​Multimessenger astronomy is an emerging field which aims to study astronomical objects using different ‘messengers’ or sources, like electromagnetic radiation (light), neutrinos and gravitational waves. This field gained enormous recognition after the joint detection of gravitational waves and gamma ray bursts in 2017. Gravitational waves can be used to identify the sky direction of an event in space and alert conventional telescopes to follow-up for other sources of radiation. However, following up on prompt emissions would require a rapid and accurate localisation of such events, which will be key for joint observations in the future.
 
The conventional method to accurately estimate the sky direction of gravitational waves is tedious—taking a few hours to days—while a faster online version needs only seconds. There is an emerging capacity from the LIGO-Virgo collaboration to detect gravitational waves from electromagnetic-bright binary coalescences, tens of seconds before their final merger, and provide alerts across the world. The goal is to coordinate prompt follow up observations with other telescopes around the globe to capture potential electromagnetic flashes within minutes from the mergers of two neutron stars, or a neutron star with a black hole—this was not possible before. The University of Western Australia’s SPIIR team is one of the world leaders in this area of research. Determining sky directions within seconds of a merger event is crucial,as most telescopes need to know where to point in the sky. In our recently accepted paper [1], led by three visiting students (undergraduate and Masters by research) at the OzGrav-UWA node, we applied analytical approximations to greatly reduce the computational time of the conventional localisation method while maintaining its accuracy. A similar semi-analytical approach has also been published in another recent study [2].
 
The results from this work show great potential and will be integrated into the SPIIR online pipeline going forward in the next observing run. We hope that this work complements other methods from the LIGO-Virgo collaboration and that it will be part of some exciting discoveries.
 
Written by OzGrav PhD student Manoj Kovalam, University of Western Australia.
 
[1] https://doi.org/10.1103/PhysRevD.104.104008
[2] https://doi.org/10.1103/PhysRevD.103.043011
This work is now accepted by PRD: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.104008

​The first evidence of the existence of black holes was found in the 1960s, when strong X-rays were detected from a system called Cygnus X-1. In this system, the black hole is orbited by a massive star blowing an extremely strong wind, more than 10 million times stronger than the wind blowing from the Sun. Part of the gas in this wind is gravitationally attracted towards the black hole, creating an ‘accretion disc’, which emits the strong X-rays that we observe. These systems with a black hole and a massive star are called ‘high-mass X-ray binaries’ and have been very helpful in understanding the nature of black holes.

After nearly 60 years since the first discovery, only a handful of similar high-mass X-ray binaries have been detected. Many more of them were expected to exist, especially given that many binary black holes (the future states of high-mass X-ray binaries) have been discovered with gravitational waves in the past few years. There are also many binaries found in our Galaxy that are expected to eventually become a high-mass X-ray binary. So, we see plenty of both the predecessors and descendants, but where are all the high-mass X-ray binaries themselves hiding?

One explanation states that even if a black hole is orbited by a massive star blowing a strong wind, it does not always emit X-rays. To emit X-rays, the black hole needs to create an accretion disc, where the gas swirls around and becomes hot before falling in. To create an accretion disc, the falling gas needs ‘angular momentum’, so that all the gas particles can rotate around the black hole in the same direction. However, we find it is generally difficult to have enough angular momentum falling onto the black hole in high-mass X-ray binaries. This is because the wind is usually considered to be blowing symmetrically, so there is almost the same amount of gas flowing past the black hole both clockwise and counter-clockwise. As a result, the gas can fall into the black hole directly without creating an accretion disc, so the black hole is almost invisible.

But if this is true, why do we see any X-ray binaries at all? In our paper, we solved the equations of motion for stellar winds and we found that the wind does not blow symmetrically when the black hole is close enough to the star. The wind blows with a slower speed in the direction towards and away from the black hole, due to the tidal forces. Because of this break of symmetry in the wind, the gas can now have a large amount of angular momentum, enough to form an accretion disc around the black hole and shine in X-rays. The necessary conditions for this asymmetry are rather strict, so only a small fraction of black hole + massive star binaries will be able to be observed.

The model in our study explains why there are only a small number of detected high-mass X-ray binaries, but this is only the first step in understanding asymmetric stellar winds. By investigating this model further, we might be able to solve many other mysteries of high-mass X-ray binaries.
 
Written by OzGrav Postdoc Ryosuke Hirai, Monash University

​The authors Debatri Chattopadhyay (Swinburne University) and Isobel Romero-Shaw (Monash University)—who are both completing their PhDs in astrophysics with OzGrav—are determined to educate children and young people about the pivotal scientific discoveries and contributions made by women scientists. They also want to encourage more girls, women, and minorities to take up careers in Science, Technology, Engineering, Mathematics and Medicine (STEMM), which is a male-dominated field.

Debatri, who is originally from India, came to Australia pursuing her PhD at Swinburne University in 2017. She was acutely aware of the lack of women in STEMM fields, as both of her parents worked in biological sciences. “My father is a scientist, so I was aware that this was a field I could go into, and he would talk about amazing biologists like Barbara McClintock, but there was almost no representation of female scientists on TV or in newspapers,” she recalls.

“This colouring book will help children learn about the colourful lives and brilliant minds of these amazing women scientists. As a colouring book, it encourages creative minds to think about scientific problems - which is very much needed for problem solving”, says Isobel, who designed the book and illustrated each of the featured scientists. “These women, who made absolutely pioneering discoveries, used their creativity to advance the world as we know it.”

“I did intense research for the biographies of the women featured in the book and at every nook and crevice was amazed at the perseverance they showed. It is for them and countless others, unfortunately undocumented, that we can do what we do today,” says Debatri. “With Christmas approaching, this book is a perfect gift for young children who have a hunger for science. It’s both fun and educational!” she added.

Last year, both Isobel and Debatri were also selected to participate in Homeward Bound, a global program designed to provide cutting-edge leadership training to 1000 women in STEMM over 10 years. To raise awareness of climate change, this journey will also take Isobel and Debatri all the way to Earth’s frozen desert, Antarctica.

The initiative aims to heighten the influence and impact of women with a science background in order to influence policy and decision making as it shapes our planet. In 2017-18, OzGrav Chief Investigator Distinguished Prof Susan Scott was also selected to participate in this program and embark on the journey to Antarctica.

“The saying that ‘you can’t be what you can’t see’ is addressed in this new colouring book,” says Distinguished Prof Scott. “The important women scientists depicted in the book come to life as role models as they are coloured in. Women are underrepresented in physics education and work in Australia. Educating children about women scientists throughout history is an important step in encouraging more girls and women to take up STEMM careers and boost diversity.”

In their “day jobs”, Isobel tries to figure out how the collapsed remains of supergiant stars—black holes and neutron stars—meet up and crash together. She does this by studying the vibrations that these collisions send rippling through space-time—these are called gravitational waves. She also recently published an illustrated book, available on Amazon, called Planetymology: Why Uranus is not called George and other facts about space and words. Planetymology explains the ties between ancient history, astronomy, and language, and introduces the reader to the harsh realities of conditions on other planets.

Debatri is involved in doing simulations in supercomputers of dead stars in binaries or in massive collections of other stellar systems - called globular clusters. Her detailed theoretical calculations help us to understand the astrophysics behind the observations of gravitational waves and radio pulsars, as well as predict what surprising observations might be made in the future. Debatri is also a trained Indian classical dancer and was a voluntary crew member of the Melbourne-based tall ship `Enterprize’.  She has recently submitted her thesis and joined as a postdoctoral fellow at the Gravity Exploration Institute, Cardiff University.

GIVEAWAY: To celebrate the launch of Women in Physics, OzGrav is giving away free copies of the colouring book to three lucky winners. Simply share the book via Twitter (re-tweet) and tag @ARC_OzGrav to be in the draw. Winners will be announced Monday 20 December!

Gravitational waves are cosmic ripples in the fabric of space and time that emanate from catastrophics events in space, like collisions of black holes and neutron stars--the collapsed cores of massive supergiant stars. Extremely sensitive gravitational-wave detectors on Earth, like the Advanced LIGO and Virgo detectors, have successfully observed dozens of gravitational-wave signals, and they’ve also been used to search for dark matter: a hypothetical form of matter thought to account for approximately 85% of all matter in the Universe. Dark matter may be composed of particles that do not absorb, reflect, or emit light, so they cannot be detected by observing electromagnetic radiation. Dark matter is material that cannot be seen directly, but we know that dark matter exists because of the effect it has on objects that we can observe directly.

Ultralight boson particles are a new type of subatomic particle that scientists have put forward as compelling dark matter candidates. However, these ultralight particles are difficult to detect because they have extremely small mass and rarely interact with other matter -- which is one of the key properties that dark matter seems to have.

The detection of gravitational waves provides a new approach to detecting these extremely light boson particles using gravity. Scientists theorise that if there are certain ultralight boson particles near a rapidly spinning black hole, the extreme gravity field causes the particles to be trapped around the black hole, creating a cloud around the black hole. This phenomenon can generate gravitational waves over a very long lifetime. By searching for  these gravitational-wave signals, scientists can finally discover these elusive boson particles, if they do exist, and possibly crack the code of dark matter or rule out the existence of some types of the proposed particles. 

In a recent international study in the LIGO-Virgo-KAGRA collaboration, with OzGrav Associate Investigator Dr Lilli Sun from the Australian National University being one of the leading researchers, a team of scientists carried out the very first all-sky search tailored for these predicted gravitational wave signals from boson clouds around rapidly spinning black holes. 

“Gravitational-wave science opened a completely new window to study fundamental physics. It provides not only direct information about mysterious compact objects in the Universe, like black holes and neutron stars, but also allows us to look for new particles and dark matter,” says Dr Sun.
Although a signal was not detected, the team of researchers were able to draw valuable conclusions about the possible presence of these clouds in our Galaxy. In the analysis, they also took into consideration that the strength of a gravitational wave signal depends on the age of the boson cloud:  the boson cloud shrinks as it loses energy by sending out gravitational waves, so the strength of the gravitational wave signal would decrease as the cloud ages. 

“We learnt that a particular type of boson clouds younger than 1000 years is not likely to exist anywhere in our Galaxy, while such clouds that are up to 10 million years old are not likely to exist within about 3260 light-years from Earth,” says Dr Sun.
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“Future gravitational wave detectors will certainly open more possibilities. We will be able to reach deeper into the Universe and discover more insights about these particles”.
Also featured on Cosmos Magazine and Sci Tech Daily.