On August 1, 2017 VIRGO, the European detector of gravitational waves, officially joined Observational Run 2 to acquire data jointly with the two US-based LIGO detectors. This event has concluded a multi-year programme to significantly improve VIRGO sensitivity successfully implemented by the VIRGO consortium. “Several last months were devoted to testing the improved VIRGO detector. We were eagerly awaiting the conclusion of the tests to start acquire scientific data jointly with the LIGO detectors during that very exciting time for our community” – said Jo van den Brand (Nikhef, VU University in Amsterdam, The Netherlands), VIRGO collaboration spokesman. – “With its still inferior sensitivity, the VIRGO detector is however able to confirm observations made by the LIGO detectors and to help to more precisely locate the point in the Universe from which the detected waves arrived”.

Sensitivity of the current version of the VIRGO detector is significantly better than that of the previous version operated until 2011. VIRGO is now a quite new instrument composed of numerous modern subsystems. It took almost a year to properly align all these subsystems. “The project to improve the VIRGO detector was quite ambitious one. Several years of intense efforts were needed to attain its goals. These objectives could not be achieved without generosity of the VIRGO consortium members, dedicated service of the European Gravitational Observatory (EGO) employees, as well as involvement of the cooperating labs”, said Federico Ferrini, EGO Director.

Observational Run 2 started on November 30, 2016 and is scheduled to finish on August 25, 2017. Gravitational waves generated in the process of merging two black holes each of a mass equal to several Sun masses were detected on Earth for the 3^rd time ever during the run. The signal observed on January 4, 2017 was named GW170104. The discovery was announced jointly by LIGO and VIRGO on June 1, 2017. Both collaborations are still analysing the observed data. From August 1, 2017 new data are acquired jointly by three modern instruments. David Shoemaker from MIT, LIGO Scientific Collaboration spokesman, said: “Even if LIGO instruments alone have supplied plenty of valuable scientific information, observations conducted simultaneously by three detectors may open up quite new possibilities. Tight cooperation between VIRGO and LIGO will help to make maximum use of the possibilities”.

"Months ahead us will be for us both demanding and exciting. We are fully aware that the planned improvements will not only boost sensitivity of the detector, but will also make it a more complex instrument. We will have to learn how to utilize every bit of capabilities of the new technologies installed in the detector”, said Alessio Rocchi from INFN at the Tor Vergata University in Rome, VIRGO consortium coordinator responsible for getting the instrument ready for OR3.

„Network of three modern second-generation detectors allows us to more precisely locate sources of gravitational waves. It’s already a great achievement, but the best is still ahead us: gradually improved sensitivity of the detectors and/or new detectors to join the network during the few coming years will open up quite fascinating perspectives in the so-called multi-messenger astronomy” – said Giovanni Losurdo from INFN in Pisa, former leader of the ”Advanced VIRGO'” project.

More than 280 physicists and engineers from 20 European labs have been conducting research within the VIRGO consortium framework. They come from 6 teams from Centre National de la Recherche Scientifique (CNRS), France; 8 teams from Istituto Nazionale di Fisica Nucleare (INFN), Italy; two teams from Nikhef, the Netherlands; the MTA Wigner RCP team (Hungary); the POLGRAW team (Poland); and European Gravitational Observatory (EGO). The VIRGO detector is located at EGO premises close to Pisa, Italy. A team from Valencia (Spain) joined the consortium recently.

Polish scientists in the VIRGO project

The POLGRAW team cooperating with the VIRGO consortium since 2009 is composed of scientists and engineers from Polish Academy of Sciences Mathematics Institute, Polish Academy of Sciences Copernicus Astronomical Centre, NCBJ, AGH University of Science & Technology in Cracow, University in Białystok, Jagiellonian University in Cracow, Copernicus University in Toruń, Warsaw University, Wrocław University and Zielona Góra University. Polish researchers are developing data analysis algorithms, constructing electronic circuitry, conducting noise analyses. They are also hoping to observe some electromagnetic signals emitted by sources of gravitational waves – it might happen for example at merger of two neutron stars.

How the VIRGO detector is able to detect gravitational waves

The VIRGO detector consists of two perpendicular arms buried in tunnels each about 3 km long. Each detector arm is terminated on both ends with mirrors. Laser beams traversing the tunnels reflect many times from the mirrors. Gravitational waves slightly change relative optical path lengths, differently in each tunnel. Such changes may be observed by means of interference of two superimposed laser beams.