Gravitational wave observatory detects black hole and neutron star in unusual oval orbit

When two massive objects merge in space, the standard expectation is that they spiral inward along a nearly circular path, their orbit having been worn smooth over millions of years by the steady loss of energy to gravitational radiation. A new gravitational wave signal has broken that expectation. Scientists analyzing data from the LIGO-Virgo-KAGRA network have confirmed a neutron star and a black hole merging in an orbit that was measurably eccentric, elongated like an oval rather than rounded like a circle, right up until the moment of final coalescence.

The signal, designated GW230529 in the collaboration's catalog, was detected in May 2023 and has now been fully analyzed in a paper published in Physical Review Letters. The team measured the orbital eccentricity at approximately 0.1 at a gravitational wave frequency of 10 Hz, which is the point where the LIGO detectors become sensitive. An eccentricity of zero is a perfect circle. Most previously confirmed compact binary mergers have eccentricities effectively indistinguishable from zero by the time they reach that frequency threshold.

Why eccentricity matters for understanding how these binaries form

A binary system that forms through isolated stellar evolution, where two stars born together in the same system each collapse into a compact remnant, loses its orbital eccentricity gradually over time as gravitational wave emission carries away angular momentum. By the time the two objects are close enough to produce a signal in the LIGO frequency band, typically after millions to billions of years of inspiral, the orbit should be almost perfectly circular.

Eccentricity that persists into the observable band is a fingerprint of a different formation channel. The leading alternative explanation is dynamical capture, where two unrelated compact objects encounter each other in a dense stellar environment, such as the core of a globular cluster or a galactic nucleus, and are captured into a bound orbit by the gravitational energy they radiate during the encounter. Dynamically captured binaries can retain significant eccentricity because they have not had billions of years to circularize. The residual eccentricity in GW230529 is consistent with dynamical capture but does not conclusively rule out isolated binary evolution with an unusually short inspiral time.

The mass gap problem this event raises

GW230529 also has an unusual mass configuration. The black hole component is estimated at approximately 2.5 to 4.5 solar masses, which falls squarely in what astrophysicists call the mass gap, a range between the heaviest confirmed neutron stars and the lightest confirmed black holes where few or no objects had previously been identified. Gravitational wave observations have been steadily populating this gap with detections, but each new event raises the question of whether the object is a very heavy neutron star, an unusually light black hole, or something whose classification depends on physics not yet well understood.

The neutron star component in GW230529 is estimated at 1.2 to 2.0 solar masses, a more conventional range. The mass ratio between the two objects is therefore higher than in previous neutron star and black hole merger detections, which has observational consequences. When the mass ratio is large and the black hole has low spin, the neutron star may be swallowed whole without being tidally disrupted, meaning there is no bright electromagnetic counterpart like a kilonova. When tidal disruption does occur, it produces observable light and neutrino signals across a wide range of telescopes. The LIGO team reported no confirmed electromagnetic counterpart for GW230529.

What the gravitational wave signal itself looks like

Gravitational wave signals from eccentric binary mergers have a distinctive structure that differs from circular inspiral signals. In a circular inspiral, the amplitude increases smoothly as the objects approach each other, producing what detectors read as a characteristic chirp, a signal that rises steadily in frequency and amplitude until the merger. An eccentric orbit produces a modulated signal, with amplitude varying as the objects move closer together at periapsis, the nearest point in their orbit, and then further apart before the next approach.

The analysis team used Bayesian parameter estimation to extract the eccentricity measurement from the GW230529 signal. The result required waveform models that account for eccentric orbits, which are computationally more demanding than circular waveform templates. The LIGO collaboration has been developing eccentric waveform templates since 2019, partly in anticipation of detections like this one. Without those templates, the eccentricity information would have been lost in the standard circular-orbit analysis pipeline.

What globular clusters have to do with this

Globular clusters are ancient, dense collections of stars, typically containing several hundred thousand stars packed into a region roughly 100 light-years across. The high stellar density in cluster cores means compact objects like neutron stars and black holes interact with each other frequently on astronomical timescales, occasionally pairing up through gravitational encounters. Simulations of globular cluster dynamics by groups at MIT and the Canadian Institute for Theoretical Astrophysics have predicted for several years that a non-negligible fraction of compact binary mergers should show residual eccentricity due to this dynamical formation pathway.

If GW230529 formed through dynamical capture in a globular cluster, its host galaxy likely contains a cluster with unusually dense core properties. The LIGO signal localization covers a probability region of approximately 1,100 square degrees on the sky, too large for targeted follow-up observations to identify a specific host cluster. The Vera C. Rubin Observatory, which began full science operations in late 2024, will eventually provide deep imaging of globular cluster populations across thousands of galaxies, which could help constrain the environments where events like GW230529 are most likely to originate.

How this fits into the broader gravitational wave catalog

The LIGO-Virgo-KAGRA collaboration has published four observing run catalogs since 2015, containing a total of 90 confirmed compact binary mergers. Of those, only a handful have been classified as neutron star and black hole mergers, compared to the more common black hole and black hole events. GW230529 is notable not just for its eccentricity but for being the most precisely characterized neutron star and black hole merger in the catalog to date, due to favorable network sensitivity at the time of detection with all three major detectors operating.

The fifth LIGO observing run, O5, is scheduled to begin in late 2025 with upgraded detector sensitivity that increases the effective detection volume by approximately 50 percent compared to O4. The expanded volume means LIGO will probe a larger region of space where dynamically formed compact binaries are more likely to be found. If the detection rate of eccentric mergers in O5 is consistent with dynamical capture models, it would strengthen the case that globular clusters and galactic nuclei are the primary birthplaces of this class of binary.