May 24, 2022

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Black hole ‘pool’ may explain strange aspects of 2019’s black hole merger

Black hole 'pool' may explain strange aspects of 2019's black hole merger
Zoom / Illustration of a swarm of small black holes in a gas disk orbiting a supermassive black hole.

Samsing / Niels Bohr Institute

In 2019, the LIGO/VIRGO collaboration picked up a gravitational wave signal from a black hole merger that proved to be one of the record books. Dubbed ‘GW190521’, it was the most massive and farthest yet detected, producing the most energetic signal detected to date, appearing in the data as more ‘loud’ than the usual ‘chirp’.

Furthermore, the new black hole resulting from the merger was about 150 times more powerful than our sun’s weight, making GW190521 the first direct observation of an intermediate-mass black hole. Even stranger, the two black holes that merged were locked in an elliptical (not circular) orbit, and their spin axes were tilted more than usual compared to those orbits.

Physicists like nothing more than to present an intriguing puzzle that doesn’t immediately seem to fit in well-established theory, and GW190521 gave them just that. New theoretical simulations suggest that all of these strange aspects can be explained by the presence of a single, third black hole beating the last dance of the binary system to produce a “messy dance,” according to new paper Published in Nature magazine.

as we are I mentioned earlierOn May 21, 2019, the collaboration’s detectors picked up the telltale signal of a black hole binary merger: four short vibrations lasting less than a tenth of a second. The shorter the signal, the more mass the black holes are merging – in this case, 85 and 66 solar masses, respectively. The black holes merged to form a new black hole larger than about 142 solar masses, emitting the equivalent of eight solar masses in the process – thus, the strong signal was picked up by the detectors.

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What made this event so unusual is that the measurement of 142 solar masses lies in the middle of what is known as the “mass gap” of black holes. Most of these objects fall into two groups: stellar-mass black holes (ranging from a few solar masses to tens of solar masses) and supermassive black holes, such as those in the middle of our Milky Way (ranging from hundreds of thousands to billions of solar masses). The former is caused by the death of massive stars in a supernova collapsing in its core, while the process of forming the latter remains a mystery.

Artist's concept of a hierarchical scheme for merging black holes.  Scientists hypothesize that the two black holes were the result of a previous merger of two small black holes.
Zoom / Artist’s concept of a hierarchical scheme for merging black holes. Scientists hypothesize that the two black holes were the result of a previous merger of two small black holes.

LEGO / Caltech / MIT / R. Hurt (IPAC)

The fact that a black hole ancestor weighed 85 solar masses is also unusual since this conflicts with current models of stellar evolution. The types of stars that would give rise to black holes between 65 and 135 solar masses would not turn out to be supernovae, and therefore would not end up as black holes. Instead, these stars will become unstable and shed much of their mass. Only then will they become a supernova — but the result will be a black hole less than 65 solar masses.