Scientists have uncovered the first robust evidence of a black hole and neutron star crashing together but orbiting in an oval path rather than a perfect circle just before they merged. This discovery challenges long-standing assumptions about how these cosmic pairs form and evolve.
Researchers from the University of Birmingham, Universidad Autónoma de Madrid, and Max Planck Institute for Gravitational Physics published their findings today (11 Mar) in The Astrophysical Journal Letters.
Most neutron star-black hole pairs are expected to adopt circular orbits long before merging. But the analysis of the gravitational-wave event GW200105 shows that this system travelled on an oval orbit long before merging to form a black hole 13 times more massive than the Sun. An oval orbit is something never seen before in this kind of collision.
Dr Patricia Schmidt, from the University of Birmingham, said: “This discovery gives us vital new clues about how these extreme objects come together. It tells us that our theoretical models are incomplete and raises fresh questions about where in the Universe such systems are born.”
The researchers analysed data from LIGO and Virgo detectors using a new gravitational‑wave model developed at the University of Birmingham’s Institute of Gravitational Wave Astronomy. This allowed them to measure both how ‘oval’ the orbit was (eccentricity) and any spin‑induced wobbling (precession). This is the first time these two effects have been measured together in a neutron star–black hole event.
Geraint Pratten, a Royal Society University Research Fellow from the University of Birmingham, said: “The orbit gives the game away. Its elliptical shape just before merger shows this system did not evolve quietly in isolation but was almost certainly shaped by gravitational interactions with other stars, or perhaps a third companion.”
A Bayesian analysis comparing thousands of theoretical predictions to the real data, showed that a circular orbit is extremely unlikely, ruling it out with 99.5% confidence.
Past analyses of GW200105, which assumed a circular orbit, underestimated the black hole mass and overestimated the neutron star mass. The new study corrects these values and finds no compelling evidence of precession, indicating that the eccentricity was imprinted by its formation rather than by spins.
Gonzalo Morras, from the Universidad Autónoma de Madrid and the Max Planck Institute for Gravitational Physics, said: “This is convincing proof that not all neutron star–black hole pairs share the same origin. The eccentric orbit suggests a birthplace in an environment where many stars interact gravitationally.”
This discovery challenges the prevailing view that all neutron star–black hole mergers arise from a single dominant formation channel and highlights the need for more advanced waveform models capable of capturing the full complexity of these systems.
The study helps to explain the growing diversity seen in compact-binary mergers and opens the door to identifying even more unusual pathways as the number of gravitational-wave detections continues to grow.