Until now, gravitational waves produced either by two black holes or by two neutron stars have been detected, but the Virgo (located in Italy), LIGO (with two facilities in the US) and the Japanese KAGRA detectors report today in The Astrophysical Journal Letters from the first direct observation of ‘combined’ pairs formed by a black hole and a neutron star.
A cosmic dance confirms an Einstein prediction
The finding was made in January 2020 when two gravitational signals emitted by two systems were detected, in which a black hole and a neutron star, rotating around each other, merged into a single compact object. The existence of these pairs was predicted by the astronomical community several decades ago, but to date they have never been observed for sure, either by electromagnetic or gravitational signals.
The events GW200105 and GW200115 detected in January 2020 represent the first observations of gravitational waves generated by a combination of a neutron star and a black hole, the so-called ‘lost binary system’
On January 5, 2020, the Advanced LIGO detector in Livingston (Louisiana, USA) and the Italian Advanced Virgo detector, observed a gravitational wave produced by the last orbits (spiral phase), before the merger, of a pair formed by a neutron star (EN) and a black hole (AN). Altogether, ENAN.
Just 10 days later, a second gravitational wave signal from the spiral and fusion phase of a similar binary system was observed by the two LIGO detectors (both in Livingston and another operating in Hanford, Washington state) and that of Virgo.
These two events, dubbed GW200105 and GW200115 for the dates of their detections, represent the first observations of gravitational waves generated by a combination of a neutron star and a black hole, since two previous gravitational signals (GW190814 and GW190426) had been considered ENAN candidates, but without a sufficiently high level of confidence.
Double neutron star systems were first observed in the Milky Way in 1974 by monitoring pulses of radio waves emitted by neutron stars, known as radio pulsars.
“The astronomical community has spent decades searching for radio pulsars orbiting black holes, but none have been found in the Milky Way so far,” says Astrid Lamberts, CNRS researcher and member of the Virgo collaboration at Artemis and Laboratories. Lagrange, in Nice.
The lost binary system
“The black hole and neutron star pairs were in fact for astronomers the ‘missing binary system’ – he emphasizes -. With this new discovery, we can finally begin to understand how many of these systems exist, how often they merge, and why we haven’t seen examples in the Milky Way yet. ”
The gravitational signals detected in January encode valuable information about the physical characteristics of the systems, such as the mass and distance of the two pairs of ENANs, as well as about the physical mechanisms that have generated these objects and caused them to collapse.
Analysis of the signal has shown that the black hole and the neutron star that originated GW200105 are, respectively, around 8.9 and 1.9 times as massive as our Sun and that their merger took place around 900 million years ago. years, hundreds of millions of years before the first dinosaurs appeared on Earth.
The black hole and neutron star that originated GW200105 are, respectively, around 8.9 and 1.9 times as massive as our Sun and that their merger took place around 900 million years ago.
In the case of the GW200115 event, Virgo and LIGO scientists estimate that the two compact objects had masses of about 5.7 (for AN) and 1.5 (for EN) times the mass of the Sun and that they merged ago. almost a billion years.
The most massive mass estimate in both cases falls within the predicted fit interval for black holes formed in stellar evolution models. The lighter mass is also consistent with neutron stars, and those results indicate that both detected systems are ENAN pairs, even if they have different confidence levels.
In this sense, although the statistical significance of GW200105 is not that high, the “shape” of the signal, as well as the parameters inferred from the analyzes, lead researchers to believe that it has an astrophysical origin.
“A great deal of work and computational resources has been devoted to estimating these parameters. In fact, a crucial aspect in analyzing the data recorded by gravitational wave detectors is to extract useful information, which is always mixed with noise, “adds Giancarlo Cella, INFN researcher and coordinator of the Virgo data analysis group.
“We need to get our best estimates of the properties of the sources and at the same time we want to know what the probability is that an identified signal is simply due to random fluctuation.”
An additional test of the detection of a mixed system of a neutron star and a black hole could have been the detection of electromagnetic radiation along with gravitational waves. In fact, if the masses of the two compact objects are roughly comparable, the neutron star, as it approaches the black hole, is subject to such tidal forces that it fragments.
In this case, in addition to the gravitational emissions, a spectacular flare of electromagnetic radiation could also be observed, due to the disintegration of stellar matter around the black hole: a mechanism similar to what leads to the formation of accretion discs around giant black holes at the center of galaxies.
This probably happened neither for GW200105 nor for GW200115, since in both cases the mass of the black hole was very large, so once the separation between the two objects was small enough, the black hole, so to speak, swallowed his partner in a single bite.
“We have evidence that our sensitivity is now above the threshold required to detect systems of this type,” says Cella, “and we expect that we will do this routinely in future observation periods.”
Drawing a new cosmic landscape “The fact that we have now detected all three types of binary systems will help us develop theories to explain the properties of all of them in a consistent way,” says Lamberts.
“In fact, this discovery allows us to deepen our understanding of the most extreme phenomena in the Universe, helping us to better understand what mechanisms could have generated them.”
The result announced today, along with the dozens of detections made by Virgo and LIGO to date, allow us, for the first time, a close observation of one of the most violent and rare phenomena in the Universe, and draw an unprecedented image of the crowded ones. and chaotic regions that are one of the possible environments where these events are generated.
In addition, the detailed information that we have begun to collect on the physics of black hole and neutron star mergers offers us the opportunity to test the fundamental laws of physics under extreme conditions, which we will obviously never be able to reproduce. on earth.
“The discovery announced today is yet another gem in the treasure of LIGO-Virgo’s third observation period,” adds Giovanni Losurdo, Virgo spokesperson and INFN researcher.
“LIGO and Virgo continue to unveil catastrophic collisions, never before observed, shedding light on a genuinely new cosmic landscape. We are now upgrading the detectors with the aim of looking far deeper into the depths of the cosmos, searching for new gems, pursuing a deeper understanding. of the universe we live in. ”
The LIGO collaboration includes the Galician Institute of High Energy Physics (IGFAE) of the University of Santiago de Compostela and the University of the Balearic Islands (UIB), while the University of Valencia (UV), the Institute of Cosmos Sciences from the University of Barcelona (ICCUB) and the Institut de Física d’Altes Energies (IFAE) of Barcelona are members of Virgo.