J. Samsing / Neils-Bohr Institute
In 2019, the LIGO/VIRGO collaboration captured a gravitational-wave signal from a black hole merger that turned out to be one of the most record-breaking. Dubbed ‘GW190521’, it was the most massive and distant ever detected and produced the most energetic signal yet detected and appeared in the data as a ‘bang’ rather than the usual ‘chirp’ .
Also, the resulting new black hole from the merger was about 150 times heavier than our Sun, making GW190521 the first direct observation of an intermediate-mass black hole. Stranger still, the two black holes that merged were confined in an elliptical (rather than circular) orbit, and their axes of rotation were tilted much more than usual relative to those orbits.
Physicists love nothing more than to be faced with an intriguing puzzle that doesn’t seem to immediately fit into established theory, and GW190521 has given them just that. New theoretical simulations suggest that all these bizarre aspects can be explained by the presence of a single third black hole on the binary system’s last dance to create a “chaotic tango”. a new paper published in the journal Nature.
As we previously reportedOn May 21, 2019, the collaboration’s detectors picked up the telltale signal of a binary black hole merger: four brief oscillations lasting less than a tenth of a second. The shorter the signal, the more massive the merging black holes—in this case, 85 and 66 solar masses, respectively. The black holes merged into a new, even larger black hole of about 142 solar masses, releasing the energy equivalent of 8 solar masses, hence the strong signal picked up by the detectors.
What made this event so unusual is that the measurement of 142 solar masses falls right in the middle of a so-called “mass gap” for black holes. Most of these objects belong to two groups: stellar-mass black holes (ranging from a few solar masses to tens of solar masses) and supermassive black holes like the one at the center of our galaxy, the Milky Way (ranging from hundreds of thousands to billions of solar masses). The former are the result of the death of massive stars in a core-collapse supernova, while the formation process of the latter remains a mystery.
LIGO/Caltech/MIT/R. injured (IPAC)
The fact that one of the progenitor black holes weighs 85 solar masses is also highly unusual, contradicting current models of stellar evolution. The types of stars that would produce black holes between 65 and 135 solar masses would not become supernovae and therefore would not end up as black holes. On the contrary, these stars would become unstable and lose a significant part of their mass. Only then would they go supernova, but the result would be a black hole less than 65 solar masses.