Study finds neutron star–black hole mergers can occur in eccentric orbits
The laws of physics challenged by Black Holes and Neutron Stars prior to collision.
Scientists have found evidence that some neutron star–black hole pairs spiral toward each other in elongated, elliptical orbits, challenging previous assumptions that such systems merge from nearly circular paths.
The finding comes from an analysis of gravitational waves produced by a neutron star–black hole collision known as GW200105, detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo. The event occurred about 910 million light-years from Earth and produced a black hole roughly 13 times the mass of the Sun.
Researchers from the University of Birmingham’s Institute of Gravitational Wave Astronomy used a new gravitational-wave model to reconstruct the orbital dynamics of the system before the merger. Their analysis indicates the two objects followed an eccentric orbit, rather than the circular trajectory commonly assumed in earlier models.
“This discovery gives us new clues about how these extreme objects come together,” said Patricia Schmidt, a researcher at the University of Birmingham. “It also suggests that our theoretical models of how these systems form are incomplete.”
By examining the gravitational-wave signal, the team measured how much the objects were precessing, or wobbling, as they orbited each other. The data showed little evidence of precession, which helped reveal the system’s elliptical orbital shape shortly before the merger.
The results mark the first time astronomers have measured these orbital properties for a mixed merger involving both a neutron star and a black hole.
Researchers say the eccentric orbit suggests the system likely formed in a dense stellar environment, where gravitational interactions with nearby stars—or possibly a third companion object—altered its trajectory.
“The orbit gives the game away,” Schmidt said. “Its elliptical shape just before merger indicates the system was likely influenced by gravitational interactions with other stars.”
Earlier analyses that assumed a circular orbit underestimated the masses of the objects involved.
The study suggests that neutron star–black hole mergers may form through multiple evolutionary pathways, rather than a single dominant formation mechanism.
The findings were published March 11 in a new study in the Astrophysical Journal Letters, examining the dynamics of neutron star–black hole binary systems.
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