First Eccentric Orbit Found in a Black Hole-Neutron Star Merger

Published in The Astrophysical Journal Letters. Research by University of Birmingham, Universidad Autonoma de Madrid, and Max Planck Institute for Gravitational Physics.

How does a black hole meet a neutron star? The standard answer has been: slowly, over millions of years, their orbits grinding into perfect circles before the final plunge. But what if some of these cosmic collisions begin with a far messier geometry?

An international team of physicists has now provided the first strong evidence that a black hole and neutron star merged while traveling on an oval -- technically, eccentric -- orbit. The finding, published in The Astrophysical Journal Letters, challenges the prevailing assumption that all such mergers follow the same tidy evolutionary path.

The gravitational wave that told a different story

The event in question, catalogued as GW200105, was detected by the LIGO and Virgo gravitational wave observatories in January 2020. At the time, it was identified as only the second confirmed merger between a neutron star and a black hole -- a rare class of event that produces telltale ripples in spacetime.

Previous analyses of this signal assumed the pair had been orbiting in a circle before they collided, as theory predicted. But researchers from the University of Birmingham, Universidad Autonoma de Madrid, and the Max Planck Institute for Gravitational Physics suspected there was more information hiding in the signal.

Using a new gravitational wave model developed at Birmingham's Institute of Gravitational Wave Astronomy, the team reanalyzed GW200105, measuring both the shape of the orbit (its eccentricity) and any spin-induced wobbling (precession). This marked the first time both effects had been measured simultaneously in a neutron star-black hole event.

The result was unambiguous. A Bayesian analysis comparing thousands of theoretical predictions against the actual data ruled out a circular orbit with 99.5% confidence. The orbit was oval, and measurably so.

Why the shape of an orbit matters

In gravitational wave astronomy, orbit shape is a forensic clue. It tells you something about where and how a binary system was born.

Most theories predict that neutron star-black hole pairs form in relative isolation -- two massive stars born together, evolving side by side, with one collapsing into a neutron star and the other into a black hole. Over millions of years, gravitational wave emission drains energy from the orbit, circularizing it long before the final merger.

An eccentric orbit disrupts that narrative. It suggests the pair did not evolve quietly in isolation but was shaped by gravitational interactions with other stars -- or possibly a third companion object. Dense stellar environments like globular clusters or the centers of galaxies, where stars are packed tightly enough to frequently perturb each other, are the most likely birthplaces for such systems.

Geraint Pratten, a Royal Society University Research Fellow at the University of Birmingham, put it directly: the elliptical shape just before merger shows this system was almost certainly shaped by gravitational interactions with other stars.

Correcting the record on GW200105

The new analysis also revised key physical parameters of the system. Earlier studies, which assumed a circular orbit, had underestimated the black hole's mass and overestimated the neutron star's mass. The corrected measurements indicate the merger produced a black hole roughly 13 times the mass of the Sun.

Notably, the team found no compelling evidence of precession -- the wobbling that would occur if the spinning objects were misaligned with their orbital plane. The absence of precession, combined with the presence of eccentricity, suggests the oval shape was imprinted by the system's formation environment rather than by the spins of the individual objects.

This distinction matters for theorists trying to sort binary mergers into formation channels. A system with eccentricity but no precession points toward what physicists call a dynamical formation channel -- assembly through gravitational encounters in a crowded stellar environment, rather than the co-evolution of a stellar binary.

What existing models missed

The discovery highlights a gap in the gravitational wave community's analytical toolkit. Most waveform models used to analyze LIGO and Virgo data assume circular orbits. If eccentricity is present but unaccounted for, it can bias mass measurements and obscure the true physics of the system.

Patricia Schmidt, from the University of Birmingham, noted that the discovery provides vital new clues about how extreme objects come together, adding that theoretical models are incomplete and that the finding raises fresh questions about where such systems are born.

Gonzalo Morras, from the Universidad Autonoma de Madrid and the Max Planck Institute, described the result as convincing proof that not all neutron star-black hole pairs share the same origin.

Limits of a single detection

This is still one event. While the statistical confidence is high -- 99.5% that the orbit was non-circular -- a single detection cannot establish how common eccentric mergers are among the broader population of neutron star-black hole systems. The LIGO-Virgo-KAGRA network has detected only a handful of confirmed neutron star-black hole mergers, and this is the first to show clear eccentricity.

Future observing runs, with improved detector sensitivity, should reveal more events. If eccentric orbits turn up in a significant fraction, it would fundamentally change estimates of how many mergers arise from dynamical formation versus isolated binary evolution. If GW200105 remains an outlier, the story is different but still important -- even rare formation channels need to be accounted for in population models.

The finding also underscores the need for more advanced waveform models capable of capturing eccentricity and precession simultaneously. As the catalog of gravitational wave events grows, the ability to extract this information from every signal will become increasingly important.