The first observation of binary systems consisting of a neutron star and a black hole has been announced on June 29, 2021, with a paper appeared in The Astrophysical Journal Letters. This was made possible by the detection, in January 2020, of gravitational signals (nicknamed GW200105 and GW200115 from the dates of their detection) emitted by two systems, in which a black hole and a neutron star, rotating around each other, merged into a single compact object. The existence of these systems was predicted by astronomers several decades ago, but they had never been observed with confidence, either through electromagnetic or gravitational signals, until now. The gravitational signals encode valuable information about the physical features of the systems, such as the mass and distance of the two NSBH pairs, as well as about the physical mechanisms that have generated them and bring them to collapse.  The signal analysis has shown that the black hole and neutron star that created GW200105 are, respectively, about 8.9 times and 1.9 times as massive as our Sun and their merger happened around 900 million years ago. For the GW200115 event, the merger involved two compact objects with masses of 5.7 and 1.5 times the Sun mass, at a distance of almost 1 billion years ago.

In addition to shedding light on a class of rare phenomena and offering the possibility of studying the fundamental physics in extreme conditions not reproducible on Earth, this result opens the way to the understanding of the mechanisms responsible for the mergers of mixed binary systems in which a black hole and a neutron star orbiting around each other eventually merged, thus forming a single compact object.

The two observations are consistent with both the 'isolated evolution' scenario and the 'dynamic formation' scenario in a young star cluster or in the proximity of an active galactic nucleus. In the first case, the black hole - neutron star system would come from the evolution of a stellar binary, while in the second case it would be the result of a close interaction between components of a star cluster. At present the LIGO and Virgo detectors are being upgraded with the goal to look even further into the cosmos. We wait for further observations of mixed binaries, in order to better understand their formation scenarios.