Scientists have discovered that before black holes collide with neutron stars and merge, these extreme stellar remnants can swirl around each other in oval rather than circular orbits. The revelation demonstrates another way in which black holes and neutron stars are pushing the laws of physics, casting doubt on assumptions about the formation and evolution of these mixed binary systems.
A team of researchers challenged assumptions that black holes and neutron stars approach each other in circular orbits when they studied ripples in spacetime, or gravitational waves, emanating from just 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.
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Key to the team’s discovery was a new model of gravitational waves developed at the University of Birmingham’s Institute of Gravitational Wave Astronomy, which enabled Schmidt and colleagues to determine the orbits of the progenitor objects.
This included calculating how much the black hole and neutron star that collided to create this gravitational wave signal wobbled, or “precessed”, before the merger. The calculations revealed a lack of precession before the merger.
This is the first time these properties 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 suggest the influence of an unseen third object in this system.
“The bottom gives the game away. 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 the parent objects outside this merger, scientists had underestimated the mass of the black hole to be about 9 times the mass of the Sun, and the neutron star has a mass of about 2 solar masses.
“This is compelling evidence that not all neutron star-black hole pairs share the same origin,” said team member Gonzalo Morras, of the Universidad Autónoma de Madrid, Spain. “The eccentric orbit suggests a birthplace in an environment where many stars interact gravitationally.”
The researchers’ results indicate that there are likely multiple ways in which black hole-neutron star mergers can proceed, rather than there being a dominant formation channel.
This may help explain why astronomers are increasingly seeing diversity when it comes to merging binary stellar remnants. The team’s results were published on Wednesday (March 11) in the Astrophysical Journal Letters.
