💫 The oval dance of a black hole and a neutron star

How do black holes and neutron stars get closer before merging? Their orbits were until now assumed to be circular, with a spiraling approach before the final contact. A recent observation challenges our knowledge, and this changes everything, including about the mass of the object resulting from the merger.

This breakthrough is the result of analyzing gravitational waves captured by instruments like LIGO and Virgo. The signal, named GW200105, comes from a merger that occurred about 910 million light-years away, involving a black hole and a neutron star.


To examine the orbits before the merger, a team used a model developed at the University of Birmingham. The researchers focused on the oscillations, or "precession," and noted their absence, which betrays an oval trajectory rather than a circular one. The elliptical shape of the orbit indicates the system did not evolve in isolation. Scientists estimate it was influenced by other stars or a third object, which changes the formation scenarios for these pairs.

Starting from the initial assumption of circular orbits, the mass of the black hole resulting from the merger was estimated at about 9 solar masses. The new calculations, based on the oval orbit, reveal a higher mass, around 13 solar masses.

Published in The Astrophysical Journal Letters, these results describe several possible pathways for the formation of these binary systems. Soon, new gravitational wave detections could confirm these models. Researchers hope to learn more about the birth conditions of these extreme pairs, paving the way for more precise and dynamic astronomy.