The ripples in space-time caused by colliding black holes have taught us a lot about these enigmatic objects.
These gravitational waves encode information about black holes: their masses, the shape of their inward spirals toward each other, their spins, and their orientations.
From this, scientists determined that most of the collisions we’ve seen occurred between black holes in binary systems. The two black holes started out as a binary of massive stars that turned into black holes together, then spiraled inwards and merged.
However, of the 90 or so fusions detected so far, one stands out as very peculiar. GW19052 was detected in May 2019 and emitted space-time ripples like no other.
“The morphology and explosion-like structure are very different from previous observations,” said astrophysicist Rossella Gamba of the University of Jena in Germany.
She adds: “GW190521 was initially analyzed as the merger of two rapidly rotating massive black holes approaching each other along nearly circular orbits, but the special features led us to propose other possible interpretations.”
In particular, the short, sharp duration of the gravity wave signal was challenging to explain.
Gravitational waves are generated by the actual merger of two black holes, like ripples from a rock falling into a pond. But they are also generated by the binary inspiration spiral, and the intense gravitational interaction sends out fainter ripples as two black holes inexorably approach.
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“The shape and brevity – less than a tenth of a second – of the signal associated with the event led us to hypothesize that there is an instantaneous merger between two black holes, which occurred without a spiral phase,” explains astronomer Alessandro Nagar. from the National Institute of Nuclear Physics in Italy.
There’s more than one way to end up with a pair of black holes interacting through gravity.
The first is that the two were together for a long time, perhaps even due to the formation of baby stars from the same piece of molecular cloud in space.
The other is when two objects moving through space pass close enough for gravity to snag in what’s known as a dynamic encounter.
This is what Gamba and her colleagues thought might have happened with GW190521, so they designed simulations to test their hypothesis. They smashed pairs of black holes together and adjusted parameters such as orbit, spin and mass to try and find the weird ones gravity wave signal detected in 2019.
Their results suggest that the two black holes didn’t start in a binary star, but were caught in each other’s gravitational web, tumbling past each other twice in a wild, eccentric loop before slamming together to form one larger hole. black hole. And none of the black holes in this scenario were spinning.
“By developing accurate models using a combination of advanced analytical methods and numerical simulations, we found that a highly eccentric fusion in this case explains the observation better than any other hypothesis previously put forward,” says astronomer Matteo Breschi of the University of Jena.
“The probability of error is 1:4.300!”
This scenario, the team says, is more likely in a densely populated part of space, such as a star cluster, where such gravitational interactions are more likely.
This is consistent with previous discoveries about GW190521. One of the black holes in the merger was measured to be about 85 times the mass of the sun.
According to our current models, black holes over 65 solar masses cannot arise from a single star; the only way we know a black hole of that mass can be formed by mergers between two objects of lower mass.
The work of Gamba and her colleagues found that the masses of the two black holes in the collision are between 81 and 52 solar masses; that is slightly lower than previous estimates, but one of the black holes is still outside the collapse path of the single star core.
It is still unclear whether our models need to be modified, but hierarchical mergers – in which larger structures are formed by the continuous merging of smaller objects – are more likely in a cluster environment with a large population of dense objects.
Dynamic encounters between black holes are considered quite rare, and the gravity wave data collected so far by LIGO and Virgo seems to support this. Rare doesn’t mean impossible, though, and the new work suggests GW190521 may be the first we’ve detected.
And a first means more could follow in the coming years. The gravity wave observatories are currently being upgraded and maintained, but will come back online in March 2023 for another observing run. This time, the two detectors from LIGO in the US and the Virgo detector in Italy will be joined by KAGRA in Japan for even more sensing power.
More detections like GW190521 would be great.
The research has been published in Nature Astronomy.