Discovery Early Career Researcher Awards) are three-year funding allocations, awarded by the Australian Research Council (ARC). Its objectives are to:
This project aims to understand the unexplored population of non-active or quiescent supermassive blackholes (SMBHs) using tidal disruption events - the multi-wavelength outburst resulting from a star being ripped apart by the tidal forces of the SMBH. This project will increase our understanding of the transient and accretion properties of SMBHs in a broad range of galaxies, while the expected outcomes include novel techniques for distinguishing different types of extreme SMBH emission and characterisation of the environments where these extreme transient events occur. These outcomes will facilitate the identification of transient SMBH events and enhance the scientific return of the next generation of international optical surveys.
This project aims to build upon Australia’s already pioneering research into the workings of the universe by addressing challenges facing future gravitational wave detectors. It will develop and utilise advanced new numerical models to generate new knowledge on large-scale precision interferometry and contribute towards the design of future detectors that are essential for gravitational wave astronomy to thrive.Expected outcomes are new optimised designs for detectors and an array of innovative new open-source numerical models for exploring new designs of quantum optics experiments. This will benefit both Australian and international research teams in the global effort to realise the third generation of gravitational wave detectors.
Explosive astrophysical events are critical to understand what the Universe is made of and its physics.This project aims to single out the most exciting exploding stars and extreme events out of the millions detected each night at the world’s largest optical telescope. It will magnify Australian leadership and optimise investment in astronomical facilities by obtaining unique information before these events fade forever. Expected outcomes include improved knowledge on the nature of exploding stars and the discovery of new events and physical processes. It will benefit the Australian community at large by training young Australians in data-intensive technologies required to lead ground-breaking research and advance our innovative economy.
Pulsars—rapidly-spinning remnants of stars that flash like a lighthouse—occasionally show extreme variations in brightness. Scientists predict that these short bursts of brightness happen because dense regions of interstellar plasma (the hot gas between stars) scatter the radio waves emitted by the pulsar. However, we still don’t know where the energy sources required to form and sustain these dense plasma regions come from. To better understand these interstellar formations, we require more detailed observations of their small-scale structure, and a promising avenue for this is in the scintillation, or “twinkling,” of pulsars.
When a pulsar’s radio waves are scattered by the interstellar plasma, the separate waves interfere and create an interference pattern on the Earth. As the Earth, pulsar, and plasma move relative to each other, this pattern is observed as brightness variations in time and in frequency: the dynamic spectrum. This is scintillation, or “twinkling”. Thanks to the point-like nature of pulsar signals, the scattering and twinkling occurs in small regions of the plasma. Following specialised signal processing of the dynamic spectrum, we can observe striking parabolic features known as scintillation arcs that are related to the image of the pulsar’s scattered radiation on the sky.
One particular pulsar, called J1603-7202, underwent extreme scattering in 2006, making it an exciting target for examining these dense plasma regions. However, the pulsar’s trajectory still hasn’t been determined as it orbits another compact star called a white dwarf in a face-on orbit, and scientists don’t have alternative methods to measure it in this situation. Fortunately, scintillation arcs serve a double purpose: their curvatures are related to the pulsar’s velocity, as well as the distance to the pulsar and the plasma. How the pulsar’s velocity changes as it orbits depends on the orbit’s orientation in space. Therefore, in the case of pulsar J1603-7202, we calculated the changes in the curvature of the arcs over time to determine the orientation.
The measurements we obtained for the orbit of J1603-7202 are a significant improvement compared to previous analyses. This demonstrates the viability of scintillation in supplementing alternative methods. We measured the distance to the plasma and showed that it was about three-quarters of the distance to the pulsar, from Earth. This does not seem to coincide with the positions of any known stars or interstellar gas clouds. Pulsar scintillation studies often explore structures such as this, which are otherwise invisible. The question therefore remains open: what is the source of the plasma that scatters the pulsar's radiation?
Finally, using our orbit measurement, we are able to estimate the mass of J1603-7202’s orbital companion, which is about half the mass of the Sun. When considered alongside the highly circular orbit of J160-7202, this implies the companion is likely a stellar remnant composed of carbon and oxygen - a rarer find around a pulsar than the more common helium-based remnants.
As we now possess a near-complete model of the orbit, it’s now possible to transform scintillation observations of J1603-7202 into on-sky scattered images and map the interstellar plasma at Solar System scales. Creating images of the physical structures that cause extreme scattering of radio waves may give us a better understanding of how such dense regions form and of the role the interstellar plasma plays in the evolution of galaxies.
Link to study: https://arxiv.org/abs/2204.11077
Written by PhD student Kris Walker (ICRAR-UWA) and Dr Daniel Reardon (OzGrav-Swinburne University).
Have you registered for the National Science Quiz on 7 August yet? Don't miss out on this night of fun! You can attend in-person in Melbourne or stream live via YouTube.
Hosted by Charlie Pickering from ABC-TV’s The Weekly and joined by some of Australia’s top scientists with our special guest team captains, each team will battle it out for the honour of being this year’s National Science Quiz champions.
While the teams discuss and ponder their quiz answers – the live audience can also play along for the chance to win a $500 cash prize! The quix will also be live-streaming via The National Science Quiz YouTube channel, so you can play along at home to win $250 cash prize. Why not get a team together and make a night of it?
You can also submit a video science quiz question to put to the panellits for a chance to win $200!
They’ll be lots of science, laughs and fun and maybe even some slime! Register NOW!
Watch, join in, do both, or do either – whichever you choose the National Science Quiz will remind you how wonderful science really is!
Get your tickets here: https://www.nationalsciencequiz.com.au/about-us/
Astronomers search for X-ray signposts of the elusive continuous gravitational waves
In the last few years, astronomers have achieved an incredible milestone: the detection of gravitational waves, vanishingly weak ripples in the fabric of space and time emanating from some of the most cataclysmic events in the Universe, including collisions betweens black holes and neutron stars. So far there have been over 90 gravitational-wave detections of such events, observable for only ~0.1 to 100 seconds. However, there may be other sources of gravitational waves, and astronomers are still on the hunt for continuous gravitational waves.
Continuous gravitational waves should be easier to detect since they are much longer in duration compared to signals from compact-object collisions. A possible source of continuous waves is neutron stars, which are stellar “corpses” left over from supernova explosions of massive stars. After the initial explosion, the star collapses in on itself, crushing atoms down into a super-dense ball of subatomic particles called “neutrons” - hence the name “neutron star”. The continuous wave signal is related to how fast the neutron star is spinning, so precise measurements of the spin frequency using more conventional telescopes would greatly improve the chance of detection of these elusive waves.
In a recent study, led by OzGrav PhD student Shanika Galaudage from Monash University, scientists aimed to determine neutron stars’ spin frequencies to help detect continuous gravitational waves.
Possible sources of continuous gravitational waves
In this study, researchers hypothesised that continuous gravitational-waves indirectly come from the gradual accumulation of matter onto a neutron star from a low-mass companion star–these binary systems of a neutron star and companion star are called low mass X-ray binaries (LMXBs).
If the neutron star can maintain an accumulated "mountain" of matter, (even if only a few centimetres in height!), it will produce continuous waves. The frequency of these waves relate to how fast the neutron star is spinning. The faster you accumulate this matter, the bigger the "mountain", producing larger continuous waves. Systems that accumulate this matter more quickly are also brighter in X-ray light. Therefore the brightest LMXBs are the most promising targets for detecting continuous waves.
Scorpius X-1 (Sco X-1) and Cygnus X-1 (Cyg X-2) are two of the brightest LMXB systems–Sco X-1 ranks second in X-ray brightness compared to the Sun. In addition to their extreme brightness, scientists know a lot about these two LMXB systems, making them ideal sources of continuous waves to study. But, their spin frequencies are still unknown.
“A way we can determine how fast these neutron stars are spinning is by searching for X-ray pulsations,” says study lead Shanika Galaudage. “X-ray pulsations from neutron stars are like cosmic lighthouses. If we can time the pulse we would immediately be able to reveal their spin frequency and get closer to detecting the continuous gravitational-wave signal.”
“Sco X-1 is one of the best prospects we have for making a first detection of continuous gravitational waves, but it’s a very hard data analysis problem,” says OzGrav researcher and study co-author Karl Wette, from The Australian National University. “Finding a spin frequency in the X-ray data would be like shining a spotlight on the gravitational wave data: ‘here, this is where we should be looking’. Sco X-1 would then be a red-hot favourite to detect continuous gravitational waves.”
Searching for X-ray pulsations
The team performed a search for X-ray pulsations from Sco X-1 and Cyg X-2. They processed over 1000 hours of X-ray data collected by the Rossi X-ray Timing Explorer instrument. The search used a total of ~500 hours of computational time on the OzSTAR supercomputer!
Unfortunately, the study did not find any clear evidence of pulsations from these LMXB sources. There are a number of reasons why this could be: the LMXB could have weak magnetic fields which are not powerful enough to support detectable pulsations. Or it could be that the pulsations come and go over time, which would make them hard to detect. In the case of Sco X-1, it could possibly be a black hole, which we would not expect to produce X-ray pulsations.
The study does find the best limits on how bright these X-ray pulsations could be if they did occur; these results could mean that neutron stars cannot sustain mountains of matter under its strong gravity. Future research can build on this study by employing better search techniques and more sensitive data.
Written by OzGrav researcher Shanika Galaudage (Monash University)
Published in MNRAS: Deep searches for X-ray pulsations from Scorpius X-1 and Cygnus X-2 in support of continuous gravitational wave searches
Gravitational wave scientists from The University of Western Australia have led the development of a new laser modesensor with unprecedented precision that will be used to probe the interiors of neutron stars and test fundamentallimits of general relativity.
Research Associate from UWA’s Centre of Excellence for Gravitational Wave Discovery (OzGrav-UWA) Dr Aaron Jones,said UWA co-ordinated a global collaboration of gravitational wave, metasurface and photonics experts to pioneer anew method to measure structures of light called ‘eigenmodes’.
“Gravitational wave detectors like LIGO, Virgo and KAGRA store enormous amount of optical power and several pairs ofmirrors are used to increase the amount of laser light stored along the massive arms of the detector,” Dr Jones said.
“However, each of these pairs has small distortions that scatters light away from the perfect shape of the laser beamwhich can cause excess noise in the detector, limiting sensitivity and taking the detector offline.
“We wanted to test an idea that would let us zoom in on the laser beam and look for the small ‘wiggles’ in power thatcan limit the detectors’ sensitivity.”
Dr Jones said a similar problem is encountered in the telecoms industry where scientists are investigating ways to usemultiple eigenmodes to transport more data down optical fibres.
“Telecoms scientists have developed a way to measure the eigenmodes using a simple apparatus, but it’s not sensitiveenough for our purposes,” he said. “We had the idea to use a metasurface – an ultra-thin surface with a special patternencoded in sub-wavelength size – and reached out to collaborators who could help us make one.”
The proof-of-concept setup the team developed was over one thousand times more sensitive than the originalapparatus developed by telecoms scientists and the researchers will now look to translate this work into gravitational-wave detectors.
OzGrav-UWA Chief Investigator Associate Professor Chunnong Zhao said the development is another step forward in detecting and analysing the information carried by gravitational waves, allowing us to observe the universe in newways.
“Solving the mode sensing problem in future gravitational wave detectors is essential if we are to understand theinsides of neutron stars and further our observation of the universe in a way never before possible,” Associate ProfessorZhao said.
The breakthrough is detailed in a study published in Physical Review.
WRITTEN BY MILKA BUKILICIN - UWA RESEARCH