A strange signal in the background hum created by a merger between two black holes could be the first ever detection of a system of three stellar-mass black holes locked in a gravitational waltz.
According to a new analysis of data from the LIGO-Virgo-KAGRA collaboration, data from a 2019 binary black hole collision showed signs of an anomalous acceleration that suggests the presence of a third black hole.
“This is the first international discovery of clear evidence for a third compact object in a binary black hole merger event,” says astronomer Wen-Biao Han of the Chinese Academy of Sciences.
“It reveals that the binary black holes in GW190814 may not have formed in isolation but were part of a more complex gravitational system, offering significant insights into the formation pathways of binary black holes.”
Related: Record-Sized Collision Between Black Holes Detected by Astronomers
Since the very first detection of gravitational waves back in 2015, scientists have cataloged some 300 or so mergers – events in which binary black holes finally complete their orbital decay and collide, coalescing into one object and sending gravitational waves rippling out through the fabric of space-time.
Astronomers can analyze the signals in those ripples to determine the masses of the black holes involved, with some mergers hinting at what we call hierarchical mergers – a series of mergers leading to bigger and bigger black holes.
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This is because stellar-mass black holes have an upper mass limit at formation – the point at which a massive star goes supernova and sheds its outer layers, leaving a core that collapses under its own gravity to become a black hole.
Above a certain star mass, the whole kit and kaboodle just explodes entirely, core and all, leaving nothing behind but debris – so if a black hole is detected above that limit, scientists infer that said black hole is the product of a prior merger.
GW190814 did not involve a black hole above the mass limit. Quite the contrary, one of the black holes involved is thought to be the smallest of its kind ever detected, so small it teeters on the brink of being a neutron star – just 2.6 times the mass of our own Sun.
The other black hole involved was significantly larger, around 23 solar masses. This mass ratio is outside what is predicted for stellar evolution models – binary objects are generally expected to consist of two objects of comparable size.
A team led by Shu-Cheng Yang of the Chinese Academy of Sciences believes that this mass ratio is indicative of a complicated past; a pair of black holes that was drawn together by the gravitational pull of a third, much larger object, around which the binary orbits.
So, they took a closer look at the gravitational wave data. A pair of black holes orbiting a larger third should exhibit an extra acceleration along the line of sight due to the orbital motion around the third black hole. The researchers worked out how this would manifest, then compared their model to GW190814’s data.
According to their model, the data suggest a line-of-sight acceleration of 0.0015 times the speed of light in a vacuum, with a confidence level of around 90 percent – hinting at the presence of a third, unseen black hole.
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This result could mean that black hole mergers can, at least in some circumstances, take place in much more complicated circumstances than we knew about. There may even be more hints of these – and other – complicated circumstances hiding in the data, waiting for someone to develop the tools to discern them.
In addition, the finding provides more evidence of hierarchical mergers, by validating the existence of black hole trinaries in which mergers can take place.
The next observing run of the LIGO-Virgo-KAGRA gravitational wave observatories is expected to provide a whole wealth of new data on black hole mergers. Perhaps it will also shed some light on the environments in which they take place, and the different ways that black hole interactions can play out throughout the Universe.
The research has been published in The Astrophysical Journal Letters.
