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.”

Next-generation discoveries
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:

• The discovery of new sources of gravitational waves and extreme electromagnetic events
• Testing the boundaries or Einstein’s theory of general relativity in the strongest gravitational fields in the universe, using black holes and pulsars
• Understanding ultra-dense matter through the observation of neutron stars and their mergers
• Mapping the cosmic evolution of the universe using gravitational waves and fast radio bursts

OzGrav is also committed to strengthening equity and diversity in this sector and increasing participation and career options for under-represented groups in STEM. Through schools outreach, the Centre also aims to inspire the next generation to pursue a career in STEM, especially at an age when many young women and under-represented groups choose to not take STEM subjects.

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.

Chief Investigators:
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.

​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
https://doi.org/10.1093/mnras/stab3095
 

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.
 

​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.