University of Glasgow physicists are closing out 2018 with the publication of a wealth of new gravitational wave data collected during the first two observing runs of the LIGO and Virgo detectors. 

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In a new paper just published online, scientists from the LIGO and Virgo research collaborations present data from a total of 10 stellar-mass binary black hole mergers and one merger of neutron stars. Seven of these events had been reported before, while four of the black hole detections are newly announced. 

The paper represents a significant step forward in the fledgling field of gravitational wave astronomy, which searches for tiny ripples in spacetime left in the aftermath of massive astronomical events. 

University of Glasgow physicists have made major contributions to the search for gravitational waves, including the development and fabrication of the delicate mirror suspensions which make the detections possible and the data analysis which sorts the gravitational wave signals from the background noise of the universe. 

Professor Sheila Rowan is director of the University of Glasgow’s Institute for Gravitational Research. Professor Rowan said: “This remarkable crop of detections show just how valuable gravitational wave astronomy is in developing our understanding of the universe. 

“In less than three years gravitational wave detections have given us direct evidence of the existence of black holes and binary neutron star collisions. Today we present a wealth of new data from LIGO and Virgo to stand alongside the ground-breaking discoveries already made during their initial observing runs. It took science a century to confirm Einstein’s prediction of the existence of gravitational waves, but the pace of our discoveries since then has been exhilarating, and we’re anticipating many more exciting detections to come.” 

Professor Martin Hendry is head of the University of Glasgow’s School of Physics and Astronomy. Professor Hendry added: “The four new detections discussed in this paper are exciting, particularly the largest and most distant black hole collision we’ve seen to date, but what’s equally significant is that we now have data from 11 detections collected together as a catalogue. 

“That represents a big step forward in our understanding of the universe, and is a ringing endorsement of the effectiveness of gravitational wave astronomy.” 

The University of Glasgow’s gravitational wave research is funded by the Science and Technology Facilities Council (STFC) and in partnership with the Universities of Cardiff and Birmingham, among other UK universities. 

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 a 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 based on the dates on which they were detected. 

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. In this coalescence, which happened roughly 5 billion years ago, an equivalent energy of almost five solar masses was converted into gravitational radiation. 

The Virgo interferometer joined the two LIGO detectors on August 1, 2017, while LIGO was in its second observing run. Although the LIGO-Virgo three-detector network was operational for only three-and-a-half weeks, five events were observed in this period. Two events detected jointly by LIGO and Virgo, GW170814 and GW170817, have already been reported. 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, 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. 

“It is gratifying to see the new capabilities that become available through the addition of Advanced Virgo to the global network,” says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration. “Especially our greatly improved pointing precision will allow astronomers to rapidly find any other cosmic messengers emitted by the gravitational wave sources.” This success is made possible by exploiting the pointing capability of the network, which uses the time delays of the signal arrival at the different sites and the so-called antenna patterns of the interferometers. 

Caltech’s Albert Lazzarini, Deputy Director of the LIGO Laboratory, says: “The release of 4 additional binary black hole mergers further informs us of the nature of the population of these binary systems in the Universe and better constrains the event rate for this type of events.”

“The next observing run, starting in Spring 2019, should yield many more gravitational-wave candidates, and the science the community can accomplish will grow accordingly,” says David Shoemaker, spokesperson for the LIGO Scientific Collaboration and senior research scientist in MIT’s Kavli Institute for Astrophysics and Space Research. “It’s an incredibly exciting time.” 

“The new catalog is another proof of the exemplary international collaboration of the gravitational wave community and an asset for the forthcoming runs and upgrades,” adds EGO Director Stavros Katsanevas. 

A total of 11 confident gravitational-wave detections were obtained by three independent analyses of the O1 and O2 data. “This gravitational-wave catalog is the reward for a tremendous effort by the LIGO and Virgo collaborations. It has been an absolute privilege to be part of this endeavor and work with so many incredibly talented scientists to achieve this fantastic result,” says Patricia Schmidt, research fellow at Radboud University Nijmegen, the Netherlands. 

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.  

“The results of the first two observing runs demonstrate the huge capabilities of the gravitational-wave network to make science,” says Viviana Fafone, INFN coordinator for the Virgo Collaboration.

“This catalog marks the transition from the first pioneering detections to the systematic exploitation of gravitational waves sources for science,” adds Benoit Mours, CNRS coordinator for the Virgo Collaboration.

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First published: 3 December 2018