Astronomers may have spotted a neutron star being swallowed by a black hole for the first time, marked by a belch of gravitational waves rippling across the cosmos. If confirmed, the detection by the twin LIGO detectors in the US and the Virgo detector in Italy would be the first evidence that black holes and neutron stars can pair up in binary systems. The observations could also reveal new details about the nature of such dramatic mergers, including whether the neutron star was ripped apart before crossing the black hole’s threshold or whether it slid seamlessly into oblivion.
Patrick Brady, a spokesman for the LIGO collaboration and Professor of Physics at the University of Wisconsin-Milwaukee, said the signal from the possible collision, which took place on 26 April, needed further analysis before the team could be confident it was a real event rather than a random blip in the background noise. “It’s like listening to somebody whisper a word in a busy cafe, it can be difficult to make out the word or even to be sure that the person whispered at all,” he said. “It will take some time to reach a conclusion about this candidate.” He put the chances of the observations being a glitch in the data at 14%. LIGO and Virgo pick up the tiny ripples in the fabric of space and time that are sent out across the cosmos when two massive objects collide. The possible detection came just a day after the LIGO and Virgo detectors identified a cataclysmic merger of two neutron stars for only the second time. Since the beginning of their third observational campaign on 1 April, the detectors have also spotted three black hole mergers.
Neutron stars are the smallest, densest stars known to exist. They are about 12 miles wide, and a teaspoon of neutron star material has a mass of about a billion tonnes. They have a smooth crust of pure neutrons, 10bn times stronger than steel. They are the collapsed remnants of giant stars, after a supernova explosion – even more massive stars go on to form black holes. When two neutron stars collide, they not only send out gravitational waves but also light, meaning that if astronomers are able to swivel their optical telescopes to the right bit of sky in time they can also pick up the explosive aftermath in light waves. The location of the possible neutron star and black hole merger, which is estimated to have taken place 1.2bn light years away, has been narrowed down to about 3% of the total sky – but that is still a vast region.
“All the astronomers are now chasing an unfortunately enormous patch of the sky to see whether there’s some light that has switched on at that time,” said Prof Alberto Vecchio, director of the Institute of Gravitational Wave Astronomy, University of Birmingham. Detecting a flash of radiation could reveal crucial details about the size of the objects and the nature of the merger. Counterintuitively, the biggest black holes are the least dense and the gravitational pull at the edge of these objects is least fierce, so a neutron star colliding with a very large black hole might simply vanish from view. “The neutron star would just dive in and nothing happens, that’s it,” said Vecchio. By contrast, for a smaller black hole, the gravity close to the event horizon would be so fierce that it could shred the neutron star, gobbling it up in several chunks. “Then you’d have this extremely dense material travelling at a fraction of the speed of light,” said Vecchio, which he said could release spectacular blasts of radiation that could be spotted by telescopes on Earth. Since LIGO began observing in 2015, its sensitivity has been increased significantly meaning it is now making several detections a month. This increases the chances of it spotting previously unseen exotic objects which have been theoretically predicted, such as boson stars or mini black holes. “We’re opening a new window on the universe and this will hopefully bring us a whole new perspective on what’s out there,” said Brady.
Credit: Hannah Devlin for The Guardian, 3 May 2019.