Mixed black hole–neutron star mergers reveal eccentric orbits and hidden formation paths

Astronomers analyzing the GW200105 gravitational-wave signal found an eccentric, precession-lacking orbit for a black hole–neutron star binary just before merger, suggesting it was shaped by gravitational interactions with a third body and not by a quiet, isolated inspiral. Using a new model from Birmingham’s Institute of Gravitational Wave Astronomy, the team says there are likely multiple formation channels for such mixed mergers, and the event revised prior mass estimates (BH ~13 solar masses, NS ~2). The finding expands our understanding of how extreme binaries form and evolve and challenges the assumption of circular, isolated mergers.