International group reveals new and unexpected black hole populations

By 3 septiembre, 2020Mathematics and Physics

An international group of specialists, with the collaboration of the University of Valencia, grouped in the LIGO and Virgo projects, have announced the detection of an extraordinarily massive binary system merger: two black holes of 66 and 85 solar masses, which generated a final black hole of around 142 solar masses.Black holes

The resulting black hole is the largest ever detected with gravitational waves. It is in a range of masses in which no black hole has ever been observed before, with gravitational waves nor through electromagnetic observations, and could help explain the formation of supermassive black holes. Furthermore, the two initial black holes, if they emerged from the collapsing of stars, are in a range of masses in which their presence would be considered, theoretically, impossible, and could thus help improve our understanding on the final stages of the life of massive stars.

The scientific community of the international collaborations with the Advanced Virgo detector at the European Gravitational Observatory in Italy, and the two Advanced LIGO detectors, in the USA, have announced the detection of a black hole of around 142 solar masses, being the end result of the merging of two black holes with sizes of 66 and 85 solar masses respectively. Both the primary and resulting components are in a range of masses that has never been observed before. The gravitational wave event was detected by the three interferometers of the global network on 21 May 2019. It is believed that the source of the signal, listed as GW190521, is located some 17 billion light years from Earth. Two scientific articles with information on the discovery and its implications have been published recently in Physical Review Letters and Astrophysical Journal letters.

Beating the record mass of detections in the periods of observation of Virgo and LIGO is just one of the special features that makes this detection an unprecedented discovery. A crucial aspect for the astrophysics community is that the resulting black hole belongs to those named “intermediate-mass black holes” (from around 100 to 100,000 solar masses). The interest in this population of black holes is linked to one of the most fascinating and complex puzzles of astrophysics and cosmology: the origin of supermassive black holes. These giant monsters, which are millions to billions of times larger than the sun and are often located in the centre of galaxies, could emerge from the merging of black holes with intermediate masses.

Until today, very few intermediate-mass black hole candidates had been identified solely through electromagnetic observations, and the resulting GW190521 is the first observation of a black hole of intermediate mass observed with gravitational waves. The fact that this detection is in the range of 100 to 1,000 solar masses makes it of greater interest, as for many years this range has been a “desert of black holes” of sorts, due to the lack of candidate events in this range.

“This detection opens the door to discover many more possible new astrophysical effects”, says Thomas Dent, coordinator of the gravitational wave programme at the Galician Institute of High-Energy Physics (IGFAE in Spanish) and member of Scientific Collaboration LIGO. “It has been very difficult to interpret the signal, as it was at the limit of our technical capabilities. We will only have a clear idea of how the system that generated it was created after additional research and with future detections to compare it to.”

“I am very proud of the great implication of the Spanish LIGO-Virgo groups with this new event, with all the activity conducted over several months, including tasks of great responsibility, and the expectations that this new discovery is generating among scientists of related fields,” adds Alicia Sintes, from the University of the Balearic Islands (UIB) and member of scientific collaboration LIGO. “In particular, Thomas Dent (IGFAE) and Juan Calderón Bustillo (Chinese University of Hong Kong and previously member of the UIB), have been members of the editorial team of these articles; Sascha Husa and David Keitel, both from the IAC3-UIB have been internal revisors of the obtained results.”

The components and dynamics of coalescent binary system GW190521 offers extraordinary astrophysical outlooks. The most massive of both merged black holes is larger than any black hole observed to date by LIGO and Virgo, and even the smallest of the black holes is among the largest ever observed. In particular, the masses of the parent black holes defy the astrophysical models that describe the collapsing of the larger stars, at the end of their lives, into black holes. According to these models, the most massive stars are completely destabilised in supernova explosions due to a process known as “pair instability”, leaving behind only gas and cosmic dust. Therefore, the astrophysics community would not expect to observe a black hole in this range of masses of between 60 and 120 solar masses: exactly the range of masses that the most massive component of GW190521 is in. Thus, this detection opens new outlooks for the studying of massive stars and the mechanisms of supernovas.

“Several scenarios predict the formation of black holes in the gap of the distribution of masses due to pair instability: these could be the result of the merger of smaller black holes or of the collision of (several) massive stars, or even of more exotic processes”, adds Michela Mapelli from the University of Padova and the INFN, and member of the Virgo collaboration. “However, it is also possible that we have to revise our current understanding of the final stages of the life of a star and the restrictions on the final mass in the processes that lead to the formation of black holes.”

In fact, the detection of GW190521 by Virgo and LIGO underlines the existence of populations of black holes that have never been observed before or which are unexpected and, as a result, poses new and intriguing questions on their formation mechanisms. Despite the unusually short duration of the signal, which limits our capability to infer the astrophysical properties of the source, the most advanced analyses and currently available models suggest that the initial black holes have significant rotations, in other words, they were rotating quickly.

“The signal shows indications of precession, a rotation on the orbital plane caused by rotations of great magnitude and in specific directions,” adds Tito Dal Canton, researcher at the CNRS of the IJCLab in Orsay (France) and member of the Virgo collaboration. “The effect is weak and we can’t state that it is present in a categorical manner, but if it is true, it would support the hypothesis that the parent black holes emerge and live in very unstable and crowded cosmic surroundings, such as a dense star cluster or the accretion disk of an active galactic core.”

It has required the combination of all the capabilities of the members of our collaborations: the instrumental improvements, the development of numerical models, data analysis and astrophysical interpretation. “This event has truly taken us to our limits: the thorough analysis of this event and its comprehensive revision by the collaborations has required a large number of researchers for over 15 months. It is also worth mentioning that we still do not have complete models for these types of signals: while we can describe precession effects reasonably well, black holes in general can also have noticeably eccentric orbits, orbiting in the shape of ellipses instead of circles when they are far apart. We are working to include this effect before LIGO and Virgo observe more signals, with the help of supercomputer Mare Nostrum, one of the fastest computers in Europe,” says Sascha Husa (UIB).

Several different scenarios are still compatible with the revealed results, and the hypothesis that the parents of the merger may be primordial black holes has not yet been ruled out. We really believe that this merger took place around 17 billion light years away.

Regarding the prior detections of gravitational waves, observed signal GW190521 has a very short duration and is much harder to analyse. Due to the more complex nature of the signal, other more exotic sources have also been considered, and these possibilities are listed in a supplementary publication. However, they are less probable compared to the possibility of the source being a merger of a binary system of black holes.

“Due to the low frequency of the GW190521 signal, the “gurgle” prior to the collision, which is typical of the prior detections, is not as visible in the detectors,” adds José Antonio Font, from the University of Valencia (UV) and member of the Virgo collaboration. “The gurgle can be reduced in an efficient way due to the precession in the orbital plane, but there are also other, possibly less likely, situations where the same effect is observed, such as in collisions with significative eccentricity. The joint work conducted by Nicolás Sanchis Gual and Alejandro Torres Forné, from the Virgo group in Valencia, and Juan Calderón Bustillo, based on numerical simulations and statistical inference, reveals that there could be some confusion regarding the type of system that caused said signal.”

“The collaboration between Valencian graphic designer Raúl Rubio and the Virgo group in Valencia has made the production of dissemination material that illustrates this discovery possible”, says Isabel Cordero Carrión, from the UV and member of the Virgo collaboration.

Five groups in Spain are contributing to the astronomy of gravitational waves with LIGO-Virgo, in fields ranging from the theoretical modelling of the astrophysical sources and data analysis, to improving the sensitivity of the detector for the current and future observation periods. Two groups, at the UIB and the IGFAE of the University of Santiago de Compostela (USC) are part of scientific collaboration LIGO; whereas the UV, the Institute for Sciences of the Cosmos of the University of Barcelona (ICCUB) and the Institute of High-Energy Physics of Barcelona are members of Virgo.

On gravitational wave observatories

The Virgo collaboration is currently comprised by 580 members from 109 institutions in 13 different countries, including Belgium, France, Germany, Hungry, Ireland, Italy, Holland, Poland, Portugal and Spain. The European Gravitational Observatory houses the Virgo detector near Pisa, in Italy, and is funded by the National Centre for Scientific Research (CNRS) in France, the National Institute of Nuclear Physics (INFN) in Italy, and Nikhef in Holland. A list of the groups of the Virgo collaboration can be found here: More information is available on the website of Virgo:

LIGO is funded by the National Science Foundation (NSF) and is operated by Caltech and MIT, which created LIGO and led the project. The NSF, along with Germany (Max-Planck society), the United Kingdom (Science and Technology Facilities Council) and Australia (Australian Research Council – OzGrav) led the economic support for the Advanced LIGO project, providing significant commitments and contributions to the project. Approximately 1,300 scientists from around the world take part in the tasks of scientific collaboration LIGO, which includes the GEO collaboration. A list of the additional collaborators can be found here:

The Spanish contribution is funded by the State Research Agency, Ministry of Science, Innovation and Universities, through the AYA and FPN programmes, the Severo Ochoa and María de Maeztu excellence programmes, European Union funding programmes, ERDF funds, the European social fund, the Vice-presidency and Council for Innovation, Research and Tourism of the Council of Education and Universities of the Government of the Balearic Islands, the Council of Innovation, Universities, Science and Digital Society of the Valencian government through PROMETEO projects, the CERCA programme of the government of Catalonia, and with the support of the Spanish Supercomputing Network (RES in Spanish).