Two black holes accidentally met and created something never seen before: ScienceAlert

The ripples in spacetime created by colliding black holes have taught us a lot about these enigmatic objects.

These gravitational waves encode information about the black holes: their masses, the shape of their inner spiral relative to each other, their spins, and their orientations.

From this, scientists determined that most of the collisions we’ve seen were between black holes in binary systems. The two black holes started out as a binary set of massive stars that became black holes together, then spiraled and merged.

Of the 90 or so mergers identified so far, however, one stands out as very strange. Spotted in May 2019, GW19052 emitted space-time ripples like no other.

“Its morphology and burst-like structure is very different from previous observations,” says astrophysicist Rossella Gamba of the University of Jena in Germany.

He adds, “GW190521 was originally analyzed as the merger of two rapidly rotating supermassive black holes approaching each other along near-circular orbits, but its special features led us to suggest other possible interpretations.”

Specifically, its short, sharp duration gravitational wave signal was difficult to explain.

Gravitational waves are created by the actual merging of two black holes, like ripples from a rock falling into a lake. But they are also created by binary inspiration, and the strong gravitational interaction emits weaker ripples as two black holes inevitably approach each other.

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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, which occurred in the absence of a spiral phase,” explains astronomer Alessandro Nagar. National Institute of Nuclear Physics in Italy.

There is 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, perhaps even from the formation of baby stars from the same piece of molecular cloud in space.

The other is when two objects moving through space pass each other close enough to become gravitationally trapped in what is known as a dynamical encounter.

That’s what Gamba and her colleagues thought might be the case with GW190521, so they designed simulations to test their hypothesis. They smashed pairs of black holes together, tweaking parameters such as orbit, spin and mass, to try to reproduce the strange gravitational wave signal detected in 2019.

Their results suggest that the two black holes did not start out in a binary system, but were caught in each other’s gravitational pull, passing by each other twice in a wild, eccentric loop before colliding with each other to form a larger black hole. And none of the black holes in this scenario were rotating.

“By developing accurate models using a combination of advanced analytical methods and numerical simulations, we found that a highly eccentric merger in this case explains the observation better than any previous hypothesis,” says astronomer Matteo Breschi. the University of Jena.

“The chance of error is 1:4,300!”

This scenario, the team says, is more likely in a densely populated region of space, such as a star cluster, where such gravitational interactions are more likely.

This follows 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 form from a single star. the only way we know a black hole of this mass that can be formed is through mergers between two objects of lower mass.

The work by Gamba and her colleagues found that the masses of the two black holes in the collision are about 81 and 52 solar masses. This is slightly lower than previous estimates, but one of the black holes is still outside the core-collapse formation path of a star.

It is not yet clear whether our models need tweaking, 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.

Dynamical encounters between black holes are thought to be quite rare, and the gravitational wave data collected by LIGO and Virgo to date seem to support this. However, rare does not mean impossible, and the new work suggests that GW190521 may be the first we’ve spotted.

And a first means there could be more in the years to come. The gravitational wave The observatories are currently being upgraded and maintained, but will come back online in March 2023 for a new observing run. This time, LIGO’s two detectors in the US and the Virgo detector in Italy will join KAGRA in Japan for even greater observing capacity.

More detections like GW190521 would be amazing.

The research has been published in Astronomy of Nature.

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