For the first time, scientists have detected gravitational waves from a black hole swallowing a neutron star – and not just once, but twice.
The detections were made by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and the Virgo gravitational-wave observatory in Italy, which first captured ripples in space-time reverberating out from a cataclysmic collision between two black holes in 2015. Since then, the observatories have seen many more of these events, plus, less frequently, the weaker signals from the mergers of two neutron stars.
But on 5 January 2020, after years of waiting, scientists finally completed the trifecta by spotting the death spiral of a neutron star into a black hole. Remarkably, just 10 days later, they detected another.
“Now we’ve completed the last piece of the puzzle with the first confirmed observations of gravitational waves from a black hole and a neutron star colliding,” says astrophysicist Susan Scott from the Australian National University (ANU) and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).
In August 2019, the collaboration announced it had spotted a black hole swallowing a mystery object, around 800 million light years away. This could have been the first black hole–neutron star detection, but it was uncertain because the second object had an odd, in-between mass that would either make it the lightest black hole or the heaviest neutron star ever discovered.
The mystery still hasn’t been solved, which makes these two new detections the first unambiguous evidence of black hole–neutron star collisions.
“It’s an awesome milestone for the nascent field of gravitational-wave astronomy,” says Rory Smith, an OzGrav astrophysicist at Monash University.
“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.”
Smith and Scott were part of an international team of more than 1000 scientists involved in the detection. Their results are published in The Astrophysical Journal Letters.
Both the observed collisions occurred around one billion years ago. One event involved a black hole of nine solar masses and a neutron star of two, while the other event involved a black hole of six solar masses and a neutron star of 1.5.
Scott says these would have been spectacular events to witness.
“Each collision isn’t just the coming together of two massive and dense objects,” she says. “It’s really like Pac-Man, with a black hole swallowing its companion neutron star whole.”
If you were watching from outside the system, you would see the two objects spiralling around each other at half the speed of light.
“Of course, you can’t see a black hole, but the neutron star would be getting closer and closer to it,” says Scott. “All of a sudden it would blip out as it goes inside the event horizon, and it’d be swallowed up.”
Once the team picked up the gravitational-wave signal from these collisions, they scrambled to scour the sky with other telescopes such as ANU’s SkyMapper to see if there was a counterpart on the electromagnetic spectrum, – powerful events like this also release bright flashes of light.
But the search was unsuccessful – partly, the team suspects, because they couldn’t narrow down the sources’ location on the sky, and partly because the black holes may have swallowed the neutron stars whole instead of tearing them apart and generating flares of electromagnetic radiation.
Pairs of black holes and neutron stars have been predicted to exist for decades, but have so far been elusive. But just because detection is rare doesn’t mean the events are uncommon, according to Simon Stevenson, another OzGrav researcher at Swinburne University of Technology.
“We find that roughly one pair of neutron star–black holes merges for every 10 pairs of neutron stars,” he says. This happens within a billion light years of Earth approximately once a month.
“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.”
As gravitational wave detectors increase their sensitivity, they’ll also spot more and more of these cosmic collisions.
Scott explains that in their coming observational run next year, upgraded detectors will be able to locate weaker signals further into space. Now they have completed the trifecta of binary objects, the collaboration will also be searching for other exotic systems.
“Two of the things we’re gunning for are detecting the gravitational waves given off by a star going supernova – we want to be able to look right back into the heart of the process to understand actually how the process happens,” she says.
“And the second type of thing we’re going after with increased sensitivity is continuous wave sources.”
These are systems that generate a constant stream of weaker gravitational waves, such as an asymmetric neutron star that would emit a ‘hum’ as it spins on its axis.
“We’re definitely going after those because we know that will also help us unlock the understanding of this mysterious form of matter which makes up neutron stars,” says Scott.
“The next 15 or 20 years is going to be amazing because it’s an entirely new window – we’ve never seen stuff before that we can see with gravitational waves.”