LIGO, Virgo, and KAGRA raise their signal score to 90

Gravitational-wave catalog update lists 35 new signals

November 08, 2021

Today the LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration have published the latest version of their gravitational-wave catalog. The Gravitational-Wave Transient Catalog 3 (GWTC-3) now contains 90 signals, 35 of which were not published before and which were observed in O3b, the second half of the third joint observing run “O3”, which ended on 27 March 2020. All signals come from merging black holes and neutron stars. The new catalog contains some surprises, such as an unusual neutron-star–black-hole merger, a massive black hole merger, and binary black holes revealing information about their spins. In parallel, the researchers published companion studies of the underlying population of black holes and neutron stars and the history of the expansion of the Universe. The detectors are currently being upgraded for O4, their fourth joined observing run, which is expected to begin late in 2022.

One of the least massive neutron stars ever observed

While the first observation of a black hole swallowing a neutron star had been published previously, GWTC-3 contains one more such event. “In O3b we discovered GW191219_163120, a merger signal, which is from a black hole 32 times the mass of our Sun swallowing a neutron star of just 1.17 solar masses – one of the least massive neutron stars ever observed,” explains Alessandra Buonanno, director at Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Potsdam and professor at the University of Maryland. “The new observations continue to challenge our understanding of how stellar-mass black holes and neutron stars form, and how they come to orbit each other until they merge.”

GW200210_092254, another new O3b discovery resembles the earlier detected GW190814, in which a black hole merges with a second object that is either a very massive neutron star or a very low-mass black hole.

The majority of new discoveries are binary black hole mergers, including some especially noteworthy events. “On 20 February 2020, we probably witnessed the birth of another ‘big fish’ similar to GW190521– an intermediate mass black hole – from the merger of two heavy black holes,” says Frank Ohme, leader of an Independent Max Planck Research Group at AEI Hannover. “Moreover, we find several events in which the gravitational waves reveal details about the spins of the merging black holes.”

The mergers of compact objects discovered so far by LIGO and Virgo (in O1, O2 and all of O3). The diagram shows black holes (blue), neutron stars (orange) and compact objects of unknown nature (grey), which were detected by their gravitational-wave emission. Each merger of a binary system corresponds to three compact objects shown: the two merging objects and the result of the merger.

Taking a break for detector upgrades

“We used a one-month break in October 2019 – between O3a and O3b, the two halves of O3 – to upgrade and improve the detectors. This includes cleaning the end mirrors at LIGO Livingston, replacing vacuum equipment at LIGO Hanford, and increasing the laser power at Virgo,” says Karsten Danzmann, director at the AEI Hannover and director of the Institute for Gravitational Physics at Leibniz University Hannover. He adds “The upgrades and the continuous maintenance of our gravitational-wave instruments increased the sensitivity of the international detector network in O3b. We listened deeper into the Universe than ever before.”

Towards the end of O3, the KAGRA detector in Japan joined the observing run, followed by two weeks of simultaneous observations with the German-UK gravitational-wave detector GEO600 near Hannover,Germany, in April 2020 after O3 ended. Results from the GEO600-KAGRA run will be published separately.

What the gravitational-wave events tell us about the Universe

The researchers have also published two papers accompanying their new catalog today. One looks at what the events can tell us about the population of compact objects in our Universe, how often they merge, and how their masses are distributed. In the other paper the researchers employed the gravitational waves to better understand the expansion history of the cosmos by measuring the Hubble constant.

AEI researchers have significantly contributed to analyses presented in the three papers. They have provided accurate models of the gravitational waves from coalescing black holes that included, the precession of the black-holes’ spins, multipole moments beyond the dominant quadrupole, as well as tidal effects introduced by the potential neutron-star companion. Those features imprinted in the waveform are crucial to extract unique information about the source’s properties and the Universe, and carry out tests of general relativity. The high-performance computer clusters “Minerva” and “Hypatia” at AEI Potsdam and “Holodeck” at the AEI Hannover were employed in the development of the waveform models and their use in the events’ analyses.

Towards the summer of 2022 and O4

The LIGO, Virgo and KAGRA detectors are currently being upgraded for O4. Their fourth joined observing run is expected to begin late in 2022. The expected higher detector sensitivity will increase the rate at which new discoveries are made. The researchers expect to observe gravitational waves up to three times as often as in O3: as many as five signals each week.


LIGO Scientific and Virgo Collaborations

This material is based upon work supported by NSF's LIGO Laboratory which is a major facility funded by the National Science Foundation. LIGO is operated by Caltech and MIT, which conceived of LIGO and led the Advanced LIGO detector project. Financial support for the Advanced LIGO project was principally from 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 1,400 scientists from around the world participate in the effort to analyze the data and develop detector designs through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.

The Virgo Collaboration is currently composed of approximately 650 members from 119 institutions in 14 different countries including Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by the 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 http://public.virgo-gw.eu/the-virgo-collaboration. More information is available on the Virgo website at http://www.virgo-gw.eu.

The KAGRA detector is located in Kamioka, Gifu, Japan. The host institute is the Institute of Cosmic Ray Researches (ICRR) at the University of Tokyo, and the project is co-hosted by National Astronomical Observatory in Japan (NAOJ) and High Energy Accelerator Research Organization (KEK). KAGRA completed its construction in 2019, and later joined the international gravitational-wave network of LIGO and Virgo. The actual data-taking was started in February 2020 during the final stage of the run called "O3b." The KAGRA collaboration is composed of over 470 members from 11 countries/regions. The list of researchers is available from http://gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA/KSC/Researchers. KAGRA information is at the website https://gwcenter.icrr.u-tokyo.ac.jp/en/.

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