Black holes swallow neutron stars like "Pac-Man"

Computer generated illustration of neutron star and black hole merging.

Artist's impression of a neutron star and black hole merging. Image: Carl Knox, OzGrav/Swinburne University

Scientists have for the first time detected black holes eating neutron stars, "like Pac Man", in a discovery documenting the collision of the two most extreme and enigmatic objects in the Universe.

The collisions occurred one billion years ago. 

The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and the Virgo gravitational-wave observatory in Italy have captured the gravitational waves from the death spiral and merger of a neutron star with a black hole, not once but twice. The findings are published today.

The researchers say their observations will help unlock some of the most complex mysteries of the Universe, including the building blocks of matter and the workings of space and time.
 
More than 1,000 scientists were involved with the world-first detections, with many from Australia, including The Australian National University (ANU), leading the way.

Distinguished Professor Susan Scott, a co-author on the study based at the ANU Centre for Gravitational Astrophysics, said the events occurred about a billion years ago but were so massive that we are still able to observe their gravitational waves today.
 
"These collisions have shaken the Universe to its core and we've detected the ripples they have sent hurtling through the cosmos," she said.
 
"Each collision isn't just the coming together of two massive and dense objects. It's really like Pac-Man, with a black hole swallowing its companion neutron star whole.
 
"These are remarkable events and we have waited a very long time to witness them. So it's incredible to finally capture them."
 
One event included a black hole with a mass nine times bigger than our own sun and a neutron star with two times our sun's mass. The other event included a black hole with about six times the mass of our sun and a neutron star with 1.5 times its mass.
 
Professor Scott, also a Chief Investigator at the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), said the international team had previously captured many events involving two black holes colliding as well as two neutron stars smashing together.
 
"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," she said.
 
Dr Johannes Eichholz, from the ANU Centre for Gravitational Astrophysics and an Associate Investigator with OzGrav, said the two detections were originally made on 5 and 15 January 2020.
 
"These kind of detections are incredibly rare," he said. "We haven't detected these events once - but twice and within 10 days of each other.
 
"Like the ripples from these two events, which have been felt a billion years later, these findings will have a profound impact on our understanding of the Universe for many years to come."
 
The findings are published today in The Astrophysical Journal Letters

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