Scientists may have spotted a black hole and a colliding neutron star | Science

Gravitational hunters may have already spotted their most exotic career. On August 14 at 5:10:39 EDT, a trio of gigantic detectors in the United States and Italy, detected a pulse of gravitational waves – ripples in the very space – apparently triggered by the formation of a black hole and a neutron star about 900 million light-years away. Observers had previously spotted many black hole mergers and a fusion of neutron stars, but never a combination. The new discovery could provide new information about neutron stars, made up of the densest matter in the cosmos.

"This is an important milestone, if it is standing," said Patrick Brady, spokesman for more than 1300 scientists working with the LIGO (Laser Interferometer) Gravitational Wave Observatory, which features two Hanford, Washington, and Livingston, Louisiana. . The new observation was made by LIGO and Virgo, a gravitational wave detector located near Pisa, Italy, which hosts more than 400 scientists.

Gravitational waves are triggered when extremely massive objects collide. To detect the infinitesimal stretching of space, physicists have come up with huge L-shaped optical instruments called interferometers, endowed with arms of several kilometers long. The first detection took place in September 2015, when LIGO researchers detected an explosion of gravitational radiation from two gigantic black holes tens of times heavier than the sun melting at more than 1.3 billion. light years. Because a black hole is a pure gravitational field left behind when a massive star sinks to a point, the collision was total and produced no visible radiation .

In August 2017, Virgo joined the hunt. In the space of a few days, the three detectors have spotted an even more fruitful event: the fusion of two neutron stars – essentially gigantic atomic nuclei of about 10 kilometers in diameter, left by the l '. implosion of stars a little too small to create black holes. This collision also caused a massive explosion observed by conventional telescopes across the electromagnetic spectrum.

Now, LIGO and Virgo may have noticed the fusion of a black hole and a neutron star. "This is the third part of our collection and we of course wish to complete our collection," said Vicky Kalogera, an astrophysicist and member of the LIGO of Northwestern University in Evanston, Illinois. Seeing a single black hole shredded by a neutron star could reveal how rigid the matter of neutron stars is, explains Kalogera, which is essential for deciphering the structure of neutron stars. Theorists still do not know how common the pairs of black-neutron hole stars are or how they are formed, she says. The modeling suggests that they are more likely to form from stars born in orbiting pairs that then collapse, rather than wandering black holes and neutron stars. who meet one way or another.

The new signal was remarkably loud. Working together, the three detectors were able to locate the source in the sky within 23 degrees squared – a spot on the sky about seven times wider than the moon. In comparison, they were able to locate fused neutron stars only at 28 square degrees, although this pair is much closer, at only 130 million light-years away. The strength of the signal reflects how much the accuracy of the detectors has improved in 2 years, says Brady.

To fully understand the nature of colliding bodies, researchers hope that conventional telescopes will be able to detect evidence of any explosion, indicating the presence of neutron star material. Until now, astronomers have reported no obvious sign of such an "optical counterpart," Brady said.

This absence of optical counterpart means that the identification of objects relies entirely on their masses, which researchers estimate from gravitational waves. One is heavier than five solar masses, and the other – the presumed neutron star – is lighter than three, says Brady. But this second identification is not entirely certain, says Kalogera. "Maybe it's not a neutron star? Maybe it's a tiny black hole?" In fact, based on the earlier observation of the fusion of neutron stars, some theorists argue that a neutron star can only weigh more than 2.2 solar masses.

If astronomers can not find an optical counterpart, researchers at LIGO and Virgo will have to try to determine the nature of colliding objects only from gravitational waves. It's difficult, but not necessarily impossible, says Brady. "This is the major advantage of this observation," he says. "It's right on the edge of our discovery space and we'll have to work very hard to understand what we're seeing."