The LIGO and Virgo Collaborations unveil a new and unexpected population of massive black holes

Sep 4, 2020 Off Comments in Announcement by

On May 21 2019, the LIGO and Virgo interferometers jointly detected a new signal labeled GW190521: scientists interpreted it as the first direct evidence for a new kind of gravitational wave sources, i.e. the “intermediate-mass” black hole population, with a mass between 100 and 1,000 times that of the sun.

GW190521 was most likely generated by a source 5 Gigaparsecs away, when the universe was about only half its age, making it one of the most distant gravitational-wave sources detected. This is also the most energetic emission in gravitational waves detected so far, since a huge amount of mass, corresponding to 8 suns, was released in gravitational radiation across the universe.

“We are dealing here with a signal of unexpected characteristics, similarly to what happened for GW150914, the first gravitational wave observed”, says prof. Giovanni Prodi at the Mathematics Department of the University of Trento and INFN-TIFPA. “Here too, our data analysis methods, developed by Trento in collaboration with Florida, Padova and Zurich, played a key role: it provided the most convincing proof of detection and it tested the consistency of the interpretation models with the data. Once again we demonstrated the ability to perform unexpected discoveries with LIGO and Virgo.”

“The signal is extremely brief in duration, a little more than four wiggles lasting less than one-tenth of a second”, says Dr. Francesco Salemi at the Physics Department of the University of Trento and INFN-TIFPA. “Its interpretation has been very challenging. We believe it encodes the instant that two black holes (of about 85 and 66 solar masses each) merged. The merger created the most massive black hole ever detected with gravitational waves, about 150 solar masses.”

The remnant black in a range of mass within which a black hole has never been observed before, either via gravitational waves or electromagnetic observations, and may help to explain the formation of supermassive black holes, objects often found at the center of galaxies with masses in the range million to billion times the sun. Moreover, the most massive component of the binary system lies in a mass range forbidden by stellar evolution theory and challenges our understanding of the final stages of massive stars life. In fact, GW190521 highlights the existence of black-hole populations that either have never been observed before or are unexpected and, in doing so, raises intriguing new questions about their formation mechanisms.

Data analysis of the gravitational wave signals produced in compact binary mergers is often performed by algorithms that compare the signal with a large database of waveforms, computed for specific source properties. Instead, the data analysis method developed by our group (coherent WaveBurst) is seeking for signals of generic waveforms morphologies. This tool picked up the signal more clearly than the “specialized” algorithms that search for specific wave patterns in the data. This is most likely due to the exceptionally high masses involved in this gravitational-wave event and, possibly, by the effect of precession (a rotation of the orbital plane produced by spins with large magnitude and particular orientation) on the binary system and, hence, on the waveform.

The Virgo group at Trento is also committed in the development of the Advanced Virgo detector, operating at the European Gravitational Observatory at Cascina (Pisa), and in the modeling and interpretation of multimessenger emissions involving neutron stars’s collisions.

Virgo group composition at Trento: prof. Albino Perego, prof. Antonio Perreca, prof. Giovanni Prodi, Dr. Andrea Grimaldi, Dr. Andrea Miani, Dr. Francesco Salemi and Dr. Michele Valentini.

Scientific papers:
GW190521: A Binary Black Hole Merger with a Total Mass of 150  M⊙
Properties and Astrophysical Implications of the 150 M ⊙ Binary Black Hole Merger GW190521


The Virgo Collaboration is currently composed of approximately 580 members from 109 institutes in 13 different countries, including Belgium, France, Germany, Greece, Hungary, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Monaco and Japan. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration groups can be found at More information is available on the Virgo website at

LIGO is funded by the National Science Foundation (NSF) and operated by Caltech and MIT, which conceived of LIGO and led the project. Financial support for the Advanced LIGO project was led by the NSF, with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at

Virgo website
LIGO website