LIGO-Virgo-KAGRA to Resume O4 After Mid-Run Break

The LIGO and Virgo detectors will resume their joint observing campaign next week, aiming to detect more than 200 gravitational-wave events by the end of the fourth observing run (O4). Astronomers also eagerly hope for the observation of new multi-messenger events, which involve the simultaneous detection of gravitational and electromagnetic waves.

On 10 April 2024, the LIGO-Virgo-KAGRA Collaboration will begin the second phase of its fourth observing run, known as O4b. The European detector Virgo, located in Italy near Pisa, will join the two LIGO detectors in Hanford, Washington, and Livingston, Louisiana, in the United States. This phase of the run is scheduled to continue until early 2025.

“Gravitational wave astronomy has become a key method for observing our Universe. With the data from this observing run, we will contribute further to significantly broadening our horizons and knowledge about the darkest and most violent parts of the Universe”, says Patrick Brady, spokesperson of the LIGO Scientific Collaboration. 

Virgo spokesperson and INFN researcher Gianluca Gemme, added, "Gravitational wave observatories are cutting edge projects and, as such, they face many challenges. Today we are very pleased to be joining the new observing run. Virgo's contribution will be crucial to improve the localization of multimessenger events, which we expect to detect in this second phase of the run”.

The first phase of the run, designated as O4a, was conducted by the LIGO detectors alone starting on May 24, 2023, then pausing for maintenance and upgrades on January 16, 2024. Virgo decided not to join O4a, instead opting to continue commissioning activities to mitigate the impact of several noise sources. The KAGRA detector in Japan participated in O4a for a month before going back to commissioning. KAGRA is currently recovering from damage caused by the magnitude 7.6 earthquake that struck the Noto peninsula (120 km from the KAGRA site) on January 1, 2024.

Taking gravitational wave astronomy to the next level

The O4 run, which is scheduled to last 18 months (excluding commissioning breaks), will significantly advance gravitational-wave astronomy. In the first seven and a half months (O4a), the two LIGO detectors identified 81 gravitational-wave candidate signals with high confidence, consistent with the anticipated detection rate of a merger every 2 or 3 days. By the end of the run in February 2025, the number of gravitational-wave signals could surpass 200, assuming a similar detection rate. The vast amount of data collected during O4a is still being analyzed, and scientists from the LIGO-Virgo-KAGRA Collaboration are optimistic that O4 will fulfill its promise of taking gravitational-wave astronomy to a new level.

Detector improvements

The LIGO detectors paused observations on January 16, 2024 (the end of O4a) for scheduled maintenance. In the time since, experts at both LIGO Hanford and LIGO Livingston have been working hard to make adjustments to the detectors. One of the major efforts involved improving the optical systems that “squeeze” laser light, allowing the LIGO detectors to surpass the sensitivity limits imposed by quantum mechanics. Another effort involved tracking down and isolating noise sources in the numerous vacuum chambers in the experimental halls at the ends of the 4-kilometer arms. These and other refinements will ensure that the trend of improved sensitivity and observing uptime demonstrated throughout O4a will continue into O4b and beyond. 

The Virgo team encountered many obstacles to improving their detector’s sensitivity. "Facing challenges is an integral part of enterprises at the frontier of science and technology, such as the upgrade and commissioning of a gravitational-wave detector,” said Virgo spokesperson Gianluca Gemme. “The good news is that, after going through a long period of commissioning and many difficulties, we managed to improve the sensitivity of the detector up to 60 Mpc, which is equal to the highest levels reached by Virgo in the past. The work to further improve the sensitivity will continue during the run.” The sensitivity of gravitational-wave detectors is often couched in terms of their range to detect signals from the merger of two neutron stars. With a neutron-star binary range of 60 Mpc, i.e., 220 million light years, Virgo will allow scientists to locate nearby merger events in the sky.

The KAGRA detector in Japan, which had planned to join O4b from the beginning, will join the run in the last months of 2024 after recovering from damage caused by the Noto Peninsula earthquake (Magnitude 7.6, 120 km from the KAGRA site) on January 1, 2024. Despite only minor damages to the tunnel, vacuum system, and cryogenic system, 9 of KAGRA's 20 mirror suspension systems need repairs. This will require a delay of at least half a year compared to the previous plan. The KAGRA group will finish this recovery work as soon as possible, restart commissioning, and then join O4b with a BNS reach of about 10 Mpc.

The science goals of this observation campaign

Compared with previous observing runs, the upgraded detectors, more accurate signal models, and more advanced data analysis methods will increase the number of black-hole and neutron-star mergers that will be detected. These changes, together with the presence of Virgo data, will also allow scientists to extract more information from the detected signals, thus extending our understanding of these fascinating objects.

With every improvement in the sensitivity of the detectors, there comes new opportunities to detect signals from sources other than mergers. Scientists will use the new data to search for continuous gravitational-wave signals that are generated by rapidly rotating neutron stars in our Galaxy, for example. The detection of such objects via gravitational-wave observations could teach us about the make-up of neutron stars and complement observations that could be made in radio.

The new data from O4 will also extend our knowledge of the background produced by many overlapping gravitational waves passing Earth from all directions. These waves can be produced in the earliest stages of the Universe's birth or may come from a multitude of sources distributed across the Universe. 

As in previous observing runs, alerts about gravitational-wave detection candidates will be distributed publicly during O4b. Information about how to receive and interpret public alerts is available at https://wiki.gw-astronomy.org/OpenLVEM.

Gravitational-wave observatories

LIGO is funded by the NSF, and operated by Caltech and MIT, which conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,600 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. Additional partners are listed at https://my.ligo.org/census.php.

The Virgo Collaboration is currently composed of approximately 880 members from 152 institutions in 17 different (mainly European) countries. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and the National Institute for Subatomic Physics (Nikhef) in the Netherlands. A list of the Virgo Collaboration groups can be found at: https://www.virgo-gw.eu/about/scientific-collaboration/. More information is available on the Virgo website at https://www.virgo-gw.eu.

KAGRA is the laser interferometer with 3 km arm-length in Kamioka, Gifu, Japan. The host institute is Institute for Cosmic Ray Research (ICRR), the University of Tokyo, and the project is co-hosted by National Astronomical Observatory of Japan (NAOJ) and High Energy Accelerator Research Organization (KEK). KAGRA collaboration is composed of over 400 members from 128 institutes in 17 countries/regions. KAGRA’s information for general audiences is at the website https://gwcenter.icrr.u-tokyo.ac.jp/en/. Resources for researchers are accessible from http://gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA.