The ripples in spacetime generated 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 internal spiral towards each other, their spins and their orientations.
From there, the scientists established that most of the collisions we’ve seen have 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 turned into a spiral and merged.
However, of the approximately 90 mergers detected so far, one stands out as very particular. Detected in May 2019, GW19052 emitted space-time ripples like no other.
“Its morphology and explosion-like structure are very different from previous observations,” says astrophysicist Rossella Gamba from the University of Jena in Germany.
She adds: “GW190521 was initially analyzed as the merger of two rapidly rotating heavy black holes approaching each other in nearly circular orbits, but its peculiar characteristics have led us to propose other possible interpretations. “
In particular, the short and short duration of the gravitational wave signal was difficult to explain.
Gravitational waves are generated by the actual merging of two black holes, like the ripples of a fallen rock in a pond. But they are also generated by the binary spiral, and the intense gravitational interaction sends weaker ripples as two black holes inexorably 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 lead us to hypothesize an instantaneous merger between two black holes, which occurred in the absence of phase spiral,” says astronomer Alessandro Nagar of the National Institute 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 had been together for a long time, possibly ever since baby stars formed from the same piece of molecular cloud in space.
The other is when two objects moving through space pass each other close enough to gravitationally get stuck in what is called a dynamic encounter.
This is what Gamba and his colleagues thought might have happened with GW190521, so they designed simulations to test their hypothesis. They shattered pairs of black holes together, adjusting parameters such as trajectory, spin and mass, to try to replicate the strange gravitational wave signal detected in 2019.
Their findings suggest the two black holes didn’t start out as binary, but got caught up in each other’s gravitational web, crossing each other twice on a wild, eccentric loop before slamming together to form a larger one. black hole. And none of the black holes in this scenario were spinning.
“By developing accurate models using a combination of state-of-the-art analytical methods and numerical simulations, we found that a very eccentric fusion in this case better explains the observation than any other previously put forward hypothesis,” says astronomer Matteo Breschi. from the University of Jena.
“The probability of error is 1:4,300!”
According to the team, this scenario 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 findings on 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 of this mass can form through fusions between two objects of lower mass.
Work by Gamba and his colleagues found that the masses of the two black holes in the collision are around 81 and 52 solar masses; this is slightly lower than previous estimates, but one of the black holes is still outside the formation path of a single star’s core collapse.
It’s still unclear if our models need tweaking, but hierarchical merges – in which larger structures form by the continuous merging of smaller objects – are more likely in a cluster environment with a large population. dense objects.
Dynamic encounters between black holes are considered quite rare, and gravitational wave the data collected by LIGO and Virgo to date seem to confirm this. However, rare doesn’t mean impossible, and the new work suggests GW190521 may be the first we’ve detected.
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 be back online in March 2023 for a new observing period. This time, the two detectors from LIGO in the United States and the Virgo detector in Italy will be joined by KAGRA in Japan for even more observing power.
More detections like GW190521 would be amazing.
The research has been published in natural astronomy.
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