Gravitational waves: fourth merger of black holes observed by LIGO-Virgo

The LIGO and Virgo Collaborations, which teams from the Linear Accelerator Laboratory (CNRS [National Centre for Scientific Research]/Université Paris-Sud) are highly involved with, have just announced a fourth detection of a merger of two black holes. This advance marks the beginning of a new era for the study of gravitational waves.

Wednesday 27 September 2017, the LIGO and Virgo Collaborations announced a fourth detection of a merger of two black holes at the side lines of the “G7 Science” summit in Turin. The signal was recorded for the first time by three instruments on 14 August at around 12.31 pm. Indeed, the two “Advanced LIGO” detectors – which have been operational since September 2015 and whose data have already made it possible to identify the gravitational waves produced by three mergers of black holes – were joined on 1 August by the “Advanced Virgo” detector.

This new event, called GW170814, is described in an article accepted by the journal Physical Review Letters. 

in blue and in solid lines, the four binary black hole systems detected by LIGO and Virgo in chronological order. GW170814 is the last one on the right. © LSC/Sonoma State University/Aurore Simonnet.

This discovery strongly demonstrates the value of a three-detector network. On the one hand, the precision in locating the source in the sky is considerably improved (by a factor of about 10 for GW170814 compared to cases where only data from the two LIGO detectors is used) and on the other hand, new tests of general relativity become possible. It bodes well for the joint LIGO-Virgo “O3” data collection which should start in about a year, after a new series of upgrades to the three instruments.

These ripples in space-time

Gravitational waves are ripples in space-time specifically produced during cataclysmic events which have occurred in the cosmos, such as the merger of two black holes. GW170814 belongs to this category: two black holes weighing 31 and 25 times the mass of the sun and located around 1.8 billion light-years from Earth merged, creating a single black hole of 53 solar masses. During this phenomenon, a huge amount of energy, equivalent to that contained in three solar masses, was emitted in the form of gravitational waves.

Aerial photo of the Virgo detector site, built near the municipality of Cascina near Pisa in Italy. © The Virgo Collaboration / Giuseppe Greco.

This event is only the fourth published by the LIGO and Virgo Collaborations in two years. And, although the black holes observed on 14 August have similar properties to those detected previously, this signal is unique in that it is the first one to have been seen by three detectors: the two “Advanced LIGO” instruments and the “Advanced Virgo” one. 

This advance marks the beginning of a new era for the study of gravitational waves as the potential of a three-detector network is much greater than that of the LIGO instruments alone.

The Linear Accelerator Laboratory is a founding member of the Virgo Collaboration.

The Virgo Collaboration consists of more than 280 scientists from 20 different European laboratories – including the Linear Accelerator Laboratory (LAL, UPSud/CNRS), one of its founding members. The current activities of the Virgo-LAL group are split between two main areas: scientific exploitation of the Virgo-LIGO data and activities focusing more on the detector.

Comparison of the sky regions selected by rapid analysis of data from the LIGO detectors only (blue) and LIGO-Virgo (green). The size of the region decreases by a factor of about 10 with the addition of Virgo. © The Virgo Collaboration / Giuseppe Greco.

In data analysis, the group's contributions are currently focused on searching for long-duration transient signals emitted during accretion phenomena around a stellar black hole or by a newly-formed neutron star. The other major research topic is concentrating on signals emitted by cosmic strings. The group’s physicists are also working on a real-time alert system for the LIGO-Virgo network, as well as on “multi-messenger” analysis, which uses data from both the interferometric gravitational-wave detectors and the partner telescopes. For many years, the group has been specifically studying the coincidence of gravitational-wave signals and gamma-ray bursts.

Historically, the Virgo group at LAL has been responsible for building the large-diameter vacuum tubes which form the two 3 km arms of the interferometer, where the laser beams circulate, as well as a large part of the experiment's software (longitudinal and angular control of the interferometer, vacuum command and control, general software tools, etc.) The current “detector” activities focus on instrument control: how to bring it to its operating point and keep it there, both in the long term and with precision. At the start of September, a student in the team submitted her thesis mainly on the control of the Advanced Virgo detector. The group is also a driving force within Virgo with regard to data quality. Finally, the CALVA platform in the laboratory allows the testing of optical configurations and control techniques which may be implemented on the LIGO and Virgo instruments in the future.

Patrice Hello - Leader of the VIRGO group at Laboratoire de l'Accélérateur Linéaire - LAL (UPSud/CNRS) - hello @
Nicolas Arnaud - CNRS researcher at Laboratoire de l'Accélérateur Linéaire - LAL (UPSud/CNRS) - narnaud @

To find out more:
•    Read the full article on the LAL website:
•    Virgo Collaboration website:

GW170814: A three-detector observation of gravitational waves from a binary black hole Coalescence. The LIGO Scientific Collaboration and The Virgo Collaboration, submitted to Physical Review Letters. or