Black hole and neutron star mergers push the laws of physics with their odd orbits
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Illustration of an eccentric neutron star – black hole binary. The trajectory of the neutron star is shown in blue and the movement of the black hole in orange as the two objects rotate around each other. | Credit: Geraint Pratten, Royal Society Research Fellow, University of Birmingham
Scientists have discovered that before black holes collide with neutron stars and merge, these extreme stellar remnants can swirl around each other in oval orbits rather than circular orbits. The revelation demonstrates another way in which black holes and neutron stars push the laws of physics and cast doubt on assumptions about the formation and evolution of these mixed binary systems.
A team of scientists questioned the assumption that black holes and neutron stars would come together in circular orbits when they studied the ripples in space-time, or gravitational waves, that resulted from such a “mixed merger.” The signal from this merger, called 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, resulting in the creation of a daughter black hole with about 13 times the mass of the sun.
“This discovery gives us vital new clues about how these extreme objects come together,” said team member Patricia Schmidt of the University of Birmingham in the United Kingdom. said in a statement. “This tells us that our theoretical models are incomplete and raises new questions about where such systems arise in the universe.”
Key to the team’s discovery was a new gravitational wave model developed at the University of Birmingham’s Institute of Gravitational Wave Astronomy, which allowed Schmidt and his colleagues to determine the orbits of the progenitor objects.
This involved calculating how much the black hole and neutron star that collided to create this gravitational wave signal flickered, or “preceded,” before they merged. The calculations revealed an absence of precession before the merger.
This is the first time these features have been measured for a “mixed merger” between a black hole and a neutron star, both of which are stellar remnants created when massive stars “die” and undergo gravitational collapse. The results hint at the influence of a third invisible object in this system.
“The orbit reveals the game. Its elliptical shape just before the merger shows that this system did not evolve quietly in isolation, but was almost certainly shaped by gravitational interactions with other stars, or a third companion,” Schmidt continued.
Previously, when a circular orbit had been considered for progenitor objects beyond this merger, researchers had underestimated the mass of the black hole as being about 9 times the mass of the sun, and the neutron star having a mass of about 2 solar masses.
“This is convincing evidence that not all neutron star-black hole pairs share the same origin,” said team member Gonzalo Morras from the Autonomous University of Madrid, Spain. “The eccentric orbit suggests a birthplace in an environment where many stars interact gravitationally.”
Illustration of a mixed neutron star and black hole merger | Credit: Carl Knox, OzGrav – Swinburne University
The scientists’ results indicate that there are likely multiple ways for black holes and neutron stars to merge, rather than just one dominant formation channel.
This could help explain why astronomers are seeing more and more diversity in merging stellar remnant binaries. The team’s results were published on Wednesday March 11 in letters from the astrophysical journal.
