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Illustration of an eccentric neutron star-black hole binary. The path of the neutron star is shown in blue and the motion of the black hole in orange as the two objects orbit each other. |Image credit: Geraint Pratten, Royal Society University Fellow at the University of Birmingham
Scientists have discovered that before black holes collide and merge with neutron stars, these extreme stellar remnants can rotate around each other in elliptical orbits rather than circular ones. The discovery demonstrates another way in which black holes and neutron stars drive the laws of physics and calls into question assumptions about the formation and evolution of these hybrid binary systems.
A team of scientists has challenged the assumption that black holes and neutron stars approach each other in circular orbits while studying the ripples in space-time, or gravitational waves, produced by such “hybrid mergers.” The signal produced by this merger, known as GW200105, was detected by the gravitational wave detectors Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo. The merger occurred about 910 million light-years away, creating a daughter black hole with a mass about 13 times that of the sun.
“This discovery gives us important new clues about how these extreme objects come together,” said team member Patricia Schmidt from the University of Birmingham in the UK. said in a statement. “It tells us that our theoretical models are incomplete and raises new questions about where in the universe such systems arise.”
Key to the team’s discovery was a new gravitational wave model developed at the University of Birmingham’s Gravitational Wave Astronomy Institute, which allowed Schmidt and colleagues to determine the orbit of the original object.
This involves calculating how much a black hole and a neutron star, whose collisions produce gravitational wave signals, wobble or “precess” before merging. Calculations show a lack of precession before the merger.
This marks the first time these features have been measured in a “hybrid merger” between a black hole and a neutron star, both of which are stellar remnants produced when massive stars “die” and undergo gravitational collapse. The results hint at the influence of an unseen third object in the system.
Schmidt continued: “The orbit gives it all away. Its elliptical shape before the merger suggests that the system did not quietly evolve in isolation, but almost certainly formed through gravitational interactions with other stars or a third companion.”
Previously, when considering the circular orbits of the original objects outside this merger, researchers had underestimated the mass of the black hole at about 9 times the mass of the Sun and the mass of the neutron star at about 2 times the mass of the Sun.
“This is compelling evidence that not all neutron star-black hole pairs have the same origin,” said team member Gonzalo Moraes from the Universidad Autònoma of Madrid in Spain. “Eccentric orbits suggest that the birthplace is in an environment where many stars interact with each other through gravity.”
Illustration of a neutron star-black hole hybrid merger | Image credit: Carl Knox, OzGrav – Swinburne University
The scientists’ results suggest that the merger of a black hole and a neutron star may occur in multiple ways, rather than having just one main formation channel.
This may help explain why astronomers are increasingly seeing the diversity of binary mergers of stellar remnants. The team’s findings were published on Wednesday (March 11) in Astrophysical Journal Letters.
