First detections of a neutron star engulfed by a black hole

Astronomy is sometimes akin to zoology. She observes new stars, as we discover new species. On January 5 and 15, 2020, astronomers spotted an unprecedented object, as they announced on June 29 in The Astrophysical Journal Letters. For the first time, a pair formed of a black hole and a neutron star has been observed, spiraling around each other.

Separately, representatives of these two families had already been seen. A black hole, a star so dense that it prevents all matter and even all light from escaping its attraction, was even photographed for the first time in 2019, in our Milky Way. Likewise, neutron stars, which concentrate the mass of one or two equivalents of our Sun in a sphere of around ten kilometers in radius only, are regularly observed in our galaxy, in the form of “beacons” pulsing their radiation. like metronomes. Hence their name of pulsars. The two objects are in fact the remains of very massive stars which, running out of fuel, collapse on themselves.

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But these two heavyweights, whose behavior is still mysterious for physicists, had never been seen as a pair. In 2015, two black holes were surprised together turning around until they became one. Then in 2017, the ballet of two neutron stars was observed. “But a neutron star and black hole binary was missing from our hunting chart. We are happy to have found it! We even saw two “, enthuses Astrid Lamberts, CNRS astrophysicist at the Côte d’Azur Observatory.

A burst of energy

On January 5, 2020, a black hole the size of nine Suns engulfed a neutron star four times lighter. Ten days later, another event brought together two protagonists weighing 5.7 and 1.5 Suns respectively. In both cases, the ballets did not last long. In a few tens of seconds, the largest, the black hole, has literally devoured the weakest, the neutron star. In the end, only a black hole remains, but also a burst of energy, strong enough to shake space-time (the kind of jelly that constitutes the Universe), to generate waves, in the same way of those created by a pebble in water, a signal that arrived on Earth a billion years later.

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These jolts, light as caresses, were however enough to move the detectors, by modifying the lengths of two laser arms 3 to 4 kilometers long. For the first event, only one detector, LIGO in Livingston (Louisiana), saw this so-called gravitational wave pass. For the second, three detectors, the second LIGO located in Hanford (Washington State) and Virgo in Italy, were shaken by the wave, relic of the cataclysm. These detectors, since 2015, have revolutionized astrophysics by giving access to events invisible until then, because they do not emit electromagnetic radiation. A fourth detector built in Japan, Kagra, has joined the LIGO / Virgo collaboration and is associated with the publication, but is not yet able to spot gravitational waves.

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