'Pac-Man' moment revealed as black holes eat up neutron stars

Researchers pored over gravitational wave data to reveal the extraordinarily rare cosmic event. Photo / 123rf

Black holes have been observed engulfing a neutron star "like Pac-Man" in a world first, according to scientists.

Two of the extraordinarily rare events were witnessed by a team of researchers in January 2020, and the findings have today been published in the Astrophysical Journal Letters.

Black holes are the most extreme objects in the universe and are so dense that nothing can escape their enormous gravitational pull, not even light.

They are formed when an enormous star many times the size of the Sun reaches the end of its lifespan and simultaneously explodes — a supernova — while its core collapses in on itself to form an ultra-dense entity.

Neutron stars are formed in a similar way, but only arise at the end of the lifespan of stars up to three times the mass of our Sun. These are big enough to go supernova, but small enough to avoid becoming a black hole.

Researchers have long been focusing on understanding how these enigmatic objects impact galaxies and star systems around them, but also how they interact with one another.

Astronomers from around the world teamed up to answer this conundrum and used various telescopes and apparatus, including Ligo, the Virgo Collaboration and the Kamioka Gravitational Wave Detector (Kagra) project.

These are adept at identifying and analysing gravitational waves, which are produced by neutron stars and energetic space collisions. On January 5 and 15, 2020, two events were detected which produced new gravitational waves.

Painstaking analysis revealed these waves were created by two merging events just 10 days apart. Both were found to be the consequence of a black hole engulfing a neutron star.

The first merger to be spotted was between a black hole with the mass of nine suns, and a neutron star with the mass of 1.9 suns.

The second however involved slightly smaller objects, with a black hole equivalent to six suns, and the neutron star 50 per cent bigger than our sun.

"These collisions have shaken the universe to its core and we've detected the ripples they have sent hurtling through the cosmos," said Prof Susan Scott, a co-author on the study based at the Australia National University.

"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."

"We've now seen the first examples of black holes merging with neutron stars, so we know that they're out there," said Dr Maya Fishbach, co-author of the paper from the Ligo Scientific Collaboration (LSC).

"But there's still so much we don't know about neutron stars and black holes – how small or big they can get, how fast they can spin, how they pair off into merger partners.

"With future gravitational wave data, we will have the statistics to answer these questions, and ultimately learn how the most extreme objects in our universe are made."