On August 14, 2017 at 10∶30:43 UTC, the Advanced Virgo detector and the two Advanced LIGO
detectors coherently observed a transient gravitational-wave signal produced by the coalescence of two
stellar mass black holes, with a false-alarm rate of ≲1 in 27 000 years. The signal was observed with a
three-detector network matched-filter signal-to-noise ratio of 18. The inferred masses of the initial black
holes are 30.5þ5.7
−3.0 M⊙ and 25:3þ2.8
−4.2 M⊙ (at the 90% credible level). The luminosity distance of the source is
540þ130
−210 Mpc, corresponding to a redshift of z ¼ 0.11þ0.03
−0.04 . A network of three detectors improves the sky
localization of the source, reducing the area of the 90% credible region from 1160 deg2 using only the two
LIGO detectors to 60 deg2 using all three detectors. For the first time, we can test the nature of
gravitational-wave polarizations from the antenna response of the LIGO-Virgo network, thus enabling a
new class of phenomenological tests of gravity.
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