Astronomers have uncovered evidence that a black hole and neutron star merged while following an unusual oval-shaped orbit, challenging expectations of circular paths in such events. The discovery comes from a reanalysis of gravitational wave data from the event known as GW200105. This finding suggests the system formed in a dynamic stellar environment.
In a study published on March 11 in The Astrophysical Journal Letters, researchers from the University of Birmingham, Universidad Autónoma de Madrid, and the Max Planck Institute for Gravitational Physics analyzed data from the LIGO and Virgo detectors. They focused on the gravitational wave signal GW200105, which originated from the merger of a neutron star and a black hole. The merger resulted in a new black hole approximately 13 times the mass of the Sun.
The team used a new model developed at the University of Birmingham's Institute of Gravitational Wave Astronomy to assess the orbit's eccentricity and any spin-related precession. Their Bayesian analysis compared thousands of theoretical models to the signal, concluding with 99.5% confidence that a circular orbit was highly unlikely. This marks the first measurement of both eccentricity and precession together in a neutron star-black hole merger.
Dr. Patricia Schmidt from the University of Birmingham stated, "This discovery gives us vital new clues about how these extreme objects come together. It tells us that our theoretical models are incomplete and raises fresh questions about where in the Universe such systems are born."
Geraint Pratten, a Royal Society University Research Fellow at the University of Birmingham, added, "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."
Earlier analyses of GW200105 had assumed a circular orbit, leading to an underestimation of the black hole's mass and an overestimation of the neutron star's mass. The new study found no strong evidence for precession, indicating the eccentricity likely arose during the system's formation.
Gonzalo Morras from Universidad Autónoma de Madrid and the Max Planck Institute noted, "This is convincing proof that not all neutron star-black hole pairs share the same origin. The eccentric orbit suggests a birthplace in an environment where many stars interact gravitationally."
The research points to multiple formation pathways for these mergers, particularly in crowded stellar regions, and aligns with the growing diversity of gravitational wave detections.