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 is a little difficult to believe it, but it has been more than seven years now that GW150914, the first source of gravitational waves detected directly on Earth, was highlighted. This is the beginning of an accomplishment, both theoretical and experimental, for many researchers and engineers who have worked on this discovery and the opening of a new window of observation in astronomy, such as the prix Nobel Kip Thorne or the French Alain Brillet and Thibault Damour. But it would be necessary to quote a long list of other names like those of the Frenchwoman Nathalie Deruelle, the Italian Alessandra Buonanno or the 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 and which are caused to merge because they lost energy on their orbit in the form of gravitational waves.

Jean-Pierre Luminet, research director at the CNRS and Françoise Combes, professor at the Collège de France, talk to us regarding black holes and in particular supermassive black holes in galaxies and which are behind AGNs. © Hugot Foundation of the College de France

Stellar black holes… not stellar?

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 this hardly fits with the theories explaining the formation of stellar black holes by the collapse of stars exploding in supernovae and expelling many solar masses at the same time. In fact, the candidates for the title of black hole, which have been highlighted by the X-ray emissions from the discs 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 mode of a hierarchical growth, that is to say involving the sources detected with high masses of black holes which are the result of captures of black holes resulting themselves from the fusion of holes wandering blacks in star-dense regions. Wandering black holes coming 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 once more have a significant chance of being captured for several reasons. which Futura spoke regarding in a previous article regarding the source GW190521 discovered on May 21, 2019.

From GW190521 it is discussed once more today in an article published in Naturean open-access version of which exists on 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 once more at the analysis of the signal, which led them to the publication in Nature.

« GW190521 was initially analyzed as the merger of two rapidly rotating heavy black holes approaching each other along nearly circular orbits but its peculiarities have led us to propose other possible interpretations », Explains the physicist Scarlett GambaPhD student at the University of Jena, lead author of the study and member of the Virgo collaboration dealing with the European gravitational wave detector.

« 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 in spiral adds Alessandro Nagar, another gravitational wave theorist working with Virgo and a researcher at INFN’s Turin division.

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 obtain a huge library of signals already calculated with different masses for black holes in rotation also with different angular moments and on different initial orbits which we can then compare to the signals detected to look for the calculated signal which is closest to it, and therefore deduce many things regarding 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 by 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 fusion . This favors the hierarchical scenario which we had already discussed and we can therefore very well imagine that black holes of around 85 and 65 times the mass of the Sun had already been formed by a series of multiple mergers just following capture in the disc of a supermassive black hole, or in a dense cluster of stars, before colliding themselves in this kind of environment where this phenomenon is clearly more probable from wandering black holes.

This result was obtained thanks to significant and recent progress 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 a complete and simple solution is known, and in general relativity where everything complicates, in particular when the gravitational fields become intense and the curvature of spacetime becomes significant, precisely when two black holes are very close and regarding to enter a merger of course at that time too.

The EOB formalism shows that one can simplify the problem because gravitational wave emission calculations are equivalent to that of a simpler problem where a single body is moving in the field of another somehow . For the curious with already a substantial background in physics, there are several course videos on the subject.

Explanations by Alessandra Buonanno on the EOB formalism. To obtain 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”. Choose “French”. © Institute of Advanced Scientific Studies (IHÉS)