The nature of the mysterious source of gravitational waves GW190521 called into question

There were several anomalies in the signal from the gravitational wave source detected on Earth on May 21, 2019 by Ligo and Virgo, not to mention the surprisingly high masses of the black holes involved in producing GW190521. New analysis of the signal now strongly suggests that we were not dealing with a typical black hole collision, thereby supporting what is being called the “hierarchical black hole growth scenario.”

It’s a little hard to believe, but it’s now more than seven years since GW150914, the first source of gravitational waves detected directly on Earth, was highlighted. This is the beginning of an achievement, both theoretical and experimental, for many scientists and engineers who have worked on this discovery and the opening of a new observational window in astronomy, such as the Nobel Prize winner Kip Thorne or the Frenchman Alain Brillet and Thibault Damour. But it would be necessary to cite a long list of other names such as French Nathalie Deruelle, Italian Alessandra Buonanno or Russian Vladimir Braginsky. As expected, this source – and many of those discovered subsequently – is the product of the collision of two black holes, initially forming a binary system, causing them to merge because they lost energy on their orbits in the form of gravitational waves.


Jean-Pierre Luminet, Research Director at CNRS and Françoise Combes, Professor at Collège de France, talk to us about black holes and especially supermassive black holes in galaxies and which are behind AGNs. © Hugot Foundation at the College de France

Stellar black holes… not stars?

Quickly, astrophysicists are amazed by the masses of black holes determined by analyzing the waves passing through the detectors Ligo, Virgo and, for some time, Kagra. These masses are high, at least several tens of solar masses, and it hardly fits with the theories that explain the formation of stellar black holes by the collapse of stars that explode in supernovae and eject many solar masses at the same time. Indeed, the contenders for the title black hole, which have been highlighted by the X-ray emissions from the disks of accreted matter coming from a companion star in a binary system, have the vast majority of masses below 10 to 15 solar masses.

Scenarios will be proposed on the method of a hierarchical growth, that is, involving sources detected with high masses of black holes, which are the result of captures of black holes, which are the result of the merger of holes migrating black holes in star-dense regions. Wandering black holes that come from the collapse of stars in a binary system.

To account for the frequency of observations, it was therefore necessary to consider regions where the distances between the stars are small and therefore with high probabilities of direct capture, for example for stars in the heart of open clusters or even globular clusters. Another possibility involves the formation of black holes in the accretion disks of supermassive black holes – black holes which then also have a non-negligible chance of being captured for several reasons, as Futura talked about in a previous article about the source GW190521 discovered on 21 .May, 2019.

From GW190521 it is discussed again today in an article published in Naturean open access version is available at arXiv.

All these considerations led a team of researchers from the university and the Turin section of the National Institute for Nuclear Physics (INFN) together with colleagues from the Friedrich Schiller University (FSU) in Jena (Germany) to look again at the analysis of the signal, which led them to the publication in Nature.

GW190521 was originally analyzed as a merger of two rapidly rotating heavy black holes approaching each other along nearly circular paths, but its peculiarities have led us to propose other possible interpretations explains physicist Rossella Gamba, PhD student at the University of Jena, lead author of the study and member of the Virgo collaboration working on the European Gravitational Wave Detector.

The shape and brevity—less than a tenth of a second—of the signal associated with the event led us to hypothesize an instantaneous merger between two black holes that occurred in the absence of spiral phase adds Alessandro Nagar, another gravitational wave theorist working with Virgo and a researcher at INFN’s Turin branch.

The signal from a gravitational wave source with two black holes can be described by combining analytical calculations with numerical calculations on computers. We then get a huge library of signals already calculated with different masses for black holes in rotation also with different angular momenta and on different initial orbits, which we can then compare with the detected signals to see if the calculated signal is closest to it, and therefore infer many things about the initial black holes.

The EOB formalism, a key to the study of black hole collisions

It now appears that GW190521 is reproduced with a probability of only 1/4300 of the previous scenario compared to the one where the initial orbits are not circular but one of the black holes was in a hyperbolic orbit with direct capture and merger . This favors the hierarchical scenario that we had already discussed, and we can therefore very well imagine that black holes of about 85 and 65 times the mass of the Sun had already been formed by a series of multiple mergers just after capture in the disk of a super massive. black hole, or in a dense cluster of stars before they collide themselves in this kind of environment, where this phenomenon is clearly more likely from wandering black holes.

This result was achieved thanks to significant and recent advances using an analytical technique called the effective one-body approach or EOB formalism (effective-one-bodyin English) and whose pioneers were Thibault Damour and Alessandra Buonanno.

The EOB formalism is very technical, but it responds to the difficulties of what is called the two-body problem both in Newtonian physics, where one knows a complete and simple solution, and general relativity, where everything complicates, especially when the gravitational fields become intense, and the curvature of spacetime becomes significant precisely when two black holes are very close and of course also at that time about to enter into a merger.

The EOB formalism shows that one can simplify the problem because gravitational wave emission calculations are equivalent to the calculations for a simpler problem where a single body somehow moves inside another body. For those who are curious and already have a significant background in physics, there are several course videos on the subject.


Explanations by Alessandra Buonanno on the EOB formalism. For a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Select “French”. © Institute for Advanced Scientific Studies (IHÉS)

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