the first evidence from gravitational waves

An important new piece of understanding of the extreme universe has been added by gravitational wave research. The scientific collaborations Virgo, in which the INFN National Institute of Nuclear Physics participates, LIGO and KAGRA have in fact given today, June 29, the announcement of the first revelation of two gravitational wave events produced by the fusion of two mixed binary systems composed of a black hole and a neutron star. The result, published in The Astrophysical Journal Letters, refers to two signals recorded in January 2020 and confirms the existence of a class of phenomena predicted by astrophysicists for several decades, but never observed until now.

On January 5, 2020, the Advanced LIGO detector in Livingston, Louisiana, in the United States, and Advanced Virgo, the Italian interferometer located in Cascina, in the province of Pisa, observed the first event. Just ten days later, on January 15, a second gravitational wave, similar to the first, was detected by both Advanced LIGO interferometers, and once again by Advanced Virgo. In both cases, the shape of the recorded signal made it possible to attribute it to a coalescence event involving a black hole and a neutron star, which, at the end of a whirling cosmic approaching ballet that saw them rotate around each other, they merged into a single extremely compact celestial body.

“This discovery is another gem in the treasure represented by the third series of observations conducted by LIGO-Virgo,” says Giovanni Losurdo, Virgo’s international coordinator and INFN researcher. “LIGO and Virgo continue to unveil catastrophic events never seen before, helping to shed light on a hitherto unexplored cosmic landscape. We are now updating the detectors with the aim of looking even further into the cosmos, for a deeper understanding of the universe we live in, ”concludes Losurdo.

The signals, called GW200105 and GW200115 – codes that identify the year, month and day of the gravitational wave (GW) observation – provided important information on the physical characteristics of the systems that emitted them, such as the masses of the primary sources and the distance of the latter from our planet. The analyzes of GW200105 have in fact shown how the masses of the black hole and of the neutron star associated with it were, respectively, about 8.9 and 1.9 times that of our Sun, also allowing us to establish that their merger took place 900 millions of years ago. As for the second signal, the scientists of the Virgo and LIGO collaborations have instead estimated that GW200115 was produced by two celestial bodies of almost 5.7 (black hole) and 1.5 (neutron star) solar masses, which collided about a billion years ago.

Although the values ​​of the masses represent evidence in favor of the fact that those observed are indeed mixed systems composed of a black hole and a neutron star, as they fall within the range predicted by the theoretical models of stellar evolution, the two signals are characterized by a different statistical significance, solid for GW200115, and lower in the case of GW200105.

“This depends on the difficulty of discriminating useful information from background noise in the data recorded by gravitational wave detectors – explains Giancarlo Cella, coordinator of Virgo data analysis and researcher of the Pisa section of the INFN – What we do is in fact identify the properties of the sources as accurately as possible. At the same time, we determine what the probability is that the identified signal can be just a random fluctuation. These are operations that require competence and attention to detail, as well as considerable computational resources ».

The revelations of gravitational signals produced by the mixed systems object of the study which appeared today in The Astrophysical Journal Letters were not accompanied by any observation of counterparts in the electromagnetic spectrum at high energy, which could have provided further evidence in favor of the nature of the sources involved. in the two events. Behavior probably attributable to the considerable difference between the masses of black holes and those of the neutron stars of GW200105 and GW200115, which allowed the former, heavier, to fully capture the latter once they reached the latter, without giving them the possibility of disintegrating by emitting electromagnetic radiation.

In addition to shedding light on a class of rare phenomena and offering the possibility of studying the fundamental laws of physics in extreme contexts not reproducible on Earth, the result of the Virgo, LIGO and KAGRA collaborations opens the way towards understanding the mechanisms responsible for the phenomena of fusion of mixed binary systems. In fact, one of the scenarios hypothesized by theorists predicts that such events may occur in regions of the universe characterized by an extremely chaotic and crowded environment.

«These systems are the last missing piece to reconstruct the demography of the binaries observable by LIGO, Virgo and KAGRA», explains Michela Mapelli, professor at the University of Padua and researcher of the INFN section of Padua. “The masses of these mixed binaries and their coalescence rate are consistent with both the ‘isolated evolution’ scenario and the ‘dynamic formation’ scenario in a young star cluster or in the vicinity of an active galactic nucleus. In the first case, the black hole – neutron star system would come from the evolution of a stellar binary, while in the second case it would be the result of a close interaction between celestial bodies in a star cluster. We await with great curiosity further observations of mixed binaries, to finally understand their formation scenarios. “

The Virgo Collaboration is currently made up of around 700 members from 126 institutions in 15 countries, mainly European (Italy, France, Holland, Germany, Belgium, Ireland, Greece, Hungary, Portugal, Czech Republic, Poland). The European Gravitational Observatory (EGO) hosts the Virgo detector near Italy, in Pisa, and is funded by the INFN, the French Center National de la Recherche Scientifique (CNRS), and the Dutch National Institute for Subatomic Physics (Nikhef ).

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