One of the most enigmatic and interesting gravitational signals observed by LIGO and Virgo to date, GW190521, may have been generated by the violent collision of two black holes orbiting in an extremely dense and crowded galactic environment just before the merger. This was claimed in a paper published today in Nature Astronomy by Virgo researchers from the University and the Turin division of the National Institute of Nuclear Physics, together with colleagues from the Friedrich Schiller University (FSU) in Jena (Germany), who reinterpreted, with an original analysis, the data made available by the scientific collaboration of LIGO, Virgo and KAGRA a year ago.
On 21 May 2019, the two interferometers LIGO in the USA and Virgo in Italy detected an extraordinarily intense but extremely short gravitational signal, a sort of very powerful cosmic gong, called GW190521, from its date of detection. That gravitational wave had been generated by the merger of two black holes billions of light years away, and from that thunderous collision a black hole of over 150 solar masses was produced, the most massive black hole observed to date by LIGO and Virgo.
GW190521 was an exceptional and in many ways enigmatic observation, which has stimulated astrophysicists to imagine possible cosmic scenarios, to explain the formation mechanism of the binary pair and the characteristics of its violent merger. An important study that tries to interpret the enigmatic nature of GW190521 is published today in Nature Astronomy by a team of researchers from the University and the Turin section of the National Institute of Nuclear Physics, together with colleagues from the Friedrich Schiller University (FSU) in Jena (Germany).
“GW190521 was initially analysed as the merger of two rapidly rotating heavy black holes approaching each other along almost circular orbits,” explains Rossella Gamba, a PhD student at the University of Jena, lead author of the study and member of The Virgo Collaboration, “but its special features led us to propose other possible interpretations.”
“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 spiraling phase,” comments Alessandro Nagar, a Virgo researcher at the INFN Turin division.
An alternative astrophysical scenario compatible with the peculiar characteristics of GW190521 could be that of a merger between two black holes that occurred in a dense star cluster, where an initially free black hole was captured by the gravitational field of another black hole. In a binary system formed in this way, black holes can merge more rapidly and follow highly eccentric trajectories.
A dynamic encounter between two black holes
Binary systems composed of black holes can form through various astrophysical processes. Most of the black holes detected by LIGO and Virgo are of stellar origin: they originated from the collapse of the cores of massive stars into isolated binary systems. Systems formed in this way are expected to coalesce along circular orbits, because the possible eccentricity of the orbit (which expresses how far it deviates from the perfect circle) is reduced by the release of energy via gravitational waves during the onset of coalescence.
The source of GW190521 may have behaved differently. In the hypothesis proposed by the Turin and Jena team, the orbits of the two colliding black holes would actually be highly eccentric, i.e. very non-circular.
“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 other hypothesis previously put forward. The probability of error is 1:4300!” comments PhD student Matteo Breschi, co-author of the study that developed the analysis software infrastructure.
GW190521 may therefore be the first dynamical encounter of black holes observed. Such events were always thought to be very rare, but this would make the discovery even more important. This hypothesis could also explain the unusually high masses of the observed ‘progenitor’ black holes: in dense environments black holes could undergo multiple mergers and their masses increase after each collision.
“I look at this interesting work as an illustration of the fact that the LIGO-Virgo-KAGRA collaboration contributes to science in many ways, – said Virgo Data Analysis Coordinator and INFN researcher, Giancarlo Cella – not only by sharing with the full scientific community accurately checked data and results, but also by hosting and encouraging a community of experts that drives the scientific debate”
Read the paper on Nature Astronomy: https://www.nature.com/articles/s41550-022-01813-w