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/
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
Pioneering astrophysicist and OzGrav Director Professor Matthew Bailes has been recognised for his outstanding contributions to science by being elected a Fellow of the Australian Academy of Science. Professor Bailes has specialised in the study of pulsars, fast radio bursts and gravitation, making major contributions to establishing Australia’s high international profile in these areas.
In particular, he has played a pivotal role in the development of a new branch of astrophysics, Fast Radio Bursts, developing pioneering instrumentation and software that led to Australia’s early dominance of the field. Professor Bailes has been central in putting Swinburne University of Technology at the cutting-edge of astrophysics.
Centre for Astrophysics and Supercomputing
In 1998 he established Swinburne’s Centre for Astrophysics and Supercomputing, recognised internationally as a centre for astrophysics and virtual-reality content for public outreach. The centre hosts one of Australia’s most powerful supercomputers and has developed 3D virtual reality films for its custom 3D theatres and IMAX.
The centre has graduated over 100 PhDs and pioneered online education via Swinburne Astronomy Online, but also worked with many school children for work experience and virtual tours of the Universe in their custom 3D theatre.
ARC Centre for Excellence for Gravitational Wave Discovery (OzGrav)
In 2016 Professor Bailes was appointed the Director of the Australian Research Council Centre for Excellence for Gravitational Wave Discovery (OzGrav). Hosted at Swinburne OzGrav is a worldwide collaboration that aims to understand the extreme physics of black holes and warped space time.
Professor Bailes was named among 22 outstanding researchers from across the breadth of Australian science as a Fellow of the Academy.
Upon hearing the news of his election Professor Bailes said: “I’ve always had a burning desire to understand how the Universe works and want to thank my mentors, staff, collaborators and students for enabling the discoveries I’ve been involved with.” He nominated the discovery of the Fast Radio Bursts as his career highlight. “I couldn't sleep the night after the first one was discovered because it seemed too good to be true! Fortunately, it was true.”
Incoming President of the Australian Academy of Science Professor Chennupati Jagadish AC, congratulated the new Fellows for their contributions to science. “Fellows of the Australian Academy of Science are among the nation’s most distinguished scientists, elected by their peers for ground-breaking research and contributions that have had clear impact,” Professor Jagadish says. “We reflect a diverse and inclusive science community that recognises the widest range of talents, backgrounds, perspectives and experiences, and we are united by our contribution and commitment to scientific excellence.”
This article is an amended version of the media release originally publish on Swinburne University’s website.
At the centre of most galaxies there is a massive black hole. These black holes are very heavy – their mass can be from a million to over a billion times the mass of the Sun and, as such, are appropriately known as supermassive black holes. As galaxies move around in the Universe, they will sometimes merge. When this happens, the supermassive black holes they host tend to migrate toward each other and form a binary system. As these two black holes orbit each other, they warp the fabric of space and time around them and produce gravitational waves which ripple out into the Universe. These gravitational waves complete one full oscillation every year or so as they travel through space and are classified as low frequency gravitational waves.
The Universe is full of these supermassive black hole binary systems, and the gravitational waves they emit fill space, combining to form something known as the stochastic gravitational wave background. Scientists are trying to find a gravitational wave signal from this background using a complex network of radio telescopes called a pulsar timing array but it could be years before there is a confirmed detection.
For this reason, cosmological simulations are often used to predict what this gravitational wave signal could look like. This type of simulation helps scientists understand the structure and history of the Universe by tracking the flow of matter and energy from a time soon after the Big Bang, up until today.
A team of researchers led by postgraduate researcher Bailey Sykes (from Monash University), alongside several OzGrav scientists, including OzGrav Associate Investigator Dr Hannah Middleton, have recently made a new prediction for the strength of this gravitational wave signal. The new estimate is based on data from the MassiveBlack-II simulation, which simulates a massive region of space similar to a chunk of our own Universe.
The team made two estimates: one in which the supermassive black holes merge almost instantly once their host galaxies collide, and another in which the two black holes take time to sink towards each other once they pair up in a binary system. This second estimate is important as the gravitational wave output of a binary can change during this time due to the interactions of stars and gas nearby the supermassive binary.
The simulated gravitational wave signal using MassiveBlack-II is similar to other predictions in previous studies. It’s smaller than a signal currently detectable by pulsar timing arrays; however, as the sensitivity of telescope technology increases over time, it’s possible a confirmed detection could be just around the corner.
The results from the study add valuable insights to existing signal predictions and provide an important reference point for future pulsar timing arrays. Progressively more accurate estimates of the stochastic gravitational wave background can be used to further understand other astrophysical phenomena, including the interactions of stars and gas which impact merging supermassive black holes.
Written by Bailey Sykes (Monash University)
In 2021, Australian researchers Lyle Roberts and James Spollard, from The Australian National University (ANU), co-founded Vai Photonics: a spin-off company developing patented photonic sensors for precision navigation. The ARC Centres of Excellence for Engineered Quantum Systems (EQUS) and Gravitational Wave Discovery (OzGrav) played key roles in kickstarting Vai Photonics by providing seed funding towards fundamental LiDAR research, which translated to real-world, industry applications. Now, Advanced Navigation, one of the world’s most ambitious innovators in AI robotics and navigation technology, has announced the acquisition of Vai Photonics with aims to commercialise Roberts and Spollard’s research into exciting autonomous and robotic applications across land, air, sea and space.
“The technology Vai Photonics is developing will be of huge importance to the emerging autonomy revolution. The synergies, shared vision and collaborative potential we see between Vai Photonics and Advanced Navigation will enable us to be at the absolute forefront of robotic and autonomy-driven technologies,” said Xavier Orr, CEO and co-founder of Advanced Navigation.
Vai Photonics co-founder James Spollard explained: “Precision navigation when GPS is unavailable or unreliable is a major challenge in the development of autonomous systems. Our emerging photonic sensing technology will enable positioning and navigation that is orders of magnitude more stable and precise than existing solutions in these environments.
“By combining laser interferometry and electro-optics with advanced signal processing algorithms and real-time software, we can measure how fast a vehicle is moving in three dimensions,” said Spollard. “As a result, we can accurately measure how the vehicle is moving through the environment, and from this infer where the vehicle is located with great precision.”
The technology, which has been in development for over 15 years at ANU, will solve complex autonomy challenges across aerospace, automotive, weather and space exploration, as well as railways and logistics.
EQUS Director Professor Andrew White applauded the initiative and determination shown by Lyle and James. “Lyle and James are perfect examples of researchers achieving useful outcomes by utilising the funds, mentoring, and guidance available through EQUS’s Translation Research Program, to help pursue the real-world impacts that our research can deliver. These two are what Australia’s research future looks like,” said White.
OzGrav Director Professor Matthew Bailes said he was thrilled to see such a positive outcome for our early career researchers that were supported by OzGrav's industry seeding scheme and workshops. "It reinforces the fact that pushing the limits of instrumentation for scientific purposes can often create opportunities for Australian innovators and industry," said Bailes.
Professor Brian Schmidt, Vice-Chancellor of the Australian National University said: “Vai Photonics is another great ANU example of how you take fundamental research – the type of thinking that pushes the boundaries of what we know – and turn it into products and technologies that power our lives.
“The work that underpins Vai Photonics’ advanced autonomous navigation systems stems from the search for elusive gravitational waves – ripples in space and time caused by massive cosmic events like black holes colliding.
“The team have built on a decade of research and development across advanced and ultra-precise laser measurements, digital signals and quantum optics to build their innovative navigation technology. We are proud to have backed Vai Photonics through our Centre for Gravitational Astrophysics and business and commercialisation office. It’s really exciting to see the team take another major step in their incredible journey.”
Co-founder Dr Lyle Roberts looks forward to an autonomous future: “This is a huge win for the Vai Photonics team – together with Advanced Navigation we are able to bring our product to market much faster than originally planned. We now have access to leading research and development facilities along with strong distribution channels. We couldn’t have asked for a better outcome and look forward to navigating the future with Advanced Navigation.”
This acquisition fits into Advanced Navigation’s larger growth strategy to expand its product and solutions portfolio across deep technology fields that look to solve the world’s greatest challenges facing the autonomy revolution.
The acquisition was finalised in April 2022, subject to typical closing conditions. The Vai Photonics team has been integrated into Advanced Navigation’s research and development team, based out of the new Canberra research facility.
This article is an amended extract from the original article written by Laura Hayward published on www.advancednavigation.com
Now that we've been detecting gravitational waves (GWs), we'd like to better understand the systems that generate GWs. The GWs found so far have been from collisions of celestial bodies, like black holes and neutron stars. Once we have detected a GW, we use "Bayesian Inference" to deduce the masses and spins of the objects that shot off the GW (to understand inference, check this video by 3blue1brown). Then we can use our mass and spin deductions to answer: where do these bodies exist in the Universe? Are these colliding bodies huddled together in galaxies or isolated in space? But, it gets tricky to answer such questions if our deductions of the masses and spins are incorrect! So, in my recent study, I have built a "deep follow-up" tool to determine which masses and spins better describe a given GW event.
I have used this deep follow-up tool to study the "boxing-day" gravitational wave, GW151226. Initial work deduced that this GW was from the merger of two black holes (BHs), both with standard masses and spins (case A). However, recent work has deduced that the GW might have originated from a strange system: one BH could be much larger than the other and with a faster spin (case B)! A diagram representing these cases can be seen below on the right side of Figure 1.
Figure 1: GW151226's two personalities. (Left) The initial and new Bayesian inference results are plotted in orange and blue, respectively. We perform a deep follow-up on the pinned points, cases A and B. (Right) Illustrative versions of what cases A and B represent. Note: black hole cartoons inspired by NASA’s Field guide to black holes.
The "deep follow-up" method involves drilling into these cases to determine which binary BH system better describes the GW. First, we pin down some deduced properties of the merging black hole system, such as the mass-ratio q (the ratio of smaller BH mass divided by the bigger BH mass) and xeff (the effective spin of the binary in the z-direction). The pinned value for the initial and new results is on the left side of Figure 1. We then use Bayesian inference at these pinned values. The output allows us to compare case A and case B. We find that both the standard (case A) and irregular (case B) black hole pairs can describe GW151226, giving the event something like a dual-identity!
This dual-identity gives GW151226 much more character than initially considered. For example, we initially believed that GW151226 came from an isolated black hole pair. However, a BH pair from case B is more likely to be found at the centre of an active galaxy! So, finally, I wonder: are there other GW events with split personalities? Hopefully, our deep follow-up method will be able to settle these questions.
Written by OzGrav researcher Avi Vajpeyi, Monash University.
Less than one percent of stars in a galaxy are formed with masses exceeding ten solar masses.
Despite their rarity, massive stars are believed to play a crucial role in shaping their surroundings, ultimately determining the evolution of the star cluster or galaxy they are located in.
Simulations of massive stars are used in many fields of astrophysics, from predicting gravitational-wave event rates to studying star formation and star cluster evolution. However, their rarity and short lives, along with their more extreme properties, mean that the evolution of massive stars is riddled with many uncertainties. These uncertainties are compounded by the fact that accurate modeling of stellar lives in three dimensions is prohibitively expensive in terms of computing resources.
Therefore, stellar evolution is modeled using one-dimensional (1D) codes, with only radius or mass as the spatial coordinate. Three-dimensional (3D) processes such as rotation and mixing are approximated using 1D analogs, which generally give good results for most stars.
However, in the envelopes of massive stars (and in low-mass stars at the late stages of evolution), the use of these 1D analogs can lead to numerical challenges for stellar evolution codes. The time steps of the computation become very small (of the order of days) and 1D codes struggle to compute the further evolution of the star.
While researchers are trying to find the solution using multidimensional models, 1D stellar evolution codes adopt different pragmatic methods to push the evolution of stars beyond these numerical challenges. These methods, along with other uncertain parameters in the evolution of massive stars, can significantly alter the predictions of massive stellar models. To get an idea of how different their predictions can be, we examined models of massive stars from five different datasets, each computed using a different 1D code.
We found that certain aspects of these predictions were extremely sensitive to the modeling assumptions employed by different codes. For example, in Figure 2, the different sets of massive star models show a variation of about 20 solar masses in their predictions of the mass of the black hole formed.
We also found huge differences in the radial evolution of these stellar models and thus the ionizing radiation produced by them. These differences can directly affect binary evolution and the simulations of stellar environments, such as galaxies.
Research Highlight: Gravitational wave scientists develop new laser mode sensor with unprecedented precision
Lasers support certain structures of light called ‘eigenmodes’. An international collaboration of gravitational wave, metasurface and photonics experts have pioneered a new method to measure the amount of these eigenmodes with unprecedented sensitivity.
In gravitational wave detectors, several pairs of mirrors are used to increase the amount of laser light stored along the massive arms of the detector. However, each of these pairs has small distortions that scatters light away from the perfect shape of the laser beam. This scattering can cause excess noise in the detector, limiting sensitivity and taking the detector offline.
From the recently submitted study, Prof Freise (from Vrije Universiteit Amsterdam) says: “Gravitational wave detectors like LIGO, Virgo and KAGRA store enormous amount of optical power – in this work, we wanted to test an idea that would let us zoom in on the laser beam and look for the small wiggles in power that can limit the detectors’ sensitivity.”Lasers support certain structures of light called ‘eigenmodes’. An international collaboration of gravitational wave, metasurface and photonics experts have pioneered a new method to measure the amount of these eigenmodes with unprecedented sensitivity.
A similar problem is encountered in the telecoms industry where scientists want to use multiple eigenmodes to transport more data down optical fibres. OzGrav researcher and lead author Dr Aaron Jones (The University of Western Australia) explains: “Telecoms scientists have developed a way to measure the eigenmodes using a simple apparatus, but it’s not sensitive enough for our purposes. We had the idea to use a metasurface and reached out to collaborators who could help us fabricate one.”
In the study, the proof-of-concept setup the team developed was over 1000x more sensitive than the original way developed by the telecoms scientists. The researchers will now look to translate this work into gravitational wave detectors, where the additional precision will be used to probe the interiors of neutron stars and test fundamental limits of general relativity.
OzGrav Chief Investigator, Prof Zhao (from University of Western Australia) says: “Solving the mode sensing problem in future gravitational wave detectors is essential, if we are to understand the insides of neutron stars.”
Written by Dr Aaron Jones (The University of Western Australia).