28 September 2017

Gravitational Waves from a Binary Black Hole Merger Observed by LIGO and Virgo



The Laser Interferometer Gravitational-wave Observatory (LIGO) Scientific Collaboration and the Virgo Collaboration (Virgo) report the first joint detection of gravitational waves with both the LIGO and Virgo detectors. This is the fourth announced detection of a binary black hole system and the first significant gravitational-wave signal recorded by the Virgo detector, and highlights the scientific potential of a three-detector network of gravitational-wave detectors. A paper about the event, known as GW170814, has been accepted for publication in the journal Physical Review Letters

The three-detector observation was made on August 14, 2017 at 10:30:43 UTC. The two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, and funded by the National Science Foundation (NSF), and the Virgo detector, located near Pisa, Italy, detected a transient gravitational-wave signal produced by the coalescence of two stellar mass black holes. 

The waves were generated from the collision of two black holes which were 25 and 31 times the mass of the Sun and ultimately formed a black hole that is 49 times heavier than the Sun. 

The Chinese University of Hong Kong (CUHK), a member of the LIGO Scientific Collaboration (and thus part of the LIGO-Virgo Collaboration) has been actively involved in this field since 2016. Prof. Tjonnie G.F. Li, Assistant Professor, Physics Department of CUHK, is currently leading the collaboration’s Testing General Relativity working group, and his team members have been actively involved in analysing LIGO/Virgo data. 

Prof. Li said, “It is great to see the Virgo Collaboration, one that I was part of for four years, join the global network of detectors. The team has done a superb job launching the detector, and I wish to congratulate my former advisors (Prof. van den Brand, and Dr. Van Den Broeck), many friends and former colleagues with this wonderful achievement”. 

Advanced LIGO is a second-generation gravitational-wave detector consisting of the two identical interferometers in Hanford and Livingston, and uses precision laser interferometry to detect gravitational waves. Beginning operation in September 2015, Advanced LIGO has conducted two observing runs. The second “O2” observing run began on November 30, 2016 and ended on August 25, 2017. 

Advanced Virgo is the second-generation instrument built and operated by the Virgo collaboration to search for gravitational waves. With the end of observations with the initial Virgo detector in October 2011, the integration of the Advanced Virgo detector began. The new facility was dedicated in February 2017 while its commissioning was ongoing. In April, the control of the detector at its nominal working point was achieved for the first time. 

The Virgo detector joined the O2 run on August 1, 2017 at 10:00 UTC. The real-time detection on August 14 was triggered with data from all three LIGO and Virgo instruments. Virgo is, at present, less sensitive than LIGO, but two independent search algorithms based on all the information available from the three detectors demonstrated the evidence of a signal in the Virgo data as well. 

Overall, the volume of universe that is likely to contain the source shrinks by more than a factor of 20 when moving from a two-detector network to a three-detector network. The sky region for GW170814 has a size of only 60 square degrees, more than 10 times smaller than with data from the two LIGO interferometers alone; in addition, the accuracy with which the source distance is measured benefits from the addition of Virgo. 

“This increased precision will allow the entire astrophysical community to eventually make even more exciting discoveries, including multi-messenger observations,” says Georgia Tech professor Laura Cadonati, the Deputy Spokesperson of the LSC. “A smaller search area enables follow-up observations with telescopes and satellites for cosmic events that produce gravitational waves and emissions of light, such as the collision of neutron stars.”

“As we increase the number of observatories in the international gravitational wave network, we not only improve the source location, but we also recover improved polarization information that provides better information on the orientation of the orbiting objects as well as enabling new tests of Einstein’s theory,” says Fred Raab, LIGO Associate Director for Observatory Operations.

LIGO and VIRGO’s partner electromagnetic facilities around the world didn’t identify a counterpart for GW170814, which was similar to the three prior LIGO observations of black hole mergers. Black holes produce gravitational waves but not light.

“With this first joint detection by the Advanced LIGO and Virgo detectors, we have taken one step further into the gravitational-wave cosmos,” says Caltech’s David H. Reitze, the Executive Director of the LIGO Laboratory. “Virgo brings a powerful new capability to detect and better locate gravitational-wave sources, one that will undoubtedly lead to exciting and unanticipated results in the future.”  

LIGO

LIGO is funded by 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 1,200 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. Additional partners are listed at http://ligo.org/partners.php

Virgo Collaboration

The Virgo Collaboration consists of more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; eight 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 the University of Valencia; and EGO, the laboratory hosting the Virgo detector near Pisa in Italy.

Professor Tjonnie Li, Assistant Professor, Department of Physics, CUHK
Professor Tjonnie Li, Assistant Professor, Department of Physics, CUHK

Virgo Control Room
Virgo Control Room

Virgo laser interferometer arm
Virgo laser interferometer arm