The LIGO Scientific Collaboration and the Virgo Collaboration have announced the detection of a binary black hole merger. The detection has occurred on August the 14th, 2017. This is the fourth confirmed detection of a coalescence and merger of two binary black holes, each weighting several tens of solar masses. This merger is the first which was also seen by the Advanced Virgo detector in addition to the two LIGO detectors, thus making its localization on the sky much more precise than before. This result shows clearly that by adding more detectors it is possible to better pinpoint the source thus increasing considerably the chance to observe it with other types of astronomical observatories.

Professor Raffaele Flaminio, director of the Gravitational Wave Project Office of the National Astronomical Observatory of Japan and member of the Virgo team at the Laboratoire d'Annecy de Physique des Particules of the French CNRS, one of the authors of the paper that has been accepted by Physical Review Letters of the American Physical Society, explains:

It's like a tripod! If you want to have it pointing firmly in one direction it has to have three legs. The international gravitational wave detector just acquired its third leg. After the operation of the two Advanced LIGO detectors since 2015, the Advanced Virgo detector collected its first scientific data in August earlier this year.

On August 14th 2017, only two weeks after the start of data taking, a binary black hole merger was simultaneously detected by the two Advanced LIGO detectors and by the Advanced Virgo detector. The merger of the two black holes took place 1.8 billion years ago. But actually the detection is not exactly simultaneous. The wave arrived first at the LIGO site of Livingston (Louisiana state), then 8 ms later at the LIGO site of Hanford (Washington state) and finally 14 ms later at the site of Virgo near Pisa (Italy). These small differences in the arrival time allow to reconstruct much more precisely than before the arrival direction of the wave. With only the two LIGO detectors the position of the source is known within about a thousand square degrees i.e. about a thousand time the angular size of the full moon. By adding the data from the Advanced Virgo detector, the source position is confined within less than 100 square degrees. This shows that by adding more detectors to the network the source can be pinpointed more precisely, helping considerably the ensuing explorations with other types of astronomical observatories.

Figure: Localization of the source. In yellow the localization obtained using only the data from the two LIGO detectors. In green the localization obtained when the Virgo data are added in the online analysis and in gray using a more refined offline analysis (credit: LIGO-Virgo collaboration).

But this is not all. According to the theory of gravitation established by Albert Einstein a century ago, gravitational waves are vibrations of the space-time structure. Their effect is to deform the distance among objects. According to Einstein two kinds of vibrations are possible (known as wave polarizations). The two LIGO detectors are sensitive to only one of the two polarization states. The addition of Virgo makes it possible to sense the other polarization state as well. Within the limits imposed by the present detector sensitivities, scientists have been able to show that indeed the theory of Einstein is the one which best matches the measurements.

These results confirms how important it is to add more detectors to the network at different places on Earth and with different orientations. The Japanese KAGRA detector, currently under installation in the Kamioka mine in Hida Citiy (Gifu prefecture) is planned to join the network soon. It will allow to further improve the precision of these measurements, thus contributing to the development of gravitational wave astronomy, the exploration of the invisible Universe and, possibly, the study of the limits in the Einstein description of space-time.

LIGO press release: https://www.ligo.caltech.edu/news/ligo20170927

VIRGO press release: http://www.virgo-gw.eu/docs/GW170814/GW170814_press_release_en.pdf