Humans have been studying the light from stars since the beginning of our history; however, we’ve only just discovered in the last few decades that stars don’t like to be alone.
Binary systems—containing two stars orbiting around each other—are one of the most common type of gravitationally-bound collections of stars, yet their evolution is complex. Astronomers are trying to piece together the puzzle of different stellar observations to reveal the bigger picture. Using their understanding of binary evolution, scientists can simulate populations of stellar binaries with the stellar population synthesis code COMPAS—mostly developed by researchers from the ARC Centre of Gravitational Wave Discovery (OzGrav).
OzGrav researchers, in collaboration with the Max Planck Institute of Hannover, Monash University and University of Birmingham, recently conducted a study to understand the origin of the properties of ‘Be X-ray’ binaries observed in the Small Magellanic Cloud.
Be X-ray binaries are star systems typically composed of a neutron star orbiting around a rapidly rotating massive star. This rotation causes the massive star to produce a disk of outflowing material—some of this is accumulated by the neutron star. The neutron star then shoots off X-ray radiation that scientists can observe and measure.
The study, led by OzGrav Affiliate Serena Vinciguerra, used the COMPAS code to simulate an environment like the Small Magellanic Cloud. By comparing the orbital properties of the simulated Be X-ray binaries with the observed ones, researchers revealed the probable evolution of these star systems:
Initially, two stars are born in a tight binary system. The most massive star evolves quicker and expands. Because of the proximity between the two stars, the inflated massive star feeds its material to the smaller star. Over time, the massive star may feed and lose most of its mass; however, the smaller star may get too ‘full’ and not accept all the ‘food’ (material).
Each star’s individual ‘diet’ depends not only on their constitution and age, but also on the massive star feeding them. In Be X-ray binaries, the stars’ diets are more generous than what astronomers previously assumed. Consequently, the well-fed stars become massive and spin rapidly.
Later in their evolution, the original most massive star may explode as a supernova, leaving behind a small but very dense neutron star. If the stars survive the explosion, they form a Be X-ray system, with a neutron star orbiting a massive and rapidly rotating star.
Thermal-driven mirror for gravitational wave detectors: The illustration shows the cross-section of a thermal bimorph mirror and its constituents. Controlling the temperature of the mirror changes the curvature of the reflected wavefront. Overlaid on the cross-section is the simulated radial stress, showing a concentration of stress at the boundary of the two layers, where the adhesive holds the structure together. Credit: Huy Tuong Cao, University of Adelaide
Researchers have developed a new type of deformable mirror that could increase the sensitivity of ground-based gravitational wave detectors such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO). Advanced LIGO measures faint ripples in space time called gravitational waves, which are caused by distant events such as collisions between black holes or neutron stars.
“In addition to improving today’s gravitational wave detectors, these new mirrors will also be useful for increasing sensitivity in next generation detectors and allow detection of new sources of gravitational waves,” said research team leader Huy Tuong Cao from the University of Adelaide node of the Australian Research Centre of Excellence for Gravitational Waves Discovery (OzGrav).
Deformable mirrors, which are used to shape and control laser light, have a surface made of tiny mirrors that can each be moved, or actuated, to change the overall shape of the mirror. As detailed in The Optical Society’s (OSA) journal Applied Optics, Cao and colleagues have, for the first time, made a deformable mirror based on the bimetallic effect in which a temperature change is used to achieve mechanical displacement.
“Our new mirror provides a large actuation range with great precision,” said Cao. “The simplicity of the design means it can turn commercially available optics into a deformable mirror without any complicated or expensive equipment. This makes it useful for any system where precise control of beam shape is crucial.”
The new technology was conceived by Cao and Aidan Brooks of LIGO as part of a visitor program between the University of Adelaide and LIGO Laboratory, funded by the Australian Research Council and National Science Foundation.
Building a better mirror
Ground-based gravitational wave detectors use laser light traveling back and forth down an interferometer’s two arms to monitor the distance between mirrors at each arm’s end. Gravitational waves cause a slight but detectable variation in the distance between the mirrors.
Detecting this tiny change requires extremely precise laser beam steering and shaping, which is accomplished with a deformable mirror.
“We are reaching a point where the precision needed to improve the sensitivity of gravitational wave detectors is beyond what can be accomplished with the fabrication techniques used to make deformable mirrors,” said Cao.
Most deformable mirrors use thin mirrors to induce large amount of actuation, but these thin mirrors can produce undesirable scattering because they are hard to polish. The researchers designed a new type of deformable mirror using the bimetallic effect by attaching a piece of metal to a glass mirror. When the two are heated together the metal expands more than the glass, causing the mirror to bend.
The new design not only creates a large amount of precise actuation but is also compact and requires minimum modifications to existing systems. Both the fused silica mirrors and aluminum plates used to create the deformable mirror are commercially available. To attach the two layers, the researchers carefully selected a bonding adhesive that would maximize actuation.
“Importantly, the new design has fewer optical surfaces for the laser beam to travel through, said Cao. “This reduces light loss caused by scattering or absorption of coatings.”
Creating a highly precise mirror requires precision characterization techniques. The researchers developed and built a highly sensitive Hartmann wave front sensor to measure how the mirror’s deformations changed the shape of laser light.
“This sensor was crucial to our experiment and is also used in gravitational detectors to measure minute changes in the core optics of the interferometer,” said Cao. “We used it to characterize the performance of our mirrors and found that the mirrors were highly stable and have a very linear response to changes in temperature.”
The tests also showed that the adhesive is the main limiting factor for the mirrors’ actuation range. The researchers are currently working to overcome the limitation caused by the adhesive and will perform more tests to verify compatibility before incorporating the mirrors into Advanced LIGO.
Pulsars—a type of rotating neutron star—are well-known for their use as incredibly stable astrophysical clocks. Their regularity, used to measure their radio pulses, has led to some of the most exciting tests of Einstein’s general theory of relativity and allowed scientists to examine the behaviour of the extremely dense matter inside neutron stars.
But just like ordinary clocks here on Earth, pulsars are not perfect keepers of time. Much like how a watch loses track of a few seconds each year, the exact rate at which pulsars spin appear to randomly wander by tiny amounts over month- to decade-long timescales.
The spins of a small fraction of pulsars have also been seen to rapidly speed up—they start ‘ticking’ slightly faster than usual. These effects, called ‘spin noise’ and ‘glitches’, change from pulsar to pulsar and may tell us how neutron stars evolved over millions of years; however, this requires precision tracking of hundreds of pulsar spins over many years.
Thanks to a series of upgrades over the last decade, the Molonglo Telescope—which celebrated its 50th birthday in 2015—can perform spin-tracking observations of hundreds of pulsars every two weeks! This enabled researchers, from the ARC Centre of Gravitational Wave Discovery (OzGrav), to find three new glitch events and measure the strength of the spin noise in 300 pulsars.
In a recently published study, led by OzGrav PhD student Marcus Lower, researchers examined 280 pulsars that are most representative of normal pulsar evolution and developed a statistical method similar to the one used for analysing gravitational-wave events detected by LIGO and Virgo. The results, presented at CSIRO’s Australia Telescope National Facility colloquium, showed that spin noise seems to decrease with pulsar age and that there is a scaling relationship between spin noise strength, how quickly a pulsar spins and how fast its spin is slowing down over time.
Marcus explains: ‘As spin noise becomes more obvious the longer you stare at a pulsar, we may be able to add additional pulsars to a re-analysis of the Molonglo data set in the future. We can also apply the statistical method to data from telescopes that have been tracking pulsar spins over multiple decades’.
The combination of additional pulsars and longer data sets would improve the study’s current measurements and allow researchers to determine the exact cause of spin noise in pulsars.
17th August 2017: a date marked down in the history books—the day the LIGO/Virgo collaboration made the first detection of gravitational waves from the death spiral of two neutron stars. Just 1.7 seconds later, astronomers observed a short burst of high-energy gamma rays known as a gamma-ray burst (GRB). Global efforts by thousands of astronomers later identified the host galaxy and a supernova-like thermal transient called a kilonova. This event gave astronomers insight into several fundamental and important questions, including an unprecedented understanding of where gold and other heavy elements are produced in the Universe, as well as our best measurement of the speed of gravity. Among other things, it confirmed that neutron star mergers originate from short-duration GRBs. Despite the numerous observations, an important question remains unanswered. What was the outcome of this merger?
Typically, one expects the merger of two neutron stars to immediately produce a black hole—an object so dense, that light itself cannot escape; however, observations of other GRBs show evidence for the immediate formation of a massive, rapidly-spinning neutron star. Such merger remnants, if they exist, have important implications for the physical composition of neutron stars.
Neutron stars are the only place in the Universe where we can study the behaviour of matter at temperatures up to 100 billion times hotter than on Earth and densities greater than an atomic nucleus—these conditions could never be reproduced on Earth. Nikhil Sarin, Paul Lasky, and Gregory Ashton—three researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University—recently published a study analysing all short-duration GRBs observed by NASA’s Neil Gehrels Swift Satellite. Out of 72 GRBs analysed, 18 show evidence for the immediate formation of a massive neutron star which later collapses into a black hole. Combining information from all 18 observations, the team were able to accurately describe the physical composition of these neutron stars.
The results indicate that these neutron stars are consistent with having a freely-moving ‘quark’ composition and a composition like regular matter, i.e. composed of atomic nuclei—the building blocks of the Universe. Quarks are elementary particles that contain protons, neutrons and atomic nuclei. In regular matter, these quarks are confined inside protons and neutrons, but in the high density and high-temperature regimes seen in neutron stars, they may move around freely. Scientists must first determine the temperature and density of neutron stars to understand the movement and behaviour of quarks and matter.
OzGrav PhD student and first author Nikhil Sarin says: ‘Our observations show a slight preference for freely-moving quarks. We look forward to getting more observations to definitively solve this puzzle’.
The research also found that, before collapsing into black holes, most neutron stars produce faint gravitational waves which are not likely to be individually detected by LIGO.
‘With the construction of more sensitive gravitational-wave detectors, such as the Einstein Telescope in Europe and the Cosmic Explorer in the US, we’re confident that we’ll eventually detect individual gravitational waves from these systems,’ explains Sarin.
A team of astrophysicists led by PhD student Mike Lau, from the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav), recently predicted that gravitational waves of double neutron stars may be detected by the future space satellite LISA. The results were presented at the 14th annual Australian National Institute for Theoretical Astrophysics (ANITA) science workshop 2020. These measurements may help decipher the life and death of stars.
Lau, first author of the paper, compares his team to ‘astro-palaeontologists’: ‘Like learning about a dinosaur from its fossil, we piece together the life of a binary star from their double neutron star fossils.’
A neutron star is the remaining ‘corpse’ of a huge star after the supernova explosion that occurs at the end of its life. A double neutron star, a system of two neutron stars orbiting each other, produces periodic disturbances in the surrounding space-time, much like ripples spreading on a pond surface. These ‘ripples’ are called gravitational waves and made headlines when the LIGO/Virgo Collaboration detected them for the first time in 2015. These gravitational waves formed when a pair of black holes spiralled too close together and merged.
However, scientists still haven’t found a way to measure the gravitational waves given off when two neutron stars or black holes are still far apart in their orbit. These weaker waves hold valuable information about the lives of stars and could reveal the existence of entirely new object populations in our Galaxy.
The recent study shows that the Laser Interferometer Space Antenna (LISA) could potentially detect these gravitational waves from double neutron stars. LISA is a space-borne gravitational-wave detector that is scheduled for launch in 2034, as part of a mission led by the European Space Agency. It’s made of three satellites linked by laser beams, forming a triangle that will orbit the Sun. Passing gravitational waves will stretch and squeeze the 40 million-kilometre laser arms of this triangle. The highly sensitive detector will pick up the slowly-oscillating waves—these are currently undetectable by LIGO and Virgo.
Using computer simulations to model a population of double neutron stars, the team predicts that in four years of operation, LISA will have measured the gravitational waves emitted by dozens of double neutron stars as they orbit each other. Their results were published in the Monthly Notices of the Royal Astronomical Society.
A supernova explosion ‘kicks’ the neutron star it forms and makes the initial circular orbit oval-shaped. Usually, gravitational wave emission rounds off the orbit—that is the case for double neutron stars detected by LIGO and Virgo. But LISA will be able to detect double neutron stars when they’re still far apart, so it may be possible to catch a glimpse of the oval orbit.
How oval the orbit is, or the eccentricity of the orbit, can tell us a lot about what the two stars looked like before they became double neutron stars. For example, their separation and how strongly they were ‘kicked’ by the supernova.
Our understanding of binary stars—stars that are born as a pair—is plagued with many uncertainties. Scientists hope that by the 2030s, LISA’s detection of double neutron stars will shed some light on their secret lives.
Astronomers regularly observe gravitational waves (GW)—ripples in space and time—that are caused by pairs of black holes merging into one. Einstein’s theory of gravity predicts that GW, which squeeze and stretch space as they pass, will permanently distort space, leaving a ‘memory’ of the wave behind. However, this memory effect has not yet been detected as it’s extremely small, leaving the faintest traces.
Researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University have finally developed a method to search and detect GW memory. Led by OzGrav PhD student Moritz Huebner, the recently published paper explains the tricky conquest of searching for memory by analysing data from numerous different observations. Huebner will be presenting these results at the Australian National Institute for Theoretical Astrophysics (ANITA) in Canberra this Thursday 6 February 2020.
The scientific models expect memory to leave an extremely faint trace on the detectors which is far smaller than the waves from the black hole collision itself. Therefore, data needs to be combined from many different gravitational wave events. To do this, the team used some of the most precise GW and memory models developed from the study of black hole mergers.
‘Our algorithms carefully comb through the data and measure the exact evidence for the existence of GW memory,’ said Huebner.
For each individual observation, this painstaking method can take hundreds of hours on a normal computer chip to explore all the possibilities of how a GW signal came about—this prompted the researchers to focus on fine-tuning the setting to reduce the amount of computing hours without compromising the search. So far, the results of the search applied to the first ten black-hole collisions—detected by LIGO and Virgo between 2015 and 2017—have proven inconclusive. LIGO and Virgo are not yet sensitive enough to make any statements about GW memory.
So, will we ever be able to detect memory?
‘Thankfully, we can now use data from the first ten black-hole collisions and have a decent idea of how many observable GW events there will be in the future. We can also calculate how much evidence of memory can be detected in each event,’ said Huebner.
Throughout the study, the researchers also discovered that their new search method must take data from approximately 2000 black hole mergers to detect memory. While this might sound implausible, the team expects to hit this number by the mid-2020s.
Plus, LIGO and Virgo are continuously being upgraded and have seen more than 40 mergers since April 2019, when the third observation run started. With further technological advances and the Japanese KAGRA observatory soon coming online, the team is confident that they’ll detect multiple binaries every day which will finally lead to revealing GW memory.
Link to journal: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.101.023011
Scientists from the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav) reveal the eccentricity of binary black holes: the shape of the orbit formed when two black holes fall into a dance as they spiral towards each other and eventually collide. While the most common orbit is thought to be circular, about one in 20 are in egg-shaped eccentric orbits, which can indicate completely different binary life histories.
Since the first detection of gravitational waves (GW) in September 2015, LIGO and its European counterpart Virgo have published the discovery of ten merging black-hole binaries. The latest run has already uncovered more than 30 new detections, with more forecast by April 2020.
OzGrav PhD student (and first author) Isobel Romero-Shaw recently published a study on the origins of GW 190425 – an event which was only announced this month (January 2020) by the LIGO/Virgo collaboration.
The GW signals provide a wealth of information about the pre-merger binaries; however, no one has yet deciphered how these black holes pair up in the first place.
New research, published in the journal Monthly Notices of the Royal Astronomical Society, reveals an important clue to how these black hole binaries are formed, how long they’ve been ‘together’ and what happens when they finally collide.
The study, led by Romero-Shaw, OzGrav Chief Investigator Eric Thrane and Associate Investigator Paul Lasky—all from Monash University—looked at data from the first and second rounds of observation of LIGO and Virgo, in particular, the ten black hole collisions that these two observation runs confirmed. They found that the orbits of all ten of these systems were remarkably circular, which is consistent with the expectation that about one in 20 orbits are not.
Artist’s depiction of two black holes falling into a dance as they spiral towards each other and eventually collide. Credit: Isobel Romero-Shaw
The current LIGO/Virgo run has already detected more than 30 additional collision signals. According to Romero-Shaw, the large amount of data coming from the third observing run ‘will mean we are much more likely to see eccentric collisions of black holes, which will give us real insight into how these systems form’.
According to Thrane, the more common circular orbits come from black holes who have been together from when they were garden-variety stars before they exploded and became black holes. Thrane explains: ‘These binaries are like siblings if you like. They grew up together and their orbit is circular’.
Eccentric orbits occur when black holes fall under each other’s gravitational influence by chance as they are zipping around galaxies. ‘These are more like adults who meet later in life and pair up. Their orbital relationship is more interesting -- much like in life,’ he added.
Importantly, when these two objects collide, the shape of their orbit means their gravitational-wave signal looks different. These detected explosions can now be used to retrospectively study the objects that collided.
Lasky said that the current LIGO and Virgo observing run is detecting ‘large numbers of these binaries and by April 2020—when the run finishes—we will have a far greater insight into what these events mean’.
VIDEO EXPLAINER: https://www.youtube.com/watch?v=LmW4Hd4sJvg
Credit: Johan Samsing
ASTRONOMERS WITNESS THE DRAGGING OF SPACE-TIME IN STELLAR COSMIC DANCE
An international team of astrophysicists led by Australian Professor Matthew Bailes, from the ARC Centre of Excellence of Gravitational Wave Discovery (OzGrav), has found exciting new evidence for ‘frame-dragging’—how the spinning of a celestial body twists space and time—after tracking the orbit of an exotic stellar pair for almost two decades. The data, which is further evidence for Einstein’s theory of General Relativity, is published today (31 January 2020) in the prestigious journal, Science.
More than a century ago, Albert Einstein published his iconic theory of General Relativity – that the force of gravity arises from the curvature of space and time and that objects, such as the Sun and the Earth, change this geometry. Advances in instrumentation have led to a flood of recent (Nobel prize-winning) science from phenomena further afield linked to General Relativity. The discovery of gravitational waves was announced in 2016; the first image of a black hole shadow and stars orbiting the supermassive black hole at the centre of our own galaxy was published just last year.
Almost twenty years ago, a team led by Swinburne University of Technology’s Professor Bailes—director of the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav)—started observing two stars rotating around each other at astonishing speeds with the CSIRO Parkes 64-metre radio telescope. One is a white dwarf, the size of the Earth but 300,000 times its density; the other is a neutron star which, while only 20 kilometres in diameter, is about 100 billion times the density of the Earth. The system, which was discovered at Parkes, is a relativistic-wonder system that goes by the name ‘PSR J1141-6545’.
Before the star blew up (becoming a neutron star), a million or so years ago, it began to swell up discarding its outer core which fell onto the white dwarf nearby. This falling debris made the white dwarf spin faster and faster, until its day was only measured in terms of minutes.
In 1918 (three years after Einstein published his Theory), Austrian mathematicians Josef Lense and Hans Thirring realised that if Einstein was right all rotating bodies should ‘drag’ the very fabric of space time around with them. In everyday life, the effect is miniscule and almost undetectable. Earlier this century, the first experimental evidence for this effect was seen in gyroscopes orbiting the Earth, whose orientation was dragged in the direction of the Earth’s spin. A rapidly spinning white dwarf, like the one in PSR J1141-6545, drags space-time 100 million times as strongly!
A pulsar in orbit around such a white dwarf presents a unique opportunity to explore Einstein’s theory in a new ultra-relativistic regime.
Lead author of the current study, Dr Vivek Venkatraman Krishnan (from Max Planck Institute for Radio Astronomy - MPIfR) was given the unenviable task of untangling all of the competing relativistic effects at play in the system as part of his PhD at Swinburne University of Technology. He noticed that unless he allowed for a gradual change in the orientation of the plane of the orbit, General Relativity made no sense.
MPIfR’s Dr Paulo Friere realised that frame-dragging of the entire orbit could explain their tilting orbit and the team presents compelling evidence in support of this in today’s journal article—it shows that General Relativity is alive and well, exhibiting yet another of its many predictions.
The result is especially pleasing for team members Bailes, Willem van Straten (Auckland University of Tech) and Ramesh Bhat (ICRAR-Curtin) who have been trekking out to the Parkes 64m telescope since the early 2000s, patiently mapping the orbit with the ultimate aim of studying Einstein’s Universe. ‘This makes all the late nights and early mornings worthwhile’, said Bhat.
Lead author Vivek Venkatraman Krishnan, Max Planck Institute for Radio Astronomy (MPIfR):
‘At first, the stellar pair appeared to exhibit many of the classic effects that Einstein’s theory predicted. We then noticed a gradual change in the orientation of the plane of the orbit.’
‘Pulsars are cosmic clocks. Their high rotational stability means that any deviations to the expected arrival time of its pulses is probably due to the pulsar’s motion or due to the electrons and magnetic fields that the pulses encounter.’
‘Pulsar timing is a powerful technique where we use atomic clocks at radio telescopes to estimate the arrival time of the pulses from the pulsar to very high precision. The motion of the pulsar in its orbit modulates the arrival time, thereby enabling its measurement.’
Dr Paulo Freire:
‘We postulated that this might be, at least in-part, due to the so-called “frame-dragging” that all matter is subject to in the presence of a rotating body as predicted by the Austrian mathematicians Lense and Thirring in 1918.’
Professor Thomas Tauris, Aarhus University:
‘In a stellar pair, the first star to collapse is often rapidly rotating due to subsequent mass transfer from its companion. Tauris’s simulations helped quantify the magnitude of the white dwarf’s spin. In this system the entire orbit is being dragged around by the white dwarf’s spin, which is misaligned with the orbit.’
Dr Norbert Wex, Max Planck Institute for Radio Astronomy (MPIfR):
‘One of the first confirmations of frame-dragging used four gyroscopes in a satellite in orbit around the Earth, but in our system the effects are 100 million times stronger.’
Evan Keane (SKA Organisation):
‘Pulsars are super clocks in space. Super clocks in strong gravitational fields are Einstein’s dream laboratories. We have been studying one of the most unusual of these in this binary star system. Treating the periodic pulses of light from the pulsar like the ticks of a clock we can see and disentangle many gravitational effects as they change the orbital configuration, and the arrival time of the clock-tick pulses. In this case we have seen Lens-Thirring precession, a prediction of General Relativity, for the first time in any stellar system.’
From Willem van Straten (AUT):
‘After ruling out a range of potential experimental errors, we started to suspect that the interaction between the white dwarf and neutron star was not as simple as had been assumed to date.’
Click this link to see an animation depicting a neutron star orbiting a rapidly-spinning white dwarf. The white dwarf's spin drags the very fabric of space-time around with it, causing the orbit to tumble in space. Credit: Mark Myers, OzGrav ARC Centre of Excellence.
LINK TO SCIENCE PAPER:
Artificial intelligence is allowing scientists to see the sources of gravitational wave faster and more accurately than ever before. Credit: James Josephide
Following the recent overwhelming success of deep learning and artificial intelligence in several fields of research, industry and medicine, researchers from the ARC Centre of Excellence of Gravitational Wave Discovery (OzGrav) and the University of Western Australia (UWA), including PhD student and the paper’s first author Chayan Chatterjee, have built a deep learning model using an Artificial Neural Network to pinpoint where in the sky gravitational wave signals have come from. The model can localise the source of gravitational waves produced by colliding pairs of black holes potentially as much as a thousand times faster than any other technique.
Professor Amitava Datta, a scientist from UWA who contributed to the study, says: ‘This work is a very interesting example of learning patterns from simulated data for predicting the outcome of real events, in this case the location of gravitational wave sources. Perhaps this approach using deep learning will be more and more useful in basic sciences in general.’
Data intensive astronomy expert Kevin Vinsen from the International Centre for Radio Astronomy Research says: ‘This project is an excellent example of how a multi-disciplinary approach can solve the problem’.
The basic structure of an Artificial Neural Network. The circles represent the neurons or nodes and the arrows represent connections between one neuron to another. Credit: Chayan Chatterjee
Gravitational waves are small ripples in the space-time continuum caused by colossal stellar events such as colliding black holes. In September 2015, following recent advances in detector sensitivity, the LIGO Scientific Collaboration detected gravitational waves for the first time. This was a landmark achievement in human discovery and heralded the birth of the new field of gravitational wave astronomy.
The need for speed and accuracy is particularly important in the context of gravitational wave localisation—scientists need to tell a global network of telescopes where to point on the sky as quickly as possible, so they can see any electromagnetic light that may also have come from the gravitational wave event. The current algorithm used to locate gravitational wave sources in real time takes a few seconds to process. More accurate methods usually take several hours to compute. The light generated by gravitational wave events can be very short-lived at certain wavelengths, like short gamma ray bursts, which last a mere 2-3 seconds, so scientists need methods that can rapidly process huge data as fast and accurately as possible.
The idea behind deep learning is simple: it’s an algorithm designed to mimic the functioning of neurons in our brain to carry out tasks, like categorising observed stimuli. This is done by making the network learn the correlations between a labelled input dataset and the output it is trying to predict. Just like electric signals or synapses flow through neurons in our brain, the input information provided to an Artificial Neural Network travel through layers of nodes (usually several layers deep), with each layer introducing some non-linearity to the input. This non-linearity helps the network learn complex features of the data. The ‘learning’ happens through a rigorous ‘training’ of the network. During the training, the predictions of the network are compared with the true values, and the parameters of the network are adjusted to minimise any erroneous gaps.
In their recently published paper, Chatterjee and the team from UWA successfully trained the Artificial Neural Network to learn the input data for source localisation. The data was pre-processed to extract the important physical Physics parameters from simulated gravitational wave signals, injected into ‘random noise’. The network classified these signals into several classes and accurately identified the source direction of the gravitational waves. The model localised the test samples much faster than other methods and at a low computational cost. The researchers plan to extend this work for pairs of merging neutron stars and neutron star-black hole systems.
Chatterjee says: ‘Hopefully the methods we introduce can also be translated to other areas of research and industry and help further untap the seemingly limitless potential of deep learning and artificial intelligence’.
OzGrav’s Chief Investigator Professor Linqing Wen who led the study says: ‘The future is wide open for gravitational wave discovery using the machine learning technique’.
This paper is an outcome of a multi-disciplinary collaboration of UWA’s Gravitational Wave Astronomy Group led by OzGrav’s Chief Investigator Professor Linqing Wen, data intensive astronomy expert Kevin Vinsen from International Centre for Radio Astronomy Research (ICRAR), and Professor Amitava Datta of UWA’s Computer Science and Software Engineering.
Link to publication: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.100.10302
Artist's impression of the binary neutron star merger producing GW190425. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet.
A new collaborative study with the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) reveals a possible collision of two neutron stars earlier in 2019—only the second time this type of cosmic event had ever been detected. The gravitational-wave observatory network, that includes the National Science Foundation's LIGO and the European Virgo detectors, picked up what appeared to be gravitational ripples from a collision of two neutron stars back on 25 April 2019.
Gravitational waves and light were first witnessed in the same event in 2017. This second event in 2019, called GW190425, did not result in any light being detected; however, researchers have learned that the collision resulted in a merged object with an unusually high mass.
OzGrav postdoctoral researcher Simon Stevenson says: ‘This event is a perfect example of how gravitational-wave astronomy is a completely new and unique way of looking at the Universe. Binaries with similar masses to this event may not exist in the Milky Way or may be completely invisible to conventional radio telescopes’.
Neutron stars are the remnants of dead stars that exploded. When two neutron stars spiral together, they undergo a violent merger that sends gravitational waves shuddering through the fabric of space and time.
The gravitational waves first detected in 2015 were generated by the fierce collision of two black holes. Since then, scientists have registered dozens of new candidate black hole mergers. The first detection of a neutron star merger took place two years later, in 2017.
OzGrav Postdoctoral Researcher Vaishali Adya says: ‘This detection manifests the importance of continued improvement of the already amazingly sensitive gravitational wave detectors, as this event would not have been observable prior to the latest upgrades. OzGrav played a vital role in these upgrades, one of which involved reducing the quantum noise in the detectors’.
OzGrav Postdoctoral Researcher Xingjiang Zhu says: ‘The combined mass of the merging objects is surprisingly high, much greater than any previously known double neutron star binaries including the one detected in 2017. This provokes us to think about the nature of this event and how the source might have been formed’.
The combined mass of the merged bodies in this event is about 3.4 times that of the mass of the Sun. Typically, neutron star collisions are only known to happen between pairs of neutrons stars with a total mass up to 2.9 times that of the Sun. One possibility for the unusually high mass is that the collision took place not between two neutron stars, but a neutron star and a black hole, since black holes are heavier than neutron stars. But if this were the case, the black hole would have to be exceptionally small for its class. Instead, the scientists believe it is more likely that the event was a shattering of two neutron stars and that their merger resulted in a newly formed black hole.
Neutron star pairs are thought to form either early in life—when companion massive stars successively die one by one—or when they come together later in life within dense, busy environments. The data from the 2019 event do not indicate which of these scenarios is more likely—more data and new models are needed to explain the unexpectedly high mass. The discovery suggests that we may have detected an entirely new population of binary neutron star systems.
OzGrav Associate Investigator Greg Ashton says: ‘This event was really interesting. The chirp-like signal was seen by two of the three detectors for about 128 seconds before the final merger. Unfortunately, one of the detectors was not observing at the time, which meant that the sky localization was poor. Perhaps because of this, and because it was so far away, no electromagnetic light was observed from this event. Nevertheless, we saw it very clearly in the gravitational wave data and could use that to calculate the masses, spins, and orientations of the objects’.
‘Additional exciting and unexpected discoveries can be expected as the sensitivity of the LIGO detectors improves. OzGrav is working closely with LIGO to improve their sensitivity, developing new instrumentation and analysis techniques’, says Professor Peter Veitch, University of Adelaide OzGrav Node Leader
The results were announced today at the American Astronomical Society meeting in Honolulu, Hawaii.
The full scientific article will be available here post-embargo: https://dcc.ligo.org/P190425/public