Scientists have found the strongest evidence yet that a black hole and a neutron star collided while traveling along elliptical orbits, rather than the near-perfect circle that scientists typically expect before such a merger. The discovery challenges long-held ideas about how these extreme cosmic pairs form and evolve.
The research was carried out by scientists from the University of Birmingham, the Autonomous University of Madrid and the Max Planck Institute for Gravitational Physics. Their findings were published on March 11th. Astrophysics Journal Letter.
Unusual elliptical orbit discovered after black hole and neutron star merge
Astronomers generally expect neutron star-black hole pairs to settle into circular orbits long before they merge. However, a new analysis of gravitational wave event GW200105 reveals that the system was still traveling along an elliptical path just before the two objects merged. This merger ultimately produced a black hole about 13 times the mass of the Sun. The detection of such an elliptical orbit for this type of event has not previously been reported.
Dr Patricia Schmidt from the University of Birmingham explained: “This discovery gives us important new clues about how these extreme objects come together. This discovery shows that our theoretical models are incomplete and raises new questions about where in the universe such systems could originate.”
Gravitational wave data reveals the shape of the orbit
To investigate this phenomenon, the team used a new model developed at the University of Birmingham’s Institute for Gravitational Wave Astronomy to study data from the LIGO and Virgo gravitational wave detectors. This approach allowed the researchers to measure how elongated the orbit is (eccentricity) and determine whether the system exhibits spin-related wobble (precession). This is the first time scientists have measured both effects simultaneously in a neutron star and black hole phenomenon.
Geraint Platten, a Royal Society research fellow at the University of Birmingham, said: “The orbits are the deciding factor. The elliptical shape just before the merger shows that this system did not evolve quietly in isolation, but was almost certainly formed by gravitational interactions with other stars, or perhaps a third companion.”
New analysis questions previous assumptions
The research team performed a Bayesian analysis that compared thousands of theoretical models to actual gravitational wave signals. Their results show that a circular orbit is very unlikely and rule it out with 99.5% confidence.
In previous studies of GW200105, the orbit was assumed to be circular. Because of that assumption, they underestimated the mass of the black hole and overestimated the mass of the neutron star. The new analysis corrects these measurements and finds no strong evidence of precession, suggesting that the elliptical orbits were not caused by spin effects and likely arose during the system’s formation.
“This is convincing evidence that not all neutron star-black hole pairs share the same origin. The eccentric orbits suggest that many stars were born in an environment where they interacted with each other gravitationally,” said Gonzalo Moras of the Autonomous University of Madrid and the Max Planck Institute for Gravitational Physics.
A more complex picture of cosmic mergers
The discovery challenges the widely held idea that all neutron star and black hole mergers occur through a single dominant formation pathway. Instead, the study suggests there may be multiple formation scenarios, some of which could have been formed by a crowded stellar environment where gravitational interactions are common.
The study also helps explain the increasing variety of compact binary mergers observed through gravitational waves. As the detectors continue to identify more events, astronomers hope to discover additional unusual systems that will reveal new paths through which these powerful cosmic collisions occur.
