The First Neutron Star-Black Hole Mergers Confirmed

Long before humans were around, before mammals became dominant, even before the dinosaurs roamed the Earth, a violent event occurred in the Universe that would shake the very fabric of space-time. 10 days later, it happened again.

Waves from these events travelled for over a billion years, before eventually passing over our world in January 2020 – stretching and squeezing the planets, as well as all of us – as they washed past us at the speed of light. You likely didn’t feel any different during either of these days, because the distance you were stretched and squeezed was roughly equal to 1/10,000^th the diameter of a proton. In other words, a glitch on an atomic scale.

The things that caused these distortions across all of your atoms, as well as that of the Earth and other planets, stars, and moons, were gravitational waves – ripples that emanated from a powerful event that involved extremely massive and exotic objects in the Universe.

Now, we’re normally flooded with these waves washing over us from all directions, caused by a number of different phenomena and sources – like merging binary stellar-mass black holes, or inspiral supermassive black hole binaries – but these two cases were special.

That’s because, for the first time, these particular gravitational waves that passed over the Earth on the 5^th of January 2020, then again (remarkably) on the 15^th of January 2020 were caused by the inspiral of two merging neutron star-black hole systems, as announced by astronomers from the LIGO/Virgo Scientific Collaboration (LSC) today.  

“It’s an awesome milestone for the nascent field of gravitational-wave astronomy. 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 the nature of space and time, as well as how stars are born, live, and die,” said Dr Rory Smith, an astrophysicist at the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University, who co-led the international team of scientists in this discovery.  

Scientists from the LSC have been catching direct evidence of merging binary black holes and merging neutron star events which create gravitational waves since 2015 (and we’ve had indirect evidence of gravitational radiation from a pulsar-neutron star system since the mid-1970s) but these two separate events were the first confirmations of a neutron star merging with a black hole, which resulted in a larger black hole. 

These discoveries now also add to the LSC catalogue of detections based within the gravitational wave (GW) frequency the array of global interferometers can observe in – which tends to be in the higher range part of the GW spectrum, produced by massive compact objects in inspiral short-period orbits (seconds to milliseconds).  

Deeply involved in the discovery was an army of Australian scientists and institutions, which form part of the global gravitational wave astronomy community – especially from across OzGrav. "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. This included the interpretation of the signal in real-time, as well as the rapid positioning of optical telescopes to determine if a visual counterpart to the events could be observed.

As part of the University of Western Australia (UWA), the SPIIR (Summed Parallel Infinite Impulse Response) pipeline made the detection of the merger event for the first time. As Australia’s only real-time gravitational-wave search pipeline (one of five pipelines that alert astronomers around the world within seconds of gravitational events), these real-time alerts are generated and distributed to astronomers, allowing them to turn their telescopes towards the location to catch any other signals, helping reveal more about the event.

Additionally, the Zadko telescope (also based at UWA) was one of Australia’s optical facilities that conducted a well-organised search but unfortunately came up empty-handed. Several scientists from across OzGrav also participated in the data analysis and research that both merger events generated, joining thousands of international scientists as part of the LSC.

“Observing a neutron star-black hole merger fills in another missing piece in our collection!” says 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 1,000 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.”

Since Einstein’s General Theory of Relativity in the early 20^th century, black holes and neutron stars have captivated the imagination of scientists and the public alike. These mysterious objects tend to defy the laws of physics with extreme gravity, density, and rotation – unlike anything we will ever come to experience in our everyday lives around us.

“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.”

“[And] 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,” added OzGrav Postdoctoral Researcher Dr Meg Millhouse, from the University of Melbourne.

Astronomers have for decades been trying to learn more about these compact massive objects, to help answer questions to physics that are yet to be resolved – such as how gravity can work on a quantum scale at the heart of a black hole, or how matter behaves under extreme pressures, such as those found in the internal layers of a neutron star. Additionally, how these objects affect their environments and any binary companions they might have, as well as the evolution of these types of systems.

The special results outlined in this paper, however, also present a sample of two systems in which the binary pair is made up of a black hole and a neutron star – something, that to date, has not yet been observed, in particular through the gravitational wave spectrum.

"This is a confirmation of a long-standing prediction from binary stellar evolution theory which predicted these systems should exist!" said Dr Simon Stevenson, OzGrav Postdoctoral Researcher at Swinburne University of Technology.

"We find that roughly one pair of neutron star-black holes merges for every ten pairs of neutron stars. This raises the possibility of observing a neutron star-black hole containing a pulsar - a rapidly rotating neutron star pulsing radio waves - in our own Milky Way using radio telescopes like the Australian Parkes radio telescope and the future Square Kilometre Array," he said.

The first detection, made on the 5^th January 2020 (and named GW200105) included a primary object – the black hole with a mass of approximately nine times that of our Sun. The secondary mass in this system, the neutron star, weighed in at almost two solar masses. 

The second detection, occurring only 10 days later on 15^th January 2020 (and named GW200115) featured a smaller black hole, almost six times the mass of our Sun, and a slightly smaller neutron star, about 1.5 solar masses. 

Both events were located just under one billion light-years away from Earth when they occurred and would have created a higher-mass black hole in each system, after the merger event (neutron stars have an upper mass limit of roughly two and a half solar masses, so anything above this value would be considered a black hole).

Whilst analysis of the data from these events did not place any stronger constraints on general relativity tests compared to the other LSC documented events (as the signal-to-noise ratio for both events was too low), these detections also didn’t show any deviations from historical findings, adding further support to Einstein’s theory.

Within the last few moments prior to the merger of neutron star-black hole events, there’s a possibility that the black holes deform the neutron stars companion through tidal forces, potentially forming an accretion disc and relativistic jets that emanate from these systems. These events could theoretically have created an electromagnetic counterpart signal for both GW200105 and GW200115, but unfortunately due to both the large distance from us, and the large uncertainty in localising them, it would have been near impossible to have detected any electromagnetic emission.

Catching this counterpart signal has proved extremely fruitful for scientists across the world in the past, as was the result of the merging double neutron star event in 2017 (GW170817) in which both the gravitational wave signal and the electromagnetic emission from the subsequent explosion were studied in high detail.

Optical telescopes located around the world, much like the Skymapper telescope in central NSW, keep a close eye on the sky and are activated to look for a bright flash in the vast cosmos, as soon as a GW candidate event is announced.

Skymapper itself is a fully automated wide-angle optical telescope, operated by Australian National University (ANU) and features a 1.35-metre diameter, with a 1.1-arcsecond angular resolution. It’s located at the Siding Spring Observatory in NSW, and in 2014 discovered one of the oldest stars in the Universe, aged at 13.6 billion years old. 

"It would’ve been so exciting for SkyMapper to help pin down the visible counterpart to one of these events. Unfortunately, it was the clouds that aligned, rather than the stars,” said Dr Christopher Onken, Research Fellow and SkyMapper Operations Manager at ANU. 

“The next time that a neutron star-black hole merger is discovered with gravitational waves, SkyMapper will hopefully have a clear, dark sky to search for the flash of light. When we eventually find one, it will be a wonderful new way to learn about the physical properties of the densest stars in the Universe.”

Both neutron stars and stellar-mass black holes are formed through a similar mechanism – when a massive progenitor star ends its life it explodes in a violent detonation known as a  supernova, casting the majority of the former star’s matter outwards into the surrounding region, whilst simultaneously collapsing and crushing the remaining core inwards.

What differentiates between which of these (a black hole or a neutron star) becomes the remnant object is the original mass of the progenitor star – with a rough range of 8 – 25 solar masses usually resulting in the remnant core becoming a neutron star and 20 – 25+ solar masses, usually resulting in the remnant core turning into a black hole. Important to note, that the entire mass of the progenitor star does not become the remnant object, only the core component that collapses inwards. 

The lower mass range that results in the formation of a neutron star doesn’t quite get up to the collapsing power required to overcome neutron degeneracy pressure, which thus produces the neutron star. However, the larger mass range of the progenitor star’s core is well suited to press right through this barrier, creating the black hole.

What is equally as fascinating is how these systems came to be in the first place. It’s well known that most massive stars in the Universe are likely in a binary pair (or sometimes in a system of several stars) – so a pathway to the evolution of these systems must be considered. Double neutron star systems have been detected since 1975, but a neutron star and black hole system has never been announced, until now.

One pathway could be that the more massive of the stars expands and explodes in a supernova, that results in a black hole – whilst the secondary star remains as a normal star. Eventually, the secondary star also explodes in a supernova, but this time resulting in a neutron star. This scenario opens up the possibility that at some point, one or the other objects (during its phase as a dying, expanding giant star or as a compact massive object) started to steal and accrete matter from its companion.

Another pathway could be that each of these stars evolved individually and not within any form of a system – living their lives as massive stars, before exploding and becoming remnant objects. Then, after some cosmic migration – the two crossed paths, locking into each other’s gravity and thereby sealing their fate forever, resulting in the merger events that have been detected and announced.

Gravitational-wave studies of binary pairs have allowed scientist to constrain the evolution of these systems, and model how they came to be by looking at a range of parameters like the eccentricity of orbits of the pre-merger objects or studying them in multiple wavelengths.

When the array of global interferometers are individually activated due to apparent passing gravitational waves, scientists need to ensure that the detection that is filtering through is not a localised terrestrial source, but rather astrophysical in origin.

Having several detectors across the planet helps cut out local events – esp. when two or more detectors register the same wave signature at nearly the same timestamp (depending on which direction it originates from), meaning that there is a higher likelihood that the signal is legitimate.

However, even candidate detections need to be thoroughly analysed to ensure that a true detection is pulled out of the wide range of different reasons that can produce noise (both terrestrial and astrophysical - and ranging from the quantum to the macro scale).

There’s a complication to this process however, in that this analysis needs to be completed as fast as possible following on from the candidate gravitational wave registering with the interferometers. The reason for this is that if a true gravitational wave event is occurring in the cosmos, and it has a counterpart electromagnetic signal, astronomers from across the world want to turn their instruments towards the region where the gravitational waves are localised, in the hopes of quickly catching electromagnetic data before it fades away.

And this is where search pipelines like SPIIR (the Summed Parallel Infinite Impulse Response pipeline) step in.

“When large astronomical events happen, such as the neutron star-black hole merger, large amounts of energy are emitted in a variety of forms including gravitational waves but also electromagnetic signals such as radio waves or visible light,” said UWA Masters of Engineering student, Victor Oloworaran.

“These can reach Earth in times ranging from a fraction of a second to months after the event. In order to catch these, we need the fastest detection possible. Compared to the telescopes which can only point to a small region of the sky, gravitational wave search pipelines are integral as they can monitor the whole sky guide the telescopes on where to point quickly,” he said. 

SPIIR itself is a processing and analysis pipeline that continually reviews data from GW interferometers around the world, looking for the unique signature produced by coalescing binary neutron stars and black holes. 

SPIIR is only one of five detection pipelines around the world that is conducting this analysis, and the only one of its kind operating in Australia, placing our scientists and researchers at the forefront of gravitational wave astronomy.

“UWA's SPIIR pipeline is Australia's only real-time gravitational wave detection pipeline. It is developed and operated by scientists in Perth and Melbourne - in collaboration with the Gravitational Wave Data Center,” said OzGrav Postdoctoral Researcher Dr Fiona Panther, from the University of Western Australia. 

“The SPIIR method is very unique: we can analyze data faster than the other real-time detection pipelines because we use graphical processing units - the kind of computer processors used for PC gaming - to check GW data against hundreds of thousands of templates of GW signals every second, and we do all our analysis in the 'time domain', so we don't have to take the extra step of converting to the frequency domain for the filtering process.”

“From detection pipelines and parameter estimation to EM follow-up and developing GW detectors, Australia takes part in every part of GW discovery,” she said.

To achieve its goals, the SPIIR pipeline combines data from the network of GW detectors, runs this data through powerful computers, and then applies time-domain filtering as well as coherent analytical methods to pull out the elusive signal amongst the noise. 

“The SPIIR pipeline combines a range of fields including data science, engineering, physics, and software to be able to analyse gravitational waves,” added Victor. “Though this may sound like a lot, you can simply imagine it as a sequence of functions applied to gravitational wave data as it streams in. These can range from denoising data to writing it into a file. At the end of the pipeline, we will predict when and where astronomical events may have happened”

“The goal of SPIIR is speed - during the third observing run, the median latency of the pipeline (the time from getting the data to identifying and uploading GW events to a central database) was nine seconds,” said Dr Panther. 

“During the next observing run, this will hopefully be reduced to as little as 5 seconds. UWA PhD student Manoj Kovalam is working hard on something called the 'Early Warning' pipeline - this version of the pipeline uses special templates that can detect neutron star mergers up to a minute before the neutron stars finally merge.”

“Faster alerts - especially these early warning alerts - will enable scientists around the world to use radio and gamma-ray telescopes to look for the light that is thought to be emitted just before and during the merger of neutron stars, and maybe even events like these NSBH mergers.”

“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,” she said.

The addition of these two new gravitational wave events, the first of their kind, now adds to the growing number of detections made that are changing our perspective of the Universe – especially in the context of high-mass compact objects like black holes and neutron stars.

To date (and since 2015) over 50 binary black hole events have been confirmed through gravitational wave detections, which includes a mass range from as low as a few solar masses all the way up to tens of solar masses.  

There have also been two binary neutron star merger events, which include GW170817 (and its wonderful electromagnetic counterpart signal), as well as GW190425 that was classed as a BNS event but didn’t showcase any EM emissions.

Another notable detection has been GW190521 – in which two high mass black holes merged to form (and result in) an intermediate-mass black hole - which are of a class higher than that which can be produced by a single stellar object (there is an upper limit to how big stars can be). 

This latest discovery now adds to the collection of growing knowledge about some of the most exotic objects in the Universe, which only 100 years back, were theoretical concepts in Einstein’s General Relativity.

There are now two unique systems of these binary compact objects to study – each containing a stellar-mass black hole, and a neutron star which eventually merged to create a bigger black hole. It took over a billion years for the minuscule gravitational waves to reach our planet, crossing the cosmos as they stretched and squeezed everything in their path.

Thankfully, human technology is now at the level where we can build incredibly sensitive machines, instruments and algorithms, that can detect these tiny sub-atomic distortions, and separate them from the cascade of background noises that occur naturally around us, as well as artificially by us.

Australian scientists have once again demonstrated that even though we don’t have our own gravitational wave interferometer here (though, we are the perfect location for one), we are still leading in the field of gravitational-wave astronomy, including in the development of instrumentation, processing of data or building pipelines that give us real-time turnaround announcements of when these events are occurring.

“Lots of GW science is happening all around Australia right now, and Australia-based scientists make important contributions to the international LIGO-Virgo collaboration from building the detectors to finding and understanding the physics of gravitational waves. No matter your interest in engineering, physics, astronomy, maths, computer science, or even art, there are opportunities in many aspects of GW science,” said Dr Panther.

“For example, the SPIIR group at UWA brings together some diverse expertise to develop and run our pipeline and work on projects in the area of GW astronomy, from computer science and machine learning to gamma-ray and radio astronomy.”

With the next gravitational wave observing sessions scheduled to begin in the not-too-distant future, there is excitement that more black hole-neutron star binary system detections will take place and the possibility that we might be able to catch the ultimate combination: a black hole-pulsar system in both gravitational and electromagnetic emissions.

Until then, just sit back and relax – distant massive beasts are merging and colliding in the Universe around you, as the waves from these events pass over you. You might not ever feel a thing from these happenings, but the Universe certainly does.

It shakes, rattles and rolls. 

Video Credits
Opening video: Animation by Carl Knox, OzGrav-Swinburne University
Catalogue Video: Zoheyr Doctor