The Stellar-Mass Graveyard Just Doubled In Size

Over the last few years, the emergence of gravitational wave astronomy has really opened up a whole new perspective on studying the Universe. Unlike regular electromagnetic light that arrives at our telescopes, this new method allows us to sort of ‘feel’ events that are occurring in far off locations in the cosmos.

Gravitational waves are produced by one of the most violent events in the Universe - when massive objects are caught in an orbital death spiral, right before merging. It’s during the inspiral phase that the very fabric of space-time is distorted, stretched and squeezed, as the high-mass objects accelerate towards their eventual doom.

Technically, all masses which accelerate generate gravitational waves. Stretch out your arm and move it in the shape of a giant circle, and you’ll be generating them. But at this scale, these are absolutely feeble and undetectable. Give something enough mass – like that of a black hole or a neutron star – and you really start to mess with the Universe.

Even still, at the massive scale of neutron stars and black holes, the detections we make today are tiny – a shift in dimensions that measures roughly 1/10,000^th the size of a proton’s diameter. We’ve only been able to achieve direct evidence of these events because of massive, and remarkably advanced instruments known as interferometers, with the first discovery only made in 2015. Since then, we’ve confirmed roughly 50 gravitational wave events, with many signals traversing the vast cosmos for billions of years, before arriving at our detectors.

Now, Australian researchers are amongst an international collaboration of scientists who have today announced the detection of 35 new gravitational wave events, boosting the population of the stellar-mass graveyard to 90, including 32 detections of merging binary black hole pairs, and three likely to have come from merging of neutron stars with black holes.

Using the giant interferometers (of which there are now several located around the world including the two LIGO detectors in the US, the Virgo detector in Italy, and the Kagara detector in Japan), scientists were conducting their observations as part of the third observing run from November 2019 through to March 2020. It was during this period, known as “O3b”, which all 35 new detections were made.

Forming part of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), Australian researchers from Swinburne University of Technology, the Australian National University, Monash University, University of Adelaide, the University of Melbourne and the University of Western Australia, amongst other local collaborators were all involved.

“Each new observing run brings new discoveries and surprises. The third observing run saw gravitational wave detection becoming an everyday thing, but I still think each detection is exciting!" said Dr Hannah Middleton, postdoctoral researcher at OzGrav, University of Melbourne.

And with each new detection comes a wealth of information that can be further analysed to determine the properties pre, during and post the merger event. Remarkably, this all occurs through interpretation of the gravitational wave signal, and not any electromagnetic counterpart signal.

“It’s fascinating that there is such a wide range of properties within this growing collection of black hole and neutron star pairs”, says study co-author and OzGrav PhD student Isobel Romero-Shaw (Monash University). “Properties like the masses and spins of these pairs can tell us how they’re forming, so seeing such a diverse mix raises interesting questions about where they came from.”

There's a really interesting mystery that has started to emerge from all these detections. Compact remnant objects (the name for things like white dwarfs, neutron stars, pulsars, and black holes) are the end product of when a star transitions away from the main part of its life (known as the main sequence phase) during which it burned and fused hydrogen in its core, and into the post-main-sequence phase.

Which of these compact remnant objects is the outcome solely depends on the mass of the progenitor star – a star as massive as our Sun (i.e., one solar mass) will usually puff away its outer layers leaving behind a white dwarf. A progenitor of roughly 8 – 25 solar masses, explodes in a violent supernova event to leave behind a neutron star or pulsar. And finally, those who are 25 solar masses and higher, usually explode but leave behind the mysterious black holes.

The remnant objects also have limits themselves on how massive they can be (this is because gravity crushes them further into the next class of remnants). For a white dwarf, the upper barrier (known as the Chandrasekhar limit) is 1.44 solar masses. For neutron stars, this upper bound is known as the TOV (Tolman–Oppenheimer–Volkoff) limit and is approximately 2.5 – 3 solar masses.

In the case of black holes, however, these masses can range from approximately 5 solar masses (the results of stellar-mass progenitors transitioning away from the main sequence phase) through to many billions of times the mass of our Sun (like the supermassive black holes that reside deep in the hearts of galaxies).

This presents an interesting concept – where are the populations of remnant objects that have masses that reside between 3 – 5 times that of the Sun? This is known as the mass-gap problem and challenges what theoretical models portray as upper limits on neutron star masses, and lower limits on the lightest black holes.

There have been a handful of cases in which objects have been found to reside in this mass-gap range, but the population numbers are low (relative to other confirmed detections and observations) and so more data is needed to uncover details about the properties of objects in this gap.

And with every new detection confirmed, such as these newly discovered 35 items, scientists are able to delve a little further to either place further constraints and limits, or completely reshape theoretical models about this mystery.

One such example from this new batch of detections is GW200210 which is the merger of a 24-solar mass black hole, with an object of 2.8 solar masses. This resulted in a new, larger black hole of 27 solar masses, but it is the smaller object that looks to be exciting – it could be an extremely heavy neutron star, or it could be an extremely light black hole.

It’s no easy feat to detect such feeble signals that originate billions of light-years away in the Universe and requires a global approach to tackling such a complicated task. To start with, once the interferometers have made a likely detection (there are algorithms running which help quantify and rule out terrestrial noise false alarms), a public alert is issued to global scientists.

This is vitally important because we don’t ‘see’ gravitational waves (it’s closer to ‘feeling’ them) – but there is a chance that the event that has taken place does indeed have a counterpart electromagnetic signal. This was the case for GW170817, in which two neutron stars merged and gave off both gravitational and electromagnetic radiation.

The public alert then triggers global observatories (of the electromagnetic nature) to turn their telescope eyes (across all frequencies) to the region of space where the gravitational wave interferometers think the event occurred. This is the important aspect – to catch a counterpart electromagnetic signal, along with the gravitational wave signal, is a holy grail of astrophysics – the ability to observe and analyse an event in multi-wavelength data.

This dual approach then goes on to help provide a greater understanding of the properties of both the progenitor and post-merger objects. In the case of GW170817, a wealth of new knowledge – from confirmations to new discoveries were made through the analysis of both the electromagnetic and gravitational wave signal (resulting in hundreds of published papers on the event).

“It’s exciting to see 18 of those initial public alerts upgraded to confident gravitational wave events, along with 17 new events,” said Dr Aaron Jones, co-author and postdoctoral researcher from The University of Western Australia.

With further fine-tuning and sensitivity continually being added to the gravitational wave detectors, these alerts are becoming more common as new waves roll past the Earth, stretching and squeezing the arms of the interferometers.

“Upgrades to the detectors, in particular squeezing and the laser power, have allowed us to detect more binary merger events per year, including the first-ever neutron star-black hole binary recorded in the GWTC-3 catalogue,” said Disha Kapasi, OzGrav student from the Australian National University.

“This aids in understanding the dynamics and physics of the immediate universe, and in this exciting era of gravitational wave astronomy, we are constantly testing and prototyping technologies that will help us make the instruments more sensitive.”

Some of the highlights from these 35 new detections include two merger events that are likely between a neutron star and black hole (GW191219 and GW200115) – including one of the least massive neutron stars ever reported in GW191219. Another is a massive pair of binary black holes, whose combined mass is 145 times that of the Sun (GW200220), whilst another massive pair of binary black holes (112 solar masses) appears to be spinning upside down (GW191109). On the other end of the scale, there’s even a light pair of binary black holes, weighing in at a (relatively) small combined mass of 18 times that of the Sun (GW191129).

Each of these systems, both pre and post the merger event, has its own set of properties – its own story – to tell about what conditions were like before and after succumbing to gravity’s relentless attraction.

“By studying these populations of black holes and neutron stars we can start to understand the overall trends and properties of these extreme objects and uncover how these pairs came to be,” said OzGrav PhD student (and SpaceAustralia.com’s social media manager!) Shanika Galaudage from Monash University.

“There are features we are seeing in these distributions which we cannot explain yet, opening up exciting research questions to be explored in the future”.

The next step is the fourth observing run (known as “O4”) for the global conglomerate of scientists working in gravitational-wave astronomy – and with higher sensitivity and further fine-tuning, an entirely new population of merging compact remnant events awaits discovery, with hopefully further exciting properties of objects from the mass-gap revealed.

Opening video: Animation by Carl Knox, OzGrav-Swinburne University.