They observe, for the first time, the merger between two black holes and two neutron stars




Long ago, in two galaxies some 900 million light years apart, two black holes devoured their companions, two neutron stars, unleashing gravitational waves in the process that finally reached Earth in January 2020.

Now, and for the first time, scientists have managed to detect this authentic ‘cosmic feast’. Something that by the way, according to the researchers, happened in a way quite similar to the famous Pac Man game. The finding, which has just been published in
‘The Astrophysical Journal Letters’,
It was made by studying two collisions between these two types of objects, considered the most extreme and enigmatic in the Universe. The two events occurred just ten days apart.

“Until now,” explains Chase Kimball, one of the authors of the study, “gravitational waves had allowed us to detect collisions of pairs of black holes and pairs of neutron stars, but the mixed collision of a black hole with a neutron star was the missing piece to understand the fusions of these compact objects. (…) With these detections, we finally have measurements of the fusion rates in the three categories of compact binary fusions. ‘

On both occasions, and just before meeting, the black holes and neutron stars embarked on a true ‘death spiral’, orbiting each other faster and faster until they collided with each other and thus produced gravitational waves that could be captured. by the LIGO observatories, in the United States, and Virgo, in Italy. The scientists, including two from the Galician Institute of High Energy Physics (IGFAE), believe their observations will help uncover some of the most complex mysteries of the Universe, including the basic components of matter and the functioning of space and time.

According to the study, the two collision events, detected on January 5 and 15, 2020, occurred almost a billion light-years from us, that is, almost a billion years ago, but they were so violent and massive that they still at that distance the team was able to observe the gravitational waves they generated.

“These collisions,” says Susan Scott, of the Australian National University and co-author of the study, “have shaken the Universe to its foundations, and we have managed to detect the waves that they sent at full speed through the cosmos. Each collision is not just the union of two massive and dense objects. It’s really like Pac-Man, with a black hole completely swallowing its companion neutron star. These are remarkable events and we have waited a long time to witness them. So it’s amazing that we were finally able to capture them. “

Gravitational waves are disturbances in the curvature of space-time created by massive moving objects, and they contain valuable information about the events that produce them. During the five years since the waves were first measured, a finding that led to the 2017 Nobel Prize in Physics, researchers have identified more than 50 gravitational wave signals from the merging of pairs of black holes and pairs of stars. of neutrons, corpses of massive stars that exploded in the form of supernovae, but he had never been able to observe a ‘mixed collision’.

The image shows the collision between two neutron stars
The image shows the collision between two neutron stars – NASA

Two stars devoured

One of the two collisions occurred between a black hole with a mass nine times that of the Sun and a much smaller neutron star with about two solar masses. The other took place between a black hole six times more massive than the Sun and a neutron star of 1.5 solar masses. In both cases, the neutron stars were completely eaten up by their dark companions. The mass differences between the components of the two systems indicate that they are in fact ‘mixed binaries’: the mass of the heavier objects correspond to those of two black holes, while the mass of the lighter objects is consistent with those of two neutron star

Astronomers around the world were immediately alerted to the two events to try to find flashes of light associated with them in the sky. But it was not possible. Which is not surprising given the great distance at which these mergers occurred, which means that whatever light comes from them, regardless of wavelength, it would be very dim and difficult to detect even with the most powerful telescopes.

The difference between the two masses could also explain why the telescopes did not detect light signals. At other times, when a neutron star approaches a black hole, it can be ripped apart by its gravitational forces, causing flares of electromagnetic radiation. However, in the two cases observed, the much more massive black holes devoured the neutron stars in one bite, leaving no trace of them.

Several hypotheses have been proposed to explain the formation of this class of mixed binary systems. The first involves the evolution of a pair of stars in orbit around each other. Towards the end of their lives, one of the stars could turn into a black hole and the other into a neutron star, never ceasing to orbit each other. According to the other hypothesis, that of dynamic interaction, the two components of the pair form independently in a very dense stellar medium before joining.

Until now, numerous collisions between two black holes, or between two neutron stars, had been detected, but never one in which the two types of objects were mixed. “Now,” says Scott, “we have completed the last piece of the puzzle with the first confirmed observations of gravitational waves. generated by the collision of a black hole and a neutron star ».

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