and release their first catalog of gravitational-wave events
On Saturday, December 1, LIGO and Virgo scientists presented new results from searches for coalescing cosmic objects, such as pairs of black holes and pairs of neutron stars, by the National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based VIRGO gravitational-wave detector. The LIGO and Virgo collaborations have now confidently detected gravitational waves from a total of 10 stellar-mass binary black hole mergers and one merger of neutron stars, which are the dense, spherical remains of stellar explosions. Six of these events had been reported before, while four of the black hole detections are newly announced.
From September 12, 2015, to January 19, 2016, during the first LIGO observing run since undergoing upgrades in a program called Advanced LIGO, gravitational waves from three binary black hole mergers were detected. The second observing run, which lasted from November 30, 2016, to August 25, 2017, yielded one binary neutron star merger and seven additional binary black hole mergers, including the four new gravitational-wave events being reported now. The new events are known as GW170729, GW170809, GW170818, and GW170823, in reference to the dates they were detected. All of the events are included in a new catalog, also released on Saturday.
Among the events in this catalog are ones that break new ground. The new event GW170729, detected in the second observing run on July 29, 2017, is the most massive and distant gravitational-wave source ever observed. “It also provides the strongest indication of fast spinning black holes prior to the collision” – says Prof. Imre Bartos, the UF LIGO group member working on black hole and neutron star astrophysics. In this coalescence, which happened roughly 5 billion years ago, an equivalent energy of almost five solar masses was converted into gravitational radiation. This event was discovered by the search algorithm Coherent Waveburst invented at UF. Coherent Waveburst was also the algorithm that detected gravitational waves for the very first time, discovering the binary black hole merger GW150914 in 2015. “So far Coherent Waveburst discovered the first, the heaviest, the farthest and possibly the fastest spinning black holes observed by LIGO” – continues Imre Bartos. The development of cWB was initiated and led by UF professors Sergey Klimenko and Guenakh Mitselmakher. “The key feature of Coherent Waveburst is that by using wavelets it explores the time-frequency structure of the data and finds signals without relying on a source model. Such search has the potential to discover new and possibly unexpected astrophysical sources of gravitational waves” – says Sergey Klimenko.
The University of Florida was the first institution beyond Caltech and MIT which joined the LIGO collaboration and took responsibility of one of the most complex and diverse optical systems, the Input Optics (IO), of the initial LIGO detectors. The group continued to be responsible for all upgrades of the IO until completion of installation of the advanced LIGO detectors. The expertise gained over the years was also used to influence the design of advanced LIGO and allowed the University of Florida to contribute to other LIGO systems as well.
Another new event GW170814 was the first binary black hole merger measured by the three-detector network and allowed for first tests of gravitational-wave polarization (analogous to light polarization). Three days later, the event GW170817 was detected. This was the first time that gravitational waves were ever observed from the merger of a binary neutron star system. What’s more, this collision was seen in gravitational waves and light, and marked an exciting new chapter in multi-messenger astronomy, in which cosmic objects are observed simultaneously in different forms of radiation. One of the new events, GW170818, detected by the global network formed by the LIGO and Virgo observatories (located in the United States and Italy, respectively), was very precisely pinpointed in the sky. The position of the binary black holes, located 2.5 billion light-years from Earth, has was identified in the sky with a precision of 39 square degrees. That makes it the next best localized gravitational-wave source after the GW170817 neutron star merger.
The scientific paper describing these new findings, which is being published today on the arXiv repository of electronic preprints, presents detailed information in the form of a catalog of all the gravitational wave detections and candidate events of the two observing runs. Thanks to more advanced data processing and better calibration of the instruments, the accuracy of the astrophysical parameters of the previously announced events increased considerably.
LIGO is funded by NSF and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by 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. More than 1,200 scientists from around the world participate in the effort 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 consists of more than 300 physicists and engineers belonging to 28 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; 11 from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with IFAE and the Universities of Valencia and Barcelona; two in Belgium with the Universities of Liege and Louvain; Jena University in Germany; and the European Gravitational Observatory (EGO), the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef. A list of the Virgo Collaboration can be found at http://public.virgo-gw.eu/the-virgo-collaboration/. More information is available on the Virgo website at www.virgo-gw.eu.