Black holes ‘swallow neutron stars like Pac-Man’

·3-min read

For the first time astrophysicists have detected black holes eating neutron stars “like Pac-Man”, in a discovery documenting the collision of the two entities.

Two instances of this violent cosmic event were detected using the Advanced LIGO and Virgo gravitational wave detectors.

While previous gravitational wave detections have spotted black holes colliding, and neutron stars merging, this is the first time scientists have detected a collision from one of each.

Dr Vivien Raymond, from Cardiff University’s Gravity Exploration Institute, said: “After the detections of black holes merging together, and neutron stars merging together, we finally have the final piece of the puzzle: black holes swallowing neutron stars whole.

“This observation really completes our picture of the densest objects in the universe and their diet.”

Gravitational waves are produced when celestial objects collide and the energy this creates causes ripples in the fabric of space-time which travel all the way to the detectors we have here on Earth.

More than 1,000 scientists were involved with the world-first detections, with many from Australia, including The Australian National University, leading the way.

Distinguished Professor Susan Scott, a co-author on the study based at the ANU Research School of Physics in the 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.

She explained: “These collisions have shaken the universe to its core and we’ve detected the ripples they have sent hurtling through the cosmos.

“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.”

On January 5 last year, the Advanced LIGO (ALIGO) detector in Louisiana in the US and the Advanced Virgo detector in Italy observed gravitational waves from this entirely new type of astronomical system.

They picked up the final throes of the death spiral between a neutron star and a black hole as they circled ever closer and merged together.

But on January 15 a second signal was again coming from the final orbits and smashing together of another neutron star and black hole pair.

Researchers from Cardiff University, who form part of the LIGO Scientific Collaboration helped to analyse both events, unpicking the gravitational wave signals and painting a picture of how the extreme collisions played out.

This involved generating millions of possible gravitational waves and matching them to the observed data.

This enabled the experts to determine the properties of the objects that produced the signals in the first place, such as their masses and their location in the sky.

From the data they were able to infer that the first signal, dubbed GW200105, was caused by a 9-solar mass black hole colliding with a 1.9-solar mass neutron star.

Analysis of the second event, GW200115, which was detected just 10 days later, showed that it came from the merger of a 6-solar mass black hole with a 1.5-solar mass neutron star, and that it took place at a slightly larger distance of around one billion light-years from Earth.

Since the first ever direct detection of gravitational waves in 2015, astronomers have predicted that this type of system – a black hole and neutron star merger – could exist, but without any compelling observational evidence.

Now that gravitational wave scientists have finally witnessed the existence of this new type of system, their detection provides new clues about how black holes and neutron stars form.

This will be helped by a new £9.4 million grant for gravitational wave research awarded to UK universities and institutes by the Science and Technology Facilities Council (STFC), £3 million of which will go to Cardiff University over the next three years.

The findings are published in The Astrophysical Journal Letters.

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