The fledgling scientific discipline of gravitational wave astronomy has made an important step forward with the detection of a gravitational wave signal using a network involving the European Virgo detector.

In a new paper accepted today by the journal Physical Review Letters, scientists outline details of the first-ever gravitational wave signal received by the Virgo detector in Italy, alongside the LIGO detectors in the USA.

This is the fourth announced detection of a binary black hole system. While this new event is of astrophysical relevance, its detection comes with an additional asset: this is the first significant gravitational wave signal recorded by the Virgo detector, which has recently completed its upgrade to Advanced Virgo.

As with previous detections since the historic first detection in September 2015, researchers from the University of Glasgow’s Institute for Gravitational Research played a key role in the analysis of the data from both Virgo and LIGO.

Gravitational waves are ripples in the fabric of spacetime caused by massive astronomical events. Both the LIGO and Virgo detectors use sophisticated laser interferometry techniques to measure the extremely weak distortion of spacetime as they pass through the planet Earth.

The detection reported from all three observatories, - now known to scientists as as ‘GW170814’ - occurred on August 14, when ripples from the collision of two black holes reached Earth.

The detected gravitational waves were emitted during the final moments of the merger of two black holes with masses about 31 and 25 times the mass of the sun and located about 1.8 billion light-years away. The newly produced spinning black hole has about 53 times the mass of our sun, which means that about 3 solar masses were converted into gravitational-wave energy during the coalescence.

Dr John Veitch, research fellow at the University of Glasgow’s School of Physics and Astronomy, co-led a team within the collaboration working on the data analysis of the signal to determine the origins and properties of the source.

Dr Veitch said: “The addition to the network of a signal from Virgo, provided us with a lot of useful data. Having a third detector means that we can now triangulate the position of the source, and much more accurately determine the exact spot in the cosmos where the signal came from.

“We go through multiple stages of analysis. The first is filtering the data from the detectors, which provides us with triggers for possible detections, which are then checked against the data from the other detectors. When a match between detectors is found, we can begin looking in more detail at the data to determine the mass and the position of the source, and start sharing data with other partners across the world.”
The Virgo detector, which was first proposed in 1993, is based in Cascina in Italy, close to Pisa, and is supported by 20 laboratories in six countries..

Professor Sheila Rowan, director of the Institute for Gravitational Research, said: “We’re proud to have played a role in this new joint detection alongside our partners in the US and in Europe, which is an important advance for the field of gravitational wave astronomy.  

“We now are demonstrating the capabilities of a network of gravitational wave detectors, which deepens the pool of data we’ll be able to draw from in future and will help to further expand our understanding of the universe.”

Jo van den Brand of Nikhef and VU University Amsterdam, spokesperson of the Virgo collaboration, said: “It is wonderful to see a first gravitational-wave signal in our brand new Advanced Virgo detector only two weeks after it officially started taking data, That’s a great reward after all the work done in the Advanced Virgo project to upgrade the instrument over the past six years.”
 
MIT’s David Shoemaker, spokesperson of the LIGO Scientific Collaboration, said: “This is just the beginning of observations with the network enabled by Virgo and LIGO working together. With the next observing run planned for Fall 2018 we can expect such detections weekly or even more often.”

“Little more than a year and a half ago, NSF announced that its Laser Gravitational-Wave Observatory had made the first-ever detection of gravitational waves resulting from the collision of two black holes in a galaxy a billion light-years away," says France Córdova, NSF director.

"Today, we are delighted to announce the first discovery made in partnership between the Virgo Gravitational-Wave Observatory and the LIGO Scientific Collaboration, the first time a gravitational-wave detection was observed by these observatories, located thousands of miles apart. This is an exciting milestone in the growing international scientific effort to unlock the extraordinary mysteries of our Universe.”  

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.

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First published: 28 September 2017