Two neutron star-black hole collision discovery — the two most extreme objects in the Universe

Artist's impression of a neutron star and black hole about to merge. Credit: Carl Knox, OzGrav-Swinburne University.

The first observation of the neutron star-black hole merger was made on 5th January 2020 when gravitational waves -- tiny ripples in the fabric of space and time -- were detected from the collision event by LIGO and Virgo. When masses collide in space, they shake the whole Universe, sending out gravitational waves, like ripples on the surface of a pond. Detailed analysis of the gravitational waves reveal that the neutron star was around twice as massive as the Sun, while the black hole was around nine times as massive as the Sun. The merger itself happened around a billion years ago before the first dinosaurs existed, but the gravitational waves only just reached Earth.

Remarkably, on 15th January 2020 another merger of a neutron star and a black hole was observed from gravitational waves. This neutron star and black hole also collided around a billion years ago, but it was slightly less massive: the neutron star was around one and a half times as massive as the Sun, while the black hole was around five and a half times as massive.

Dr Rory Smith, an astrophysicist from the Monash University School of Physics and Astronomy who is also with the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)  co-led the international team of scientists in this discovery. “It’s an awesome milestone for the nascent field of gravitational-wave astronomy," Dr Smith said.

"Neutron stars merging with black holes are amongst the most extreme phenomena in the Universe. Observing these collisions opens up new avenues to learn about fundamental physics, as well as how stars are born, live and die.”

Thousands of international scientists teamed up for this world-first detection, with Australia playing a leading role. "From the design and operation of the detector, to the analysis of data, Australian scientists are working at the frontiers of astronomy," added Dr Smith. The SPIIR pipeline, at the University of Western Australia (UWA) -- Australia's only real-time gravitational-wave search pipeline -- detected a neutron star-black hole event in real-time for the first time. SPIIR is one of five pipelines that alerts astronomers around the world within seconds of gravitational events, so they can try to catch the potential flash of light emitted when a neutron star is torn apart by its companion black hole.

The Zadko telescope, also based at UWA, was one of the Australian facilities that searched for a counterpart to the merger event and, despite a well-organised search, the team led by Dr Bruce Gendre and Eloise Moore could not secure a localised source. “During the next observing run, many more of these events are expected, providing more opportunities for SPIIR to catch them in real-time, and for astronomers to observe the light from these extreme events,” said OzGrav Postdoctoral Researcher Dr Fiona Panther (UWA).

“Observing a neutron star-black hole merger fills in another missing piece in our collection!” said OzGrav Postdoctoral Researcher Dr Daniel Brown, from the University of Adelaide. “OzGrav expertise and technology played a crucial role in enhancing the performance of the LIGO detectors to enable these fascinating new measurements. Researchers and students spent over 1000 days at the LIGO sites during the last observation run installing new hardware and tuning up the performance. A key contribution was the installation of new hardware to generate what is known as ‘Squeezed light’, which is a special quantum state of light that reduces the noise in our measurements. The Heisenberg uncertainty principle suggests there is a limit to how sensitive we can make the detectors, but by using squeezed light we can get around it.”

Black holes and neutron stars are two of the most extreme objects ever observed in the Universe—they are born from exploding massive stars at the end of their lives. Typical neutron stars have a mass of one and a half times the mass of the Sun, but all of that mass is contained in an extremely dense star, about the size of a city. One teaspoon of a neutron star weighs as much as all of humanity.

OzGrav Postdoctoral Researcher Dr Meg Millhouse, from the University of Melbourne said: “The matter in neutron stars is much more dense than anything we can create in labs on Earth. This new observation is an opportunity to learn more about how matter behaves in these extreme conditions.”

Neutron stars and black holes orbit around each other at around half the speed of light before they collide and merge. This puts the neutron star under extraordinary strain, causing it to stretch and deform as it nears the black hole. How much a neutron star can stretch depends on what kind of matter it’s made of. The amount the star stretches can be decoded from the gravitational waves, which in turn tells us about the type of stuff they’re made of.

Black holes are even more dense objects than neutron stars: they have a lot of mass, normally at least 3 times the mass of our Sun, in a tiny amount of space. Black holes contain an “event horizon” at their surface: a point of no return that not even light can escape.

“Black holes are a kind of cosmic enigma”, said Dr Smith. “The laws of physics as we understand them break down when we try to understand what is at the heart of a black hole. We hope that by observing gravitational waves from black holes merging with neutron stars, or other black holes, we will begin to unravel the mystery of these objects.”

Pairs of neutron stars and black holes have been predicted to exist by theorists for decades, but had long avoided detection. Since their first detection in 1975, many pairs of neutron stars have been found, but never a neutron star orbiting a black hole.

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