First detections of a black hole “swallowing” a neutron star

Astrophysicists had already observed fusions of black holes between themselves, or those of neutron stars, but never yet mixed couples, as indicated in a CNRS press release.

It is the missing link that will help to understand these coalescences (mergers), with theories encompassing all of these phenomena.astrophysicist Astrid Lamberts, lead author of the study published in the journal, told AFP Astrophysical Journal Letters.

The two events were spotted ten days apart, in January 2020, by the Virgo and Ligo gravitational wave detectors.

Gravitational waves are tiny variations in space-time, caused by phenomena of magnitude, such as the fusion of two black holes. By analyzing the shape of the wave, such as that caused by a stone thrown on the surface of a pond, we can determine the distance and the masses present.

In this case, these very distinct mergers took place about 900 million and one billion light years from Earth. The black holes involved had a mass of 8.9 and 5.7 times our Sun, respectively. In front of them, the neutron stars, with a respective mass of 1.9 and 1.5 times our Sun, had no chance of escaping their fatal attraction.

If the mass difference is too big, the black hole suddenly swallows, explains Astrid Lamberts, CNRS researcher at the Côte d’Azur Observatory (OCA), and expert in massive stars, which are the progenitors of neutron stars and stellar black holes.

Stellar black holes and neutron stars are among the most exotic objects in the cosmic world.

Both result from the collapse, at the end of its life, of a massive star on itself. The mass of the black hole is so concentrated that the force of gravity prevents anything from escaping, even light. The neutron star, although extraordinarily compact, remains visible.

Observations of the two black hole / neutron star mergers complete a picture of nearly 50 binary black hole mergers and a handful of binary neutron star mergers to date.