O3 (LIGO–Virgo run)

O3 (LIGO–Virgo run)
Name	O3 (LIGO–Virgo run)
Location	United States; Italy
Established	2019
Closed	2020
Operator	LIGO Scientific Collaboration; Virgo Collaboration

O3 (LIGO–Virgo run) O3 was the third observing run of the LIGO Scientific Collaboration and Virgo Collaboration, conducted jointly with contributions from the KAGRA Collaboration and supported by institutions including Caltech, Massachusetts Institute of Technology, European Gravitational Observatory, National Science Foundation, and Italian National Institute for Nuclear Physics. The run aimed to increase sensitivity relative to prior campaigns coordinated with observatories such as GEO600 and to provide timely public alerts to projects like Fermi Gamma-ray Space Telescope, Neil Gehrels Swift Observatory, INTEGRAL, and electromagnetic follow-up networks such as Zwicky Transient Facility, Pan-STARRS, Dark Energy Survey. O3 combined advances in instrumental control from teams at MIT Kavli Institute, LIGO Hanford Observatory, LIGO Livingston Observatory, and Virgo (interferometer) with multi-messenger strategies developed alongside groups at Max Planck Institute for Gravitational Physics, University of Glasgow, University of Birmingham, Cardiff University, and Monash University.

Background and objectives

The run followed earlier campaigns including O1 (LIGO run) and O2 (LIGO–Virgo run) and built on discoveries beginning with GW150914 credited to researchers affiliated with Barry Barish, Kip Thorne, Rainer Weiss, and institutions such as California Institute of Technology and Massachusetts Institute of Technology. Goals included expanding the compact binary merger catalog used by collaborations such as LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration to inform theoretical models from groups at Cambridge University, Harvard University, Princeton University, Stanford University, Columbia University, University of Tokyo, and Riken. The run sought to improve detection rates for binary black holes, binary neutron stars, and neutron star–black hole systems relevant to modeling efforts by researchers at Max Planck Institute for Astrophysics, IPMU, CEA Saclay, INAF, and SRON. O3 also prioritized low-latency alerts for follow-up by observatories including Hubble Space Telescope, Very Large Telescope, Atacama Large Millimeter Array, Very Large Array, MeerKAT, and survey facilities like Sloan Digital Sky Survey.

Instrumentation and detectors

Detectors comprised the two LIGO interferometers at Hanford, Washington and Livingston, Louisiana, and the Virgo (interferometer) near Pisa, Italy, augmented by commissioning work at KAGRA (Kamioka). Key hardware and teams included suspension systems developed with contributions from Glasgow University, seismic isolation by groups at Caltech, optics coatings researched at University of Rochester, and laser systems refined by engineers from MIT, LIGO Laboratory, and AEI Hanover. Control systems integrated software stacks from developers at LIGO Hanford Observatory, LIGO Livingston Observatory, European Gravitational Observatory, and collaborators at Tata Institute of Fundamental Research. Environmental monitoring networks used sensors and data from partners at US Geological Survey, Istituto Nazionale di Geofisica e Vulcanologia, National Oceanic and Atmospheric Administration, and timing provided by National Institute of Standards and Technology.

Observation schedule and data collection

O3 ran in two main segments, with scheduling coordinated by the LIGO Scientific Collaboration executive structures and liaison offices at National Science Foundation and European Gravitational Observatory. The timeline overlapped observing campaigns by Fermi Gamma-ray Space Telescope, Swift Observatory, and ground telescopes including Subaru Telescope, Keck Observatory, Gemini Observatory, and robotic networks such as Las Cumbres Observatory. Data acquisition pipelines interfaced with computing resources at Open Science Grid, LIGO Data Grid, CERN, GridPP, and institutional clusters at Caltech, MIT, AEI, INFN. Public alert systems delivered notices through channels used by Gamma-ray Coordinates Network, Astronomer's Telegram, and catalog efforts coordinated with NASA Goddard Space Flight Center, ESA, and community archives like Gravitational Wave Open Science Center.

Detected events and candidate list

O3 produced an enriched candidate list including numerous binary black hole mergers and several events classified as binary neutron star or neutron star–black hole candidates, with notable triggers cross-referenced by teams at LIGO Scientific Collaboration, Virgo Collaboration, KAGRA Collaboration, Max Planck Institute for Gravitational Physics, University of Chicago, Yale University, Caltech, MIT, INAF, UCL, University of Melbourne, RIKEN, Osaka University, and NIKHEF. Individual events were assigned identifiers used by collaborative analysis groups at LALSuite, PyCBC, GstLAL, MBTA, and modeling groups at NRSurrogate Project, SXS Collaboration, Einstein Toolkit, and Black Hole Perturbation Theory teams. Alerts prompted rapid follow-up by Swift, Fermi, H.E.S.S., MAGIC, VERITAS, IceCube Collaboration, ANTARES, Auger Collaboration, LOFAR, CHIME, and optical groups at ZTF, Pan-STARRS, DES, ASAS-SN.

Data analysis and pipelines

Analysis used multiple pipelines including PyCBC, GstLAL, MBTA, and unmodeled search algorithms from cWB and teams at AEI, CARDIFF University, UCB, UIUC, and Caltech. Parameter estimation drew on Bayesian inference frameworks developed at LIGO Scientific Collaboration, Virgo Collaboration, and researchers at Stanford University, Perimeter Institute, Purdue University, University of Wisconsin–Milwaukee, Northwestern University, Niels Bohr Institute, and Max Planck Institute for Astrophysics. Waveform modeling incorporated contributions from SXS Collaboration, IMRPhenom, SEOBNR, NRAR, Einstein Toolkit, and numerical relativity groups at Cornell University, Princeton University, University of Jena, and RIT. Data quality and veto strategies involved environmental teams at USGS, ILK, INFN, ANU, and software engineering from LIGO Laboratory and EGO.

Results and scientific impact

O3 expanded the gravitational-wave catalog used by theorists at Niels Bohr Institute, Aspen Center for Physics, Institute for Advanced Study, CITA, DAMTP, Kavli Institute for Theoretical Physics, IPMU, Perimeter Institute, informing population synthesis models from Stony Brook University, Northwestern University, Monash University, UCL, University of Edinburgh, and tests of general relativity developed by Caltech, MIT, Paris Observatory, Ruhr University Bochum, and Leiden University. Multi-messenger campaigns influenced work at NASA, ESA, JAXA, CNRS, CNPq, and astronomy consortia operating VLT, ALMA, VLA, HST, and ground telescopes. Results affected constraints on the equation of state pursued at Los Alamos National Laboratory, Oak Ridge National Laboratory, RIKEN, CEA, and informed cosmological measurements intersecting projects at Planck Collaboration, DESI, Euclid, and LSST (Vera C. Rubin Observatory).

Operational challenges and upgrades

O3 faced commissioning challenges addressed by teams at LIGO Hanford Observatory, LIGO Livingston Observatory, Virgo (interferometer), KAGRA (Kamioka), with mitigation from engineers at Caltech, MIT, AEI, INFN, EGO, LIGO Laboratory, NSF, and Japan Society for the Promotion of Science. Noise sources traced by groups at University of Glasgow, Cardiff University, University of Birmingham, University of Cambridge, and University of Western Australia led to hardware and software upgrades such as improved suspensions, coatings, and seismic isolation contributed by University of Rochester, Syracuse University, University of Maryland, University of Florida, Johns Hopkins University, and University of Colorado Boulder. Lessons informed planning for future runs and facilities including A+ upgrade, Voyager (gravitational-wave detector), Einstein Telescope, and Cosmic Explorer.

Category:Gravitational-wave astronomy