​​Astronomers have for the first time used distant galaxies as ‘scintillating pins’ to locate and identify a piece of the Milky Way’s missing matter.
 
For decades, scientists have been puzzled as to why they couldn’t account for all the matter in the universe as predicted by theory. While most of the universe’s mass is thought to be mysterious dark matter and dark energy, 5 percent is ‘normal matter’ that makes up stars, planets, asteroids, peanut butter and butterflies. This is known as baryonic matter.
 
However, direct measurement has only accounted for about half the expected baryonic matter.
 
Yuanming Wang, a doctoral candidate in the School of Physics at the University of Sydney, has developed an ingenious method to help track down the missing matter. She has applied her technique to pinpoint a hitherto undetected stream of cold gas in the Milky Way about 10 light years from Earth. The cloud is about a trillion kilometres long and 10 billion kilometres wide but only weighing about the mass of our Moon.
 
The results, published in the Monthly Notices of the Royal Astronomical Society, offer a promising way for scientists to track down the Milky Way’s missing matter.
 
“We suspect that much of the ‘missing’ baryonic matter is in the form of cold gas clouds either in galaxies or between galaxies,” said Ms Wang, who is pursuing her PhD at the Sydney Institute for Astronomy.
 
“This gas is undetectable using conventional methods, as it emits no visible light of its own and is just too cold for detection via radio astronomy,” she said.
 
What the astronomers did is look for radio sources in the distant background to see how they ‘shimmered’.
 
“We found five twinkling radio sources on a giant line in the sky. Their signals show their light must have passed through the same cold clump of gas,” Ms Wang said.
 
Just as visible light is distorted as it passes through our atmosphere to give stars their twinkle, when radio waves pass through matter, it also affects their brightness. It was this ‘scintillation’ that Ms Wang and her colleagues detected.
 
Dr Artem Tuntsov, a co-author from Manly Astrophysics, said: “We aren’t quite sure what the strange cloud is, but one possibility is that it could be a hydrogen ‘snow cloud’ disrupted by a nearby star to form a long, thin clump of gas.”
 
Hydrogen freezes at about minus 260 degrees and theorists have proposed that some of the universe’s missing baryonic matter could be locked up in these hydrogen ‘snow clouds’. They are almost impossible to detect directly.
 
“However, we have now developed a method to identify such clumps of ‘invisible’ cold gas using background galaxies as pins,” Ms Wang said.
 
Ms Wang’s supervisor, Professor Tara Murphy, said: “This is a brilliant result for a young astronomer. We hope the methods trailblazed by Yuanming will allow us to detect more missing matter.”
 
The data to find the gas cloud was taken using the CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope in Western Australia.
 
Dr Keith Bannister, Principal Research Engineer at CSIRO, said: “It is ASKAP’s wide field of view, seeing tens of thousands of galaxies in a single observation that allowed us to measure the shape of the gas cloud.”
 
Professor Murphy said: “This is the first time that multiple ‘scintillators’ have been detected behind the same cloud of cold gas. In the next few years, we should be able to use similar methods with ASKAP to detect a large number of such gas structures in our galaxy.”
 
The research was done in collaboration with CSIRO, Manly Astrophysics, the University of Wisconsin-Milwaukee and the ARC Centre of Excellence for Gravitational Wave Discovery, OzGrav.

Media release written and edited by The University of Sydney media office. 

Congratulations to OzGrav researchers Adam Deller, Ryan Shannon, Cherie Day, Stefan Oslowski, Chris Flynn, Wael Farah, on receiving the Newcomb Cleveland prize from the AAAS. The award is for the best paper published in Science Magazine in the last year and was awarded for their paper that presented the discovery of the first localised one-off FRB: 'A single fast radio burst localized to a massive galaxy at cosmological distance'. 

Full media release by CSIRO here: https://www.csiro.au/en/News/News-releases/2021/In-the-blink-of-an-eye-astronomers-win-prestigious-American-science-prize 

 
A recent study by an international team of scientists—led by the Galician Institute of High Energy Physics, the University of Aveiro, and including OzGrav researchers—shows that the “heaviest black hole collision” ever observed might be something even more mysterious—dark matter.

Gravitational waves are ripples in the fabric of space-time that travel at the speed of light. Predicted in Einstein’s General Theory of Relativity, they originate in the most violent events of our Universe, carrying information about their sources. Since 2015, humankind can observe and interpret gravitational waves thanks to the two Advanced LIGO detectors (Livingston and Hanford, USA) and the Advanced Virgo detector (Cascina, Italy). To date, these detectors have already observed around 50 gravitational-wave signals which originated in the coalescence and merger of two of the most mysterious entities in the Universe—black holes and neutron stars—deepening our knowledge of the Universe.

Gravitational wave astronomy could eventually provide us with evidence for previously unobserved or unexpected objects and shed light on current open issues, like the nature of dark matter—a discovery that may have already happened.

In September 2020, the LIGO and Virgo collaborations (LVC) announced the gravitational-wave signal called GW190521. The signal was consistent with the collision of two black holes of 85 and 66 times the mass of the Sun, which produced a final 142 solar mass black hole—this was the first-ever detected intermediate-mass black hole. This discovery was extremely important as intermediate black holes were long considered the missing link between two well-known black-hole families: the stellar-mass black holes, that form from the collapse of stars, and the supermassive black holes, that hide in the centre of almost every galaxy.

Despite its significance, the observation of GW190521 posed an enormous challenge to scientists’ understanding of stellar evolution: the life and death of stars is significantly more massive than our Sun. If this is correct, the heaviest of the two colliding black holes shouldn’t have occurred as the end-result of the gravitational collapse of a massive star.
 
In an article recently published in Physical Review Letters, a team of scientists lead by OzGrav alumnus Dr Juan Calderón Bustillo, (now “La Caixa Junior Leader - Marie Curie Fellow”, at the Galician Institute of High Energy Physics) and Dr Nicolás Sanchis-Gual, a postdoctoral researcher at the University of Aveiro and at the Instituto Superior Técnico (University of Lisbon), together with OzGrav researchers from Monash University Dr Rory Smith and Avi Vajpeyi, and collaborators from the University of Valencia and The Chinese University of Hong Kong, has proposed an alternative explanation for the origin of the signal GW190521: the collision of two exotic compact objects known as boson stars. Such hypothetical stars are among the simplest exotic compact objects proposed, and present as well-founded dark matter candidates. Within this interpretation, the team estimated the mass of a new particle constituent of these stars: an ultra-light boson with a mass billionths of times smaller than that of the electron.
 
Dr Nicolás Sanchis-Gual, explains: “Boson stars are objects almost as compact as black holes, but they don’t have a ‘no-return’ surface, or event horizon. When they collide, they form a boson star that can become unstable, eventually collapsing to a black hole, and producing a signal consistent with what LIGO and Virgo observed. Unlike regular stars, which are made of what we commonly know as matter, boson stars are made up of what we know as ultralight bosons. These bosons are one of the most appealing candidates for constituting dark matter, which forms ~27% of the Universe.”
 
The team compared the GW190521 signal to computer simulations of boson-star mergers and found that these explain the data slightly better than the analysis conducted by LIGO and Virgo. The result implies that the source would have different properties than stated earlier. Dr Calderón Bustillo explains: “First, we wouldn’t be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole. Second, because boson star mergers are much weaker, we infer a much closer distance than the one estimated by LIGO and Virgo. This leads to a much larger mass for the final black hole, of about 250 solar masses, so the fact that we have witnessed the formation of an intermediate-mass black hole remains true”.
 
The team found that even though the analysis tends to favour “by design” the merging black-holes hypothesis, a boson star merger is actually preferred by the data, although in a non-conclusive way. Professor José A. Font from the University of Valencia says: “Our results show that the two scenarios are almost indistinguishable given the data, although the exotic boson-star one is slightly preferred. This is very exciting since the computational framework of our current boson-star simulations is still fairly limited and subject to major improvements. A more evolved model might lead to even larger evidence for the boson-star scenario and would also allow us to study similar gravitational-wave observations under the boson-star merger assumption”.
 
This result would not only involve the first observation of boson stars, but also that of their building block, a new particle known as ultra-light boson. Such ultra-light bosons have been proposed as the constituents of what we know as dark matter, which makes up around 27% of the observable Universe. Professor Carlos Herdeiro, from University of Aveiro says that “one of the most fascinating results is that we can actually measure the mass of this putative new dark-matter particle, and that a value of zero is discarded with high confidence. If confirmed by subsequent analysis of this and other gravitational-wave observations, our result would provide the first observational evidence for a long-sought dark matter candidate”.
 
OzGrav researcher Dr Rory Smith adds: “Gravitational-wave astronomy is still very much in its infancy. However, the fact that we are already able to start drawing connections between gravitational-wave observations and fundamental particle physics is a remarkable sign of how powerful this new field is. Even if future observations rule out boson stars as real astronomical objects, we should expect many new and exciting discoveries in the future”. 

Written by Dr Juan Calderón Bustillo. Also featured on Space Australia.

In the moments immediately following the Big Bang, the very first gravitational waves rang out. The product of quantum fluctuations in the new soup of primordial matter, these earliest ripples through the fabric of space-time were quickly amplified by inflationary processes that drove the universe to explosively expand.

Primordial gravitational waves, produced nearly 13.8 billion years ago, still echo through the Universe today. But they are drowned out by the crackle of gravitational waves produced by more recent events, such as colliding black holes and neutron stars.
​
Now a team of international scientists, including reasearchers from the Massachusetts Institute of Technology and OzGrav, has developed a method to tease out the very faint signals of primordial ripples from gravitational-wave data. Their results were published in Physical Review Letters.
Gravitational waves are being detected on an almost daily basis by LIGO and other gravitational-wave detectors, but primordial gravitational signals are several orders of magnitude fainter than what these detectors can register. It’s expected that the next generation of detectors will be sensitive enough to pick up these earliest ripples.

In the next decade, as more sensitive instruments come online, the new method could be applied to dig up hidden signals of the Universe’s first gravitational waves. The pattern and properties of these primordial waves could then reveal clues about the early universe, such as the conditions that drove inflation.

‘If the strength of the primordial signal is within the range of what next-generation detectors can detect, which it might be, then it would be a matter of more or less just turning the crank on the data, using this method we’ve developed,’ says Sylvia Biscoveanu—MIT graduate student and the study’s lead author. ‘These primordial gravitational waves can then tell us about processes in the early Universe that are otherwise impossible to probe.’ OzGrav researchers Colm Talbot, Eric Thrane and Rory Smith were also co-authors of the study. 

The hunt for primordial gravitational waves has concentrated mainly on the cosmic microwave background, or CMB, which is thought to be radiation that is leftover from the Big Bang. Scientists believe that when primordial gravitational waves rippled out, they left an imprint on the CMB, in the form of B-modes, a type of subtle polarization pattern.Physicists have looked for signs of B-modes, most famously with the BICEP Array, a series of experiments including BICEP2, which in 2014 scientists believed had detected B-modes; however, the signal turned out to be due to galactic dust.
As scientists continue to look for primordial gravitational waves in the CMB, others are hunting the ripples directly in gravitational-wave data. The general idea has been to try and subtract away the ‘astrophysical foreground’—any gravitational-wave signal that arises from an astrophysical source, such as colliding black holes, neutron stars, and exploding supernovae. Only after subtracting this astrophysical foreground can physicists get an estimate of the quieter, nonastrophysical signals that may contain primordial waves.

The problem with these methods, Biscoveanu says, is that the astrophysical foreground contains weaker signals that are too faint to discern and difficult to estimate in the final subtraction.
In their study, the researchers used a predictive model to describe the more obvious ‘conversations’ of the astrophysical foreground. The team used this more accurate model to create simulated data of gravitational wave patterns and then characterize every astrophysical signal. Once they identified distinct, nonrandom patterns in gravitational-wave data, they were left with more random primordial gravitational-wave signals and instrumental noise specific to each detector. Applying their new methods, the team was able to fit both the foreground and the background at the same time, so the background signal didn’t get contaminated by the residual foreground. 
Biscoveanu says she hopes that once more sensitive, next-generation detectors come online, the new method can be used to cross-correlate and analyse data from two different detectors, to sift out the primordial signal. Then, scientists may have a useful thread they can trace back to the conditions of the early Universe.

This article is an edited extract from the original article featured on MIT’s news website written by Jennifer Chu. 

​Astronomers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and CSIRO have just observed bizarre, never-seen-before behaviour from a ‘radio-loud’ magnetar—a rare type of neutron star and one of the strongest magnets in the Universe.
 
Their new findings, published today in the Monthly Notices of the Royal Astronomical Society (MNRAS), suggest magnetars have more complex magnetic fields than previously thought – which may challenge theories of how they are born and evolve over time.
 
Magnetars are a rare type of rotating neutron star with some of the most powerful magnetic fields in the Universe. Astronomers have detected only thirty of these objects in and around the Milky Way—most of them detected by X-ray telescopes following a high-energy outburst.
 
However, a handful of these magnetars have also been seen to emit radio pulses similar to pulsars—the less magnetic cousins of magnetars that produce beams of radio waves from their magnetic poles. Tracking how the pulses from these ‘radio-loud’ magnetars change over time offers a unique window into their evolution and geometry.
 
In March 2020, a new magnetar named Swift J1818.0-1607 (J1818 for short) was discovered after it emitted a bright X-ray burst. Rapid follow-up observations detected radio pulses originating from the magnetar. Curiously, the appearance of the radio pulses from J1818 were quite different to those seen from other radio-loud magnetars.
 
Most radio pulses from magnetars maintain a consistent brightness across a wide range of observing frequencies. However, the pulses from J1818 were much brighter at low frequencies than high frequencies – similar to what is seen in pulsars, another more common type of radio-emitting neutron star.
 
In order to better understand how J1818 would evolve over time, a team led by scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) observed it eight times using the CSIRO Parkes radio telescope (also known as Murriyang) between May and October 2020.
 
During this time, they found the magnetar underwent a brief identity crisis: in May it was still emitting the unusual pulsar-like pulses that had been detected previously; however, by June it had started flickering between a bright and a weak state. This flickering behaviour reached a peak in July where they saw it flicking back and forth between emitting pulsar-like and magnetar-like radio pulses.
 
“This bizarre behaviour has never been seen before in any other radio-loud magnetar,” explains study lead author and Swinburne University/CSIRO PhD student Marcus Lower. “It appears to have only been a short-lived phenomenon as by our next observation it had settled permanently into this new magnetar-like state.”

The scientists also looked for pulse shape and brightness changes at different radio frequencies and compared their observations to a 50-year-old theoretical model. This model predicts the expected geometry of a pulsar, based on the twisting direction of its polarised light.
“From our observations, we found that the magnetic axis of J1818 isn’t aligned with its rotation axis,” says Lower.

“Instead, the radio-emitting magnetic pole appears to be in its southern hemisphere, located just below the equator. Most other magnetars have magnetic fields that are aligned with their spin axes or are a little ambiguous.”

“This is the first time we have definitively seen a magnetar with a misaligned magnetic pole.”
Remarkably, this magnetic geometry appears to be stable over most observations. This suggests any changes in the pulse profile are simply due to variations in the height the radio pulses are emitted above the neutron star surface. However, the August 1st 2020 observation stands out as a curious exception.

“Our best geometric model for this date suggests that the radio beam briefly flipped over to a completely different magnetic pole located in the northern hemisphere of the magnetar,” says Lower.
A distinct lack of any changes in the magnetar’s pulse profile shape indicate the same magnetic field lines that trigger the ‘normal’ radio pulses must also be responsible for the pulses seen from the other magnetic pole.

The study suggests this is evidence that the radio pulses from J1818 originate from loops of magnetic field lines connecting two closely spaced poles, like those seen connecting the two poles of a horseshoe magnet or sunspots on the Sun. This is unlike most ordinary neutron stars which are expected to have north and south poles on opposite sides of the star that are connected by a donut-shaped magnetic field.

This peculiar magnetic field configuration is also supported by an independent study of the X-rays pulses from J1818 that were detected by the NICER telescope on board the International Space Station. The X-rays appear to come from either a single distorted region of magnetic field lines that emerge from the magnetar surface or two smaller, but closely spaced, regions.
​
These discoveries have potential implications for computer simulations of how magnetars are born and evolve over long periods of time, as more complex magnetic field geometries will change how quickly their magnetic fields are expected to decay over time. Additionally, theories that suggest fast radio bursts can originate from magnetars will have to account for radio pulses potentially originating from multiple active sites within their magnetic fields.
 
Catching a flip between magnetic poles in action could also afford the first opportunity to map the magnetic field of a magnetar.
 
"The Parkes telescope will be watching the magnetar closely over the next year" says scientist and study co-author Simon Johnston, from the CSIRO Astronomy and Space Science.

​The merger of two black holes in a binary system emits energy that can be detected on Earth by gravitational-wave observatories. The LIGO Scientific Collaboration and the VIRGO Collaboration have announced tens of confident detections of such mergers to date. Now, one of the main questions we can try to address concerns the origin of such merging binaries: do they come from isolated binary stars or from dense stellar environments? The answer  might not be that simple.
 
A recent study published in the Astrophysical Journal Letters, led by OzGrav Alumni (Monash University) Dr. Alejandro Vigna-Gómez—and current DARK Fellow at the Niels Bohr Institute—shows that some binary black holes can originate from triple stellar systems. A triple stellar system consists of an inner binary and a triple stellar companion orbiting around it. If the inner binary is close enough, it can become a binary black hole which rapidly merges. The product of a binary black hole merger is a single rotating black hole. The merger of the inner binary black hole transforms the initial triple system to a binary, which itself might be able to merge within the age of the Universe. However, the assembly of these triple systems is not as simple as it sounds, as they need to be formed at low metallicities. 

​Astronomers consider metals to be all elements except hydrogen and helium. Low metallicity environments are those in which hydrogen and helium compose more than approximately 99% of matter. Scientists believe that rare compact stars exist in low metallicity environments. In these environments, rapid rotation and mixing stir the stellar fuel and restrict chemically homogeneously-evolving stars from expanding. Additionally, metallicity increases alongside the age of the Universe, and therefore compact stars are more likely to be formed in the distant past.
 
Vigna-Gómez and collaborators studied the properties of such sequential binary black hole mergers and conclude that GW170729, one of the detected signals of a binary black hole merger, might be of triple stellar origin. The progenitor of GW170729 has at least one rapidly spinning black hole, plausibly from a previous merger.
 
Moreover, the masses are consistent with those of chemically homogeneously evolving stars, and the inferred formation time coincideswith the time it would take a triple system to experience two sequential mergers. Future observations from gravitational-wave observatories will help to further probe this formation channel and, more in general, understand the origin of binary black hole mergers.
 
 Written by OzGrav alumnus Dr Alejandro Vigna-Gomez 

A group of international scientists, including an Australian astrophysicist, has used knowhow from gravitational wave astronomy (used to find black holes in space) to study ancient marine fossils as a predictor of climate change.

The research, published in the journal Climate of the Past, is a unique collaboration between palaeontologists, astrophysicists and mathematicians – to improve the accuracy of a palaeo-thermometer, which can use fossil evidence of climate change to predict what is likely to happen to the Earth in coming decades.

Professor Ilya Mandel, from the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav), and colleagues, studied biomarkers left behind by tiny single-cell organisms called archaea in the distant past, including the Cretaceous period and the Eocene.

Marine archaea in our modern oceans produce compounds called Glycerol Dialkyl Glycerol Tetraethers (GDGTs). The ratios of different types of GDGTs they produce depend on the local sea temperature at the site of formation.

When preserved in ancient marine sediments, the measured abundances of GDGTs have the potential to provide a geological record of long-term planetary surface temperatures.

To date, scientists have combined GDGT concentrations into a single parameter called TEX86, which can be used to roughly estimate the surface temperature.  However, this estimate is not very accurate when the values of TEX86 from recent sediments are compared to modern sea surface temperatures. 
“After several decades of study, the best available models are only able to measure temperature from GDGT concentrations with an accuracy of around 6 degrees Celsius,” Professor Mandel said.  Therefore, this approach cannot be relied on for high-precision measurements of ancient climates.

Professor Mandel and his colleagues at the University of Birmingham in the UK have applied  modern machine-learning tools -- originally used in the context of gravitational-wave astrophysics to create predictive models of merging black holes and neutron stars -- to improve temperature estimation based on GDGT measurements. This enabled them to take all observations into account for the first time rather than relying on one particular combination, TEX86.  This produced a far more accurate palaeo-thermometer.  Using these tools,  the team extracted temperature from GDGT concentrations with an accuracy of just 3.6 degrees – a significant improvement, nearly twice the accuracy of previous models.

According to Professor Mandel, determining how much the Earth will warm in coming decades relies on modelling, “so it is critically important to calibrate those models by utilising literally hundreds of millions of years of climate history to predict what might happen to the Earth in the future,” he said.
 

A discovery that links stellar flares with radio-burst signatures will make it easier for astronomers to detect space weather around nearby stars outside the Solar System. Unfortunately, the first weather reports from our nearest neighbour, Proxima Centauri, are not promising for finding life as we know it.

“Astronomers have recently found there are two ‘Earth-like’ rocky planets around Proxima Centauri, one within the ‘habitable zone’ where any water could be in liquid form,” said Andrew Zic from the University of Sydney. Proxima Centauri is just 4.2 light years from Earth.
“But given Proxima Centauri is a cool, small red-dwarf star, it means this habitable zone is very close to the star; much closer in than Mercury is to our Sun,” he said. “What our research shows is that this makes the planets very vulnerable to dangerous ionising radiation that could effectively sterilise the planets.”

Led by Mr Zic, under the supervision of OzGrav Associate Investigator Professor Tara Murphy, astronomers have for the first time shown a definitive link between optical flares and radio bursts on a star that is not the Sun. The finding, published in The Astrophysical Journal, is an important step to using radio signals from distant stars to effectively produce space weather reports.
“Our own Sun regularly emits hot clouds of ionised particles during what we call ‘coronal mass ejections’. But given the Sun is much hotter than Proxima Centauri and other red-dwarf stars, our ‘habitable zone’ is far from the Sun’s surface, meaning the Earth is a relatively long way from these events,” Mr Zic said. “Further, the Earth has a very powerful planetary magnetic field that shields us from these intense blasts of solar plasma.”

The research was done in collaboration with CSIRO, the University of Western Australia, University of Wisconsin-Milwaukee, University of Colorado and Curtin University, with contributions from the ARC Centre for Gravitational Wave Discovery (OzGrav) and University of California Berkeley. 
Zic said: “M-dwarf radio bursts might happen for different reasons than on the Sun, where they are usually associated with coronal mass ejections. But it’s highly likely that there are similar events associated with the stellar flares and radio bursts we have seen in this study.”

Coronal mass ejections are hugely energetic expulsions of ionised plasma and radiation leaving the stellar atmosphere. “This is probably bad news on the space weather front. It seems likely that the galaxy’s most common stars – red dwarfs – won’t be great places to find life as we know it,” Mr Zic said.
In the past decade, there has been a renaissance in the discovery of planets orbiting stars outside our Solar System. There are now more than 4000 known exoplanets. This has boosted hopes of finding ‘Earth-like’ conditions on exoplanets. Recent research says that about half the Sun-like stars in the Milky Way could be home to such planets. However, Sun-like stars only make up 7 percent of the galaxy’s stellar objects. By contrast, M-type red dwarfs like Proxima Centauri make up about 70 percent of stars in the Milky Way. The findings strongly suggest planets around these stars are likely to be showered with stellar flares and plasma ejections.

 Methodology 

The Proxima Centauri observations were taken with the CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) telescope in Western Australia, the Zadko Telescope at the University of Western Australia and a suite of other instruments.

OzGrav scientist Dr Bruce Gendre, from the University of Western Australia, said the research helps understand the dramatic effects of space weather on solar systems beyond our own.

“Understanding space weather is critical for understanding how our own planet biosphere evolved – but also for what the future is,” Dr Gendre said.

Professor Murphy said: “This is an exciting result from ASKAP. The incredible data quality allowed us to view the stellar flare from Proxima Centauri over its full evolution in amazing detail.

“Most importantly, we can see polarised light, which is a signature of these events. It’s a bit like looking at the star with sunglasses on. Once ASKAP is operating in full survey mode we should be able to observe many more events on nearby stars.”

This will give us much greater insights to the space weather around nearby stars.

Other facilities, including NASA’s planet-hunting Transiting Exoplanet Survey Satellite and the Zadko Telescope observed simultaneously with ASKAP providing the crucial link between the radio bursts and powerful optical flares observed.

Mr Zic said: “The probability that the observed solar flare and received radio signal from our neighbour were not connected is much less than one chance in 128,000.”

The research shows that planets around Proxima Centauri may suffer strong atmospheric erosion, leaving them exposed to very intense X-rays and ultraviolet radiation.

But could there be magnetic fields protecting these planets?

Mr Zic said: “This remains an open question. How many exoplanets have magnetic fields like ours?”

So far there have been no observations of magnetic fields around exoplanets and finding these could prove tricky. Mr Zic said one potential way to identify distant magnetic fields would be to look for aurorae, like those around Earth and also witnessed on Jupiter.

“But even if there were magnetic fields, given the stellar proximity of habitable zone planets around M-dwarf stars, this might not be enough to protect them,” Mr Zic said.

This is an extract of a media release originally written by the Media Office at the University of Sydney. 

​In 2015, the first direct observation of gravitational waves was made by the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) (Abbott et al. 2016). The detected signal, known as GW150914, not only provided the most rigorous test of General Relativity—Einstein’s theory of gravity—it was also the first observation of two black holes merging. This confirmed the existence of black holes, binary stellar-mass black hole systems, and proved that they can merge within the current age of the Universe.
​
Since that first detection, aLIGO has detected many more black hole mergers but scientists are still puzzled: many of the black hole mergers that aLIGO has detected so far, including GW150914, involve systems with the component black holes weighing-in at more than 20 solar masses each—some much more. These are massive black holes—heavier than any previously known black holes from X-ray binary observations—which raises the question: how did they form? Our recent study seeks to answer that question.

aLIGO records (part of) the inspiral, merger, and ring-down of the orbiting black holes—as shown in figure 1[JR1] . aLIGO can only record these events if they occur within its operating lifetime—that means that the progenitor stars must collapse to black holes before they merge, and the subsequent orbiting black holes must spiral in towards each other and merge within the age of the Universe. For that to happen, the black holes must be big and  close together; however, progenitor stars big enough and close enough to produce a binary black hole (BBH) system that would spiral in and eventually merge within the age of the universe, and that could generate gravitational waves detectable by aLIGO, would be too big and too close; so these progenitor stars merge first and then collapse  into black holes.

Thus, the aLIGO detections raise the intriguing questions: how did the black holes get so massive? and how did they get so close together?
​
Chemically Homogeneous Evolution (CHE) has been proposed as a possible solution. Rapid spinning stirs a star leading to its interior becoming homogeneous, and possibly fusing entirely rather than just the core. A hot, rapidly spinning, chemically homogeneous star doesn’t expand as it ages in the way a conventionally evolving star does. The chemically homogeneous model begins with a pair of close, massive stars rotating around each other extremely rapidly—so close that they become tidally locked, causing the stars to spin rapidly on their own axes. The component stars of this massive, overcontact binary eventually collapse into massive black holes, which are now close enough to spiral in and merge within the age of the Universe.

For the first time, we simultaneously explore conventional isolated binary star evolution and chemically homogeneous evolution under the same set of assumptions. This approach allows us to constrain population properties and make simultaneous predictions about the gravitational-wave detection rates of BBH mergers for the CHE and conventional formation channels.

This joint model for the classical and CHE isolated binary evolution channels will enable simultaneous inference on binary evolution model parameters and the metallicity-specific star formation history once the full trove of observations from the aLIGO third gravitational-wave observing run is available. Ultimately, the relatively short delay times of CHE BBHs make them ideal probes of high-redshift star formation history; their high masses make them perfect targets for third-generation gravitational-wave detectors with good low-frequency sensitivity, such as the Einstein Telescope or the Cosmic Explorer.

Written by OzGrav PhD student Jeff Riley - Monash University

Link: https://arxiv.org/abs/2010.00002

Over the past five years, astronomy has been revolutionised as scientists have used ripples in the fabric of spacetime, called gravitational waves, to reveal the secrets of the previously hidden world of black holes. Gravitational waves are created when two black holes merge in a cataclysmic release of energy, but until now, there were few clues as to how and why black holes merge.

Today, researchers from the LIGO and Virgo Collaborations announced a series of discoveries providing some of the first hints as to the origin of black hole mergers. Researchers from Monash University––members of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)––helped lead the effort.

“We are announcing the discovery of 44 confirmed black hole mergers, which is a more than a four-fold increase in the number of previously known gravitational-wave signals,” explains Monash University School of Physics and Astronomy PhD student, Shanika Galaudage, who helped write one of the new LIGO papers.

“With so many black holes to study, we can start to answer deep questions about how these systems came to merge," Ms Galaudage says.

A key clue comes from the fact that black holes spin. The orientation of the black hole spins affects the gravitational-wave signal.

Study author Dr Colm Talbot, also from the School of Physics and Astronomy, says “There are two theories for how two black holes can get together. Sometimes, pairs of stars called binaries make pairs of black holes that merge, creating ripples in spacetime called gravitational waves. Alternatively, two black holes can stumble into each other.”

The verdict? “It seems there are multiple ways for two black holes to get together,” says OzGrav Chief Investigator Professor Eric Thrane from Monash University.

“Some binary black holes are born from pairs of stars. Others wander the cosmos before finding a partner to merge with. Either way, a tremendous amount of energy is released in gravitational waves.”