The ripples in space-time 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 upward spiral, their spins and their orientations.
From this, the scientists ascertained that most of the collisions we have seen have occurred between black holes in binary systems. The two black holes started out as a binary of massive stars that morphed into black holes together, then spiraled and merged.
Of the approximately 90 mergers detected so far, however, one stands out as very peculiar. 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 of the University of Jena in Germany.
He adds: ‘GW190521 was initially analyzed as the merger of two rapidly spinning heavy black holes approaching each other in nearly circular orbits, but its special characteristics led us to propose other possible interpretations.’
In particular, the short and sharp 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 rock that has fallen into a pond. But they’re also generated by the binary spiral, and the intense gravitational interaction emits fainter 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 a spiral phase”, explains 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 have been together for a long time, perhaps 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 each other closely enough that they become gravitationally entangled in what is known as 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 have smashed pairs of black holes apart, changing parameters such as trajectory, rotation 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 off in a binary system, but were trapped in each other’s gravitational web, rolling past each other twice in a wild and eccentric spin before colliding. together 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 analytical methods and numerical simulations, we found that a highly eccentric merger in this case explains the observation better than any hypothesis advanced previously,” 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 region of space, such as a star cluster, where such gravitational interactions are more likely.
This tracks previous discoveries 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 or that mass can form through mergers between two objects of lower mass.
The work of Gamba and his colleagues found that the masses of the two black holes in the collision lie at about 81 and 52 solar masses; is slightly lower than previous estimates, but one of the black holes is still outside the single-star core collapse formation path.
It’s not yet clear whether our models need tweaking, but hierarchical mergers (whereby 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 quite rare, and the gravitational wave the data collected by LIGO and Virgo to date would appear to support this. However, rare doesn’t mean impossible, and the new work suggests that 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 undergoing updates and maintenance, but will be back online in March 2023 for a new observing session. This time, the two detectors of LIGO in the United States and the Virgo detector in Italy will be joined by KAGRA in Japan for even greater observing power.
More detections like GW190521 would be surprising.
The research was published in Nature astronomy.