For years, astronomers have treated the final moments before a black hole swallows a neutron star as relatively predictable. As the two dead stellar remnants spiral together, their orbit is expected to smooth into an almost perfect circle before the violent merger sends gravitational waves rippling across the universe. However, a newly reanalyzed collision suggests that picture may be far too simple.
In a study published in The Astrophysical Journal Letters, an international team reports evidence that one black hole–neutron star pair followed a stretched, elliptical orbit right up until impact, marking the first time researchers have identified such an unusual configuration in this kind of merger.
The finding, based on gravitational-wave data collected by the LIGO and Virgo observatories, suggests some of the universe’s most extreme collisions may form in far more chaotic environments than scientists assumed.
The event, known as GW200105, was first detected in 2020. But using a new gravitational-wave model developed at the University of Birmingham, researchers found that the system’s final orbit was not circular at all. Instead, the neutron star and black hole appear to have been locked in an elongated cosmic dance before merging into a black hole roughly 13 times the mass of the Sun.
The discovery matters because orbital shape can preserve clues about how such systems formed. Circular orbits are generally thought to form when two objects evolve together in relative isolation over long periods, gradually radiating energy away. An elliptical orbit, by contrast, points to a more turbulent origin—one shaped by close gravitational encounters with other stars or perhaps even a third companion.
Neutron Star-Black Hole Pairs
Neutron stars are cosmic leftovers created during the supernova deaths of massive stars. These enormous explosions leave behind a superdense core containing roughly as much mass as our Sun, compressed into a sphere about the size of a city. Neutron stars possess extremely strong magnetic fields and rotate rapidly, making them among the most intense objects in the universe despite their small size.
However, the extreme gravitational pull of black holes is too much for these tiny but powerful stellar remnants. When a neutron star wanders too close to a black hole, it becomes captured by the black hole’s gravity and pulled into orbit. In most cases, the pair settles into a circular orbit that tightens as the neutron star spirals inward. Eventually, the neutron star is consumed by the black hole, leaving behind a larger black hole and sometimes an accretion disk formed from leftover debris.
A Gravitational Wave Observation
Records of such cosmic violence are preserved in the gravitational waves produced by these mergers. These ripples travel across the universe, creating tiny but measurable distortions in spacetime that allow scientists to reconstruct events that occurred billions of years ago.
It was through these waves that researchers identified the unusual event. The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and the Virgo interferometer in Italy both detected the gravitational-wave event known as GW200105.
According to the team’s analysis, using a gravitational-wave model developed at the University of Birmingham, the neutron star and black hole were locked in an elliptical orbit before colliding. The merger ultimately produced a black hole roughly 13 times the mass of the Sun.
A Strange Black Hole Orbit
This finding was highly unusual, as researchers had previously detected only circular orbits in similar collisions. However, measurements of the system’s orbital eccentricity and precession revealed that this event was a striking cosmic outlier, with a confidence level of 99.5 percent.
“This discovery gives us vital new clues about how these extreme objects come together,” said co-author Dr. Patricia Schmidt of the University of Birmingham. “It tells us that our theoretical models are incomplete and raises fresh questions about where in the Universe such systems are born.”
“The orbit gives the game away. Its elliptical shape just before merger shows this system did not evolve quietly in isolation but was almost certainly shaped by gravitational interactions with other stars, or perhaps a third companion,” added co-author Geraint Pratten, a Royal Society University Research Fellow from the University of Birmingham.
Rethinking a Black Hole Collision
While previous researchers had examined the GW200105 event, they were unable to measure its orbital eccentricity. Instead, earlier studies assumed a circular orbit, as had been observed in previous detections. As a result, those analyses underestimated the black hole’s mass and overestimated the neutron star’s mass.
“This is convincing proof that not all neutron star–black hole pairs share the same origin,” explained first author Gonzalo Morras, from the Universidad Autónoma de Madrid and the Max Planck Institute for Gravitational Physics. “The eccentric orbit suggests a birthplace in an environment where many stars interact gravitationally.”
The team’s findings suggest that neutron star–black hole mergers may be far more dynamic than previously thought. To better understand these events, astronomers will now need to develop more complex waveform models capable of accounting for multiple formation scenarios rather than assuming a single dominant pathway.
The paper, “Orbital Eccentricity in a Neutron++ Star–Black Hole Merger,” appeared in The Astrophysical Journal Letters on March 11, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.