Surprise merger in deep space

Print edition : September 13, 2019

Artist’s impression of a black hole devouring a neutron star, an event that creates gravitational waves. Photo: Dana Berry/NASA

A team of gravitational astronomers, including those from The Australian National University (ANU) among others, believe that they have made the first-ever detection of a black hole (BH) swallowing a neutron star (NS). Both are the super-dense remains of dead stars.

On August 14, gravitational-wave detectors, the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) comprising twin detectors in the United States and the European Gravitational Observatory called Virgo in Italy, detected ripples in space and time from a cataclysmic event that happened about 8,550 million trillion kilometres away.

Susan Scott of the ANU Research School of Physics said the achievement completed the team’s trifecta of observations, which included the merger of two BHs and collision of two NSs. The latter has already been detected among the various observations using these gravitational wave observatories since the first gravitational wave detection of a BH-merger in 2016 and an NS-merger in 2017.

“About 900 million years ago, this black hole ate a… neutron star, like Pac-man—possibly snuffing out the star instantly,” said Susan Scott, leader of the General Relativity Theory and Data Analysis Group at the ANU. “The ANU SkyMapper Telescope responded to the detection alert and scanned the entire likely region of space where the event occurred, but we’ve not found any visual confirmation,” she added.

Scientists are still analysing the data to confirm the exact size of the two objects, but initial findings indicate the strong likelihood of a BH enveloping an NS. “Scientists have never detected a black hole smaller than five solar masses or a neutron star larger than about 2.5 times the mass of our sun,” she said. “Based on this experience, we’re very confident that we’ve just detected a black hole gobbling up a neutron star. However, there is the slight but intriguing possibility that the swallowed object was a very light black hole. That would be a truly awesome consolation prize.”

Seeing a black hole eat up a neutron star could produce a wealth of information that no other type of event can provide, B.S. Sathyaprakash, a LIGO theoretical physicist at Pennsylvania State University, was quoted as saying by “Nature”. Such an observation would confirm that these long-sought systems, which result from binary stars of very different masses, do exist. The NS-BH systems can, therefore, be more powerful test beds for general relativity, said Sathyaprakash.

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