Gravitational waves: Collision on an unexpectedly elliptical path
Using new models to describe gravitational waves, the merger of a black hole with a neutron star in an elliptical orbit has been demonstrated for the first time – a strong indication that common assumptions about stable, circular orbits are not always true.
Eccentric system | The illustration shows a binary star system consisting of a neutron star (top left) and a black hole (middle). Due to external influences, their common orbit could deviate from a circular shape, as the system behind the gravitational wave signal GW200105 suggests.
In January 2020, the LIGO detector for gravitational waves in the USA provided the first reliable evidence that neutron stars are also swallowed by black holes. In the event named GW200105, an object with around nine times the mass of our Sun merged with a neutron star of around 1.9 solar masses after the two moved closer together in a circular orbit – at least that was the original assumption. In a follow-up investigation, an international team led by astronomer Gonzalo Morras from the University of Birmingham was able to provide evidence for the first time that the objects involved actually collided with each other on an elliptical orbit. This not only impacts their mass estimates, but also calls into question the previous assumption that such systems always adopt circular orbits before merging. The group published their results in the specialist journal “The Astrophysical Journal Letters”. Due to external influences, their common orbit could deviate from a circular shape, as the system behind the gravitational wave signal GW200105 suggests.
Of the numerous recorded gravitational wave signals, the pairing of two black holes accounts for by far the largest proportion – those from black holes and neutron stars are significantly rarer. What all systems have in common, however, is the continuous emission of gravitational waves as they approach, causing them to lose angular momentum and energy. Initially rather elliptical orbits, known in technical jargon as eccentric, gradually approach the ideal circular shape with an eccentricity of zero over time. Finally, they appear almost circular in the frequency range of the detectors.
It’s different with GW200105: Using a new model to describe gravitational waves, the team was able to estimate an orbital eccentricity of both objects around their center of mass of approximately e = 0.145, with an orbital period of 0.1 seconds. For comparison: Mercury shows e = 0.206 a significantly more elliptical orbit around the sun.
The shape of the path provides crucial information about the environment in which these objects interact with each other. The elliptical orbit at GW200105 indicates an origin where many gravitational interactions with other stars took place. This represents a clear alternative to the common view, according to which all neutron star-black hole mergers occur in a quiet, isolated environment and can be traced back to a single dominant formation mechanism.
Taking eccentricity into account also has an impact on the physical parameters of the objects involved. Previous analyzes based on a circular orbit resulted in completely different masses. According to the new models, the neutron star had only 1.5 times the mass of the Sun, while the black hole had an original mass of about 11.5 solar masses. After the merger, one with more than 13 solar masses remained. In addition, the emission of gravitational waves is also stronger because the elliptical shape of the orbit means that the accelerations of the masses revolving around one another are much more violent than in a circular orbit.
Given the growing diversity of merger events, the work highlights the need to develop more advanced gravitational wave models. And there is still potential with the detectors: the origin of GW200105 could be determined at a distance of around 950 million light years, but the uncertainty is quite large at up to around 40 percent. The situation is similar with the location in the sky: the collision could have occurred in an area that covers around 17 percent of the entire firmament – that corresponds to the area of 34,000 full moons.
Morras, G. et al., arXiv 10.48550/arXiv.2503.15393, 2025
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