10.11.2021

Signals from cosmic collisions hint at the life and death of stars.

Somewhere out there in the universe, neutron stars and black holes regularly collide, causing cosmic ripples in space and time – gravitational waves – to bounce their way across the cosmos to Earth.

Now, an international team of researchers, including those from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), have collaborated to present a whopping 90 detections – the largest number of gravitational wave detections to date.

Each detection represents some form of violent celestial event, such as supernova explosions or brutal crashes between stars and black holes as they hurtle along the cosmic highway.

There have been three rounds of observations, and the most recent detections were collected between November 2019 and March 2020. Of the 35 detections in the third round, 32 were caused by two black holes merging, two were likely caused by a neutron star – a dense, city-sized star that’s heavier than our own Sun – smashing into a black hole, and one had mysterious origins.

“Each new observing run brings new discoveries and surprises,” says co-author Dr Hannah Middleton, of OzGrav and the University of Melbourne.

“The third observing run saw gravitational wave detection becoming an everyday thing, but I still think each detection is exciting!

“As we continue to observe more gravitational-wave signals, we will learn more and more about the objects that produce them, their properties as a population, and continue to put Einstein’s theory of General Relativity to the test.”

This final round brings the total to 90 detections from 2015 to 2020.

“These discoveries represent a tenfold increase in the number of gravitational waves detected [observatories] since they started observing,” says OzGrav chief investigator Professor Susan Scott, from the Australian National University (ANU).

“We’ve detected 35 events. That’s massive! In contrast, we made three detections in our first observing run, which lasted four months in 2015–16.

“This really is a new era for gravitational wave detections and the growing population of discoveries is revealing so much information about the life and death of stars throughout the universe.

“Looking at the masses and spins of the black holes in these binary systems indicates how these systems got together in the first place.

“It also raises some really fascinating questions. For example, did the system originally form with two stars that went through their life cycles together and eventually became black holes? Or were the two black holes thrust together in a very dense dynamical environment such as at the centre of a galaxy?”

Top detection highlights

●     Two mergers between possible neutron star-black hole pairs. These are called GW191219_163120 and GW200115_042309, the latter of which was previously reported in its own publication. The neutron star in GW191219_163120 is one of the least massive ever observed.

●     A merger between a black hole and an object that could either be a light black hole or a heavy neutron star called GW200210_092254

●     A massive pair of black holes orbiting each other, with a combined mass 145 times heavier than the Sun (called GW200220_061928)

●     A pair of black holes orbiting each other, in which at least one of the pair is spinning upright (called GW191204_171526)

●     A pair of black holes orbiting each other which have a combined mass 112 times heavier than the Sun, which seems to be spinning upside-down (called GW191109_010717)

●     A ‘light’ pair of black holes that together weigh only 18 times the mass of the Sun (called GW191129_134029)

Secrets of life and death

The detections showed how unique each event was, which gives important clues as to how massive stars live and later die in supernova explosions.

“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, of 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.”

But each event can also be seen as a larger ‘population’.

“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,” says OzGrav PhD student Shanika Galaudage, of Monash University, who was a co-author on a companion publication released today.

“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.”

Detecting gravitational waves

All of these detections were made possible by the global coordinated efforts from the LIGO(USA), Virgo (Italy) and KAGRA (Japan) gravitational-wave observatories.

“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,” says OzGrav student Disha Kapasi, of ANU.

“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.”

The next observational round will begin in August 2022 after LIGO and Virgo have been further upgraded. In the meantime, the researchers will continue to analyse the data to learn more about undiscovered properties of gravitational waves.

The full list of detections was published in ArXiv.