The classification and definitive analysis of the 39 events detected by Virgo and LIGO in the third observation period (which ran from April to October 2019) was published today on the ArXiv online archive. Most of these are black hole mergers, the characteristics of which, however, question some established astrophysical models and open up new scenarios. A likely merger of neutron stars and two probable ‘mixed’ neutron star-black hole systems were also detected in the same period.
It took a year of work and complex analysis by the researchers of the Virgo and LIGO scientific collaborations to complete the study of all of the gravitational-wave signals that were recorded by the Virgo interferometer, installed at the European Gravitational Observatory, in Italy, and the two LIGO detectors, in the US, during the data-taking period – called ‘O3a’ – which ran from the 1st of April to the 1st of October, 2019. Events included: 36 mergers of black holes; a likely merger of a binary system of neutron stars; and two systems that were most likely composed of a black hole and a neutron star. Among these, four “exceptional events” have, during the last year, already been published, but the catalogue released today provides, for the first time, a complete picture of the extraordinarily large number of recorded gravitational-wave signals and their sources. It represents a wealth of observations and data on the physics of black holes, barely imaginable until only a few years ago.
“Since the end of the O2 observing run in August 2017, many efforts have been made to upgrade many of the technical components and different sectors of the detector, in order to boost the Virgo sensitivity across the whole frequency range”, said Ilaria Nardecchia, a researcher at the University of Roma Tor Vergata and member of the Virgo Collaboration. “We reaped the benefits of our work because we doubled the sensitivity of the detector!”
Indeed, between September 2017, and April 2019, the sensitivity of the three detectors has been significantly improved. This has led, for example, to Virgo becoming capable of observing a volume of the universe almost ten times larger than in the previous observational run (O2).
“Observations with Advanced Virgo and LIGO have exceeded expectations. As well as opening a new and exciting phase in the history of human observation of the cosmos, we are seeing events that either lacked observational evidence until now, or go beyond our current understanding of stellar evolution”, said Ed Porter, directeur de recherche CNRS at APC-Paris, and member of the Virgo Collaboration. “Just five years after the first detection of gravitational waves, we can say that gravitational astronomy is a concrete reality.”
The detection of gravitational signals allows us, in fact, for the first time, to closely observe the dynamics of extraordinary mergers of black holes and neutron stars, which release bursts of energy equivalent to several solar masses in gravitational waves. This allows us to study, as never before, the physics of black holes, the cosmic phenomena that generate them and even the characteristics of the largest populations of black holes. Actually, the results of the present catalogue raise serious questions about the validity of some of the astrophysical scenarios and models, which until now seemed the most plausible.
In particular, the masses of black holes, presented in the O3a catalogue, question various theoretical and observational limits on the mass ranges of black hole populations. Some observations, for example, indicate the presence of compact objects (which could be either black holes or neutron stars) exactly in the gap between the mass of the heaviest neutron stars and that of the lightest black holes observed by astronomers to date. This gap could therefore narrow or even disappear. Other observed black holes have a mass with a value between 65 and 120 solar masses; a range forbidden by stellar evolution models. According to these models, the very massive stars, beyond a certain threshold, are completely disrupted by the supernova explosion, due to a process called pair instability, and leave behind only gas and cosmic dust. The existence of black holes in the range prohibited by pair instability suggests other mechanisms of black hole formation, such as the merger of smaller black holes or the collision of massive stars, but may also indicate the need to revise our description of the final stages of the lives of stars.
The publication of the O3a catalogue is the conclusion of complex work involving many phases and covering detector calibration, data characterisation and data analysis. The catalogue for each observation run is only published once researchers have the final validated dataset, thus making it possible to estimate the physical parameters (such as distance, mass and spins) of the black-hole and neutron-star mergers, as well as a confident estimate of their margins of error. Of the 39 events presented in this latest catalogue, 26 were announced immediately after detection, while 13 are reported for the first time in the paper published today. These add to the 11 gravitational-wave events reported by LIGO and Virgo for the previous runs (O1 and O2). In addition to the LIGO-Virgo events catalogue, three other articles have also been released today on the arXiv server: the global analysis of the astrophysical properties of the gravitational-waves sources; new tests of the theory of general relativity; and the search for gravitational-wave signals coincident with gamma-ray bursts.”
“These papers are very important and represent a further step forward in a long and exciting journey”, said Giovanni Losurdo, INFN researcher and spokesperson for the Virgo Collaboration. “We are already looking forward to the results of the second part of the third observation period (O3b). The very high number of events still to analyse and understand promises that the next catalogue will be as exciting, if not more so, than this one. Meanwhile, we are striving to implement a substantial upgrade of the Virgo detector, aiming to pursue the next run, in 2022, again with a considerably improved sensitivity.”
The above-mentioned findings are of great importance also to the researchers of the ELHK Wigner Research Centre for Physics (Wigner FK). “Some observations indicate the presence of compact objects which fall right in the gap between the mass of the heaviest neutron stars and that of the lightest black holes observed to date. Additional observation may very well further refine the picture. In addition to gravitational-physics research, many researchers at Wigner FK deal with high-energy physics. Therefore, the potential existence of unusually heavy neutron stars may have a substantial impact on the study of extremely dense material phases, quark material and hadron material”, said Dániel Barta, researcher of Wigner FK and member of the Virgo Collaboration.
Citizen-science projects for gravitational-wave data-analysis
Two citizen-science projects, Gravity Spy for LIGO and the European project, REINFORCE for Virgo, allow everyone to contribute to the identification of spurious signals and therefore to the discovery of new gravitational-waves signals, by collaborating directly with researchers involved in the analysis of the data of the three interferometers.
In fact, although external as well as internal noise sources are minimised, the data taken by the interferometers are still plagued by some disturbances. In some cases, these are monitored by witness sensors and are then subtracted from the data in real-time. Nevertheless, the identification of other noises is more problematic and requires off-line dedicated analysis in order to flag them. This is the case with glitchy noises; those that are generated, for instance, by light scattered off the main laser beam and that then recombine with it. The careful studies required to claim a true gravitational-wave signal explain why the LIGO and Virgo Collaborations issue alerts of a candidate event to the scientific community soon after it has been measured. This can then either be confirmed by subsequent analysis and hence considered a true signal or not. Thanks to Gravity Spy and REINFORCE, citizen scientists can help researchers in this complex analysis work by directly accessing the data detected by the LIGO and Virgo interferometers.
The image shows sky localisations for the different LIGO-Virgo detections that are included in the O3a catalogue. Each localisation – represented by shaded areas on the map – is deduced on the basis of information provided by the three detectors in the network. The day and time of arrival on Earth, a scientific name and the time it took the signal to reach the Earth from wherever in the Universe it was generated, are all recorded. The smaller the shaded area in the sky map, the better the signal has been localised. Localisation is crucial in enabling follow-up searches with different messengers, such as light or neutrinos.