The ripples in spacetime created by colliding black holes have taught us much about these enigmatic objects.
These gravitational waves encode information about black holes: their masses, the shape of their inner spiral relative to each other, their spins, and their orientation.
From this, scientists have determined that most of the collisions we have seen have occurred between black holes in binary star systems. The two black holes started out as a binary system of massive stars that turned together into black holes, then spiraled into each other and merged.
Of the around 90 mergers uncovered so far, however, one stands out as very peculiar. Discovered in May 2019, GW19052 emitted space-time ripples like no other.
“Its morphology and explosive structure are very different from previous observations,” says astrophysicist Rossella Gamba of the University of Jena in Germany.
She adds: “GW190521 was originally analyzed as the merger of two rapidly spinning heavy black holes approaching in nearly circular orbits, but its peculiar features led us to propose other possible interpretations.”
In particular, the short, sharp duration of the gravitational wave Signal was difficult to explain.
Gravitational waves are created by the actual merger of two black holes, like ripples from a rock falling into a pond. But they are also generated by binary inspiration, and the intense gravitational interaction emits fainter waves as two black holes approach inexorably.
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“The shape and brevity – less than a tenth of a second – of the signal associated with the event leads us to hypothesize an instantaneous merger between two black holes that occurred without a spiral phase,” explains astronomer Alessandro Nagar of the Nationale Institution of Nuclear Physics in Italy.
There’s more than one way to end up with a pair of gravitationally interacting black holes.
The first is that the two have been together for a long time, maybe even since 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 to get stuck by gravity in what is called a dynamic encounter.
Gamba and her colleagues thought this might have happened to GW190521, so they designed simulations to test their hypothesis. They smashed pairs of black holes, tweaking parameters like trajectory, spin and mass to try to reproduce the odd gravitational wave Signal detected in 2019.
Their results suggest that the two black holes didn’t start out in a binary system, but instead were caught in each other’s gravitational web and tumbled past each other twice in a wild, eccentric loop before colliding to form a larger one black hole. And none of the black holes in this scenario rotated.
“By developing precise models using a combination of state-of-the-art analysis methods and numerical simulations, we found that a highly eccentric merger in this case explains the observation better than any other hypothesis put forward beforehand,” says astronomer Matteo Breschi from the University of Jena.
“The probability of error is 1:4,300!”
That scenario, the team says, is more likely in a densely populated region of space, like a star cluster, where such gravitational interactions are more likely.
This follows up 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 larger than 65 solar masses cannot form from a single star; the only way we know a black hole This mass can form is due to mergers between two objects of lower mass.
Gamba and her colleagues’ work found the masses of the two black holes in the collision to be around 81 and 52 solar masses; That’s slightly lower than previous estimates, but one of the black holes is still out of the path of forming the single stellar core collapse.
It is still unclear whether our models need to be optimized, but hierarchical mergers – where larger structures form through 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 fairly rare, and the gravitational 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 spotted.
And a premiere means there could be more to come in the years to come. tea gravitational wave Observatories are currently being upgraded and maintained but will come back online in March 2023 for a new observing run. This time, the two detectors from LIGO in the US and the Virgo detector in Italy are joined by KAGRA in Japan for even more observing power.
More detections like GW190521 would be amazing.
The research was published in natural astronomy.
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