Reanalysis of Landmark LIGO Signal Suggests Black Hole–Neutron Star Crash Had an Oval Orbit

When a faint shudder in spacetime washed through Earth on Jan. 5, 2020, astronomers hailed it as a long-awaited first: a black hole and a neutron star colliding in the dark, nearly a billion light-years away. The event, dubbed GW200105, quickly entered the books as a model merger on a near-perfect circular orbit.

Six years later, that orbit no longer looks so tidy.

A “smoking gun” in the orbit

A new analysis of the original LIGO and Virgo data finds that the black hole–neutron star pair was still moving on a distinctly oval path in the final seconds before they smashed together. That lingering eccentricity—a measure of how stretched an orbit is—is difficult to reconcile with the standard picture of two massive stars quietly evolving together in isolation.

Instead, researchers say, the warped orbit is a “smoking gun” that the system was forged in a cosmic crowd.

In a paper accepted this month in Astrophysical Journal Letters, an international team led by Gonzalo Morras of Universidad Autónoma de Madrid and the Max Planck Institute for Gravitational Physics reports clear statistical evidence that GW200105’s orbit was significantly eccentric when the signal entered the LIGO–Virgo detection band.

“The orbit gives the game away,” said co-author Geraint Pratten, a Royal Society University Research Fellow at the University of Birmingham. “Its elliptical shape at the moment we observe it tells us this system did not evolve quietly in isolation. It was almost certainly shaped by interactions with other stars or a third companion.”

The finding marks the first robust detection of orbital eccentricity in a merger between a black hole and a neutron star—and it rewrites the origin story of one of gravitational-wave astronomy’s landmark events.

What GW200105 looked like at first

GW200105 was detected during the third observing run of the U.S.-based LIGO and Europe’s Virgo interferometers. The signal, picked up strongly at LIGO’s Livingston, Louisiana, detector and faintly at Virgo near Pisa, Italy, showed the telltale chirp of two compact objects spiraling together and merging.

In 2021, the LIGO–Virgo–KAGRA collaboration announced that GW200105 and a second event, GW200115, were the first confident examples of mixed black hole–neutron star binaries. The initial analysis of GW200105, which assumed a circular orbit, indicated a black hole about 8.9 times the mass of the sun swallowing a neutron star roughly 1.9 solar masses, with the collision occurring about 900 million light-years from Earth.

At the time, collaboration spokespersons described the merger as a dark, clean plunge. “The black hole swallowed the neutron star whole,” Patrick Brady, then a LIGO spokesperson, said in a 2021 briefing, noting that the team did not expect—and did not see—any bright flash of light.

Those early studies followed a common simplifying assumption in gravitational-wave analysis: by the time merging objects reach the frequencies LIGO and Virgo can hear, their orbits have been circularized by the slow draining of energy through gravitational radiation. For systems born as isolated stellar binaries, most models predict eccentricities far below 0.01 by the time the signal enters the detectors’ sensitive band near 20 hertz.

Better models, a less circular story

The new work revisits that assumption.

Morras and colleagues used an updated family of waveform models that, for the first time in a neutron star–black hole case, simultaneously account for orbital eccentricity and spin-induced precession—a wobbling of the orbital plane caused by spinning objects. Previous studies either forced the orbit to be circular or treated eccentricity and precession separately, leaving room for one effect to masquerade as the other in the data.

Running a full Bayesian parameter estimation on the original strain data for GW200105, the team found that an eccentric model fits the observed signal far better than a circular one. They report a median orbital eccentricity of about 0.145 at a gravitational-wave frequency of 20 hertz, with values below 0.028 ruled out at 99.5% confidence.

Put differently, the final orbit was noticeably stretched—more like a cosmic racetrack than a nearly perfect circle—just fractions of a second before the neutron star disappeared into the black hole.

“We see convincing proof that not all neutron star–black hole pairs share the same origin,” Morras said in a statement released by the Max Planck Institute. “The eccentric orbit points to an environment where many stars interact gravitationally, such as a dense star cluster or a triple-star system.”

The more sophisticated modeling also nudged the system’s inferred masses. Allowing for eccentricity, the analysis favors a somewhat heavier black hole and a lighter neutron star than the original circular-orbit fit suggested, yielding a more uneven mass ratio. The resulting “daughter” black hole formed after the merger weighs in at roughly 13 times the mass of the sun, according to summaries of the work.

Crucially, the authors do not find strong evidence that spin precession alone can explain the observed waveform distortions. That weakens a common alternative explanation and bolsters the case that the eccentricity is genuine.

Patricia Schmidt, a co-author and associate professor at the University of Birmingham’s Institute for Gravitational Wave Astronomy, said the discovery exposes gaps in existing models of how such systems form.

“This gives us vital new clues about how these extreme objects come together,” Schmidt said. “The presence of significant eccentricity so late in the inspiral is a smoking-gun signal that some neutron star–black hole binaries must form differently than our standard models predict.”

Why an eccentric orbit matters

Astrophysicists broadly consider three main channels for creating compact binaries that eventually merge:

• Isolated binary evolution: Two massive stars are born together, exchange mass, explode as supernovae and end up as a compact pair, gradually spiraling together on a nearly circular orbit.
• Dynamical formation: Binary partners are assembled or hardened through close encounters in crowded environments such as globular clusters, young massive star clusters or galactic nuclei.
• Hierarchical triples: A distant third companion can, through Kozai–Lidov oscillations, drive the inner pair to high eccentricity and tilt its orbit.

Residual eccentricity at high frequencies is difficult to produce in the first scenario. It emerges naturally in the latter two.

“The orbit we infer for GW200105 is exactly the kind of clue we were hoping gravitational waves would provide,” Schmidt said. “It points us back to the environments where these binaries are likely to be born—the crowded stellar cities, not the quiet suburbs.”

Hints of eccentric mergers have surfaced before in the population of binary black hole events, including GW190521 and GW200208_222617, but those inferences have been hampered by shorter signals and by uncertainties in waveform modeling. GW200105, with its lower total mass and therefore longer observable inspiral, offers more orbital cycles for analysis and a clearer view.

From discovery to forensics

The new result will now feed into broader population studies that aim to quantify how many mergers carry the dynamical fingerprints of clusters or triples. Some theoretical work has suggested that if a substantial fraction of neutron star–black hole events show eccentricity at frequencies around 20 hertz, triple-star evolution could dominate their formation.

For now, GW200105 is just one compelling data point. But researchers say it illustrates how gravitational-wave astronomy is evolving from a discovery phase to something more forensic.

When the first detections arrived in 2015 and 2017, the central questions were simply whether black holes and neutron stars merged at all, and how often. As the catalog grows, attention is turning to subtler features—orbit shapes, spin tilts, environmental clues—that can reveal where these systems lived before they crashed.

“Our theoretical models are incomplete,” Schmidt said. “What studies like this show is that by going back to existing data with better tools, we can piece together a much richer story of how and where these cosmic collisions happen.”

Future observatories such as the planned Einstein Telescope in Europe and the Cosmic Explorer in the United States, with improved sensitivity at lower frequencies, are expected to catch compact binaries earlier in their inspiral, when eccentricity signals are even clearer. That, scientists say, will help determine whether GW200105 is an outlier—or the first well-identified member of a surprisingly eccentric family.