International collaborations Virgo, LIGO and KAGRA have added an element to understanding extreme cosmic phenomena: the first direct observation of a pairing comprised by a black hole and a neutron star.
This is an entirely new phenomenon, as the gravitational waves detected heretofore had been generated by pairs of black holes or neutron stars. José Antonio Font, Astronomy and Astrophysics professor at the University of Valencia (UV), and Isabel Cordero, professor at the Faculty of Mathematics, are members of the UV’s research group in the Virgo collaboration.
The discovery provides new perspectives on the complex mechanisms that may have generated these extreme and rare events and, together with the prior discoveries of Virgo and LIGO, start to shed some light on a yet unexplored cosmic landscape.
Scientific collaborations Virgo, LIGO and KAGRA announced today the first observation of binary systems comprised by a neutron star and a black hole. This has been possible thanks to the detection, in January 2020, of gravitational signals (labelled GW200105 and GW200115 due to the dates when they were detected) emitted by two systems, where a black hole and a neutron star, rotating around each other, merged in a single compact object. The existence of these systems was foretold by the astronomic community several decades ago, but they had never been observed using electromagnetic or gravitational signals, until now. The result and its astrophysical implications have been published in journal The Astrophysical Journal Letters.
“Today’s announcement once again reveals the huge discovery potential of Gravitational Wave Astronomy,” says José Antonio Font, researcher at the UV for the Virgo collaboration. “The two signals observed have once again confirmed a prediction anticipated by theoretical models. It is very exciting to imagine what can happen with other theoretical proposals as the capabilities of current observatories increase,” adds Font.
“We are at a time of transition in gravitational wave astronomy: with the first individual detections of different types of objects that we are currently living, to the numerous detections that will be made in coming years and will allow us to analyse the attributes of these objects in a global way,” says Isabel Cordero Carrión, member of the dissemination and communication team of the Virgo collaboration and UV professor. “We will reveal the distribution of the objects that we had no proof of mere years ago, analysing the possible role the have in the creation of structures in the different astrophysical scenarios.”
The gravitational waves detected in January code valuable information on the physical attributes of the systems, such as the mass and the distance of the two pairings of neutron star and black hole, as well as the physical mechanisms that generated these objects and made them collapse. The analysis of the signal showed that the black hole and neutron star that created GW200105 are, respectively, around 8.9 and 1.9 times the size of the Sun and their merger took place 900 million years ago, hundreds of million years before the first dinosaurs appeared on Earth. In the case of GW200115, the Virgo, LIGO and KAGRA scientists calculate that both compact objects were 5.7 (the black hole) and 1.5 (the neutron star) times the size of the Sun and merged almost one billion years ago.
The mass of the largest object in both cases is within the predicted adjustment range for black holes formed in the models of stellar evolution. The smallest mass is also consistent with neutron stars and these results show that both detected systems are pairs of black holes and neutron stars, even if they have different levels of confidence. In this sense, even though the statistical significance of GW200105 is not particularly high, the “form” of the signal, as well as the inferred parameters of the analyses, lead the researchers to believe it has an astrophysical origin.
The detection of electromagnetic radiation together with the gravitational waves could be additional proof of the detection of a mixed system of a neutron star and a black hole. In fact, if the mass of the two compact objects are comparable, the neutron star, while getting closer to the black hole, is subjected to such tidal force that it becomes fragmented. In this case, as well as the gravitational emissions, the researchers also observed a spectacular flare of electromagnetic radiation due to the disintegration of the stellar matter around the black hole: a similar mechanism to the one that leads to the formation of accretion disks around giant black holes at the heart of galaxies. This probably did not happen with GW200105 nor GW200115, as the mass of the black hole was very large in both cases, which is why once the separation between both items is small enough, the black hole “swallowed” its companion in a single bite.
Abbot et al. 2021, ApJL, 915, L5. DOI: 10.3847/2041-8213/ac082e URL: https://iopscience.iop.org/article/10.3847/2041-8213/ac082e