GW170608: Observation of a 19 Solar-mass Binary Black Hole Coalescence

On 2017 June 8 at 02:01:16.49 UTC, a gravitational-wave (GW) signal from the merger of two stellar-mass black holes was observed by the two Advanced Laser Interferometer Gravitational-Wave Observatory detectors with a network signal-to-noise ratio of 13. This system is the lightest black hole binary so far observed, with component masses of {12}_-2⁺⁷ {M}_☉ and {7}_-2⁺² {M}_☉ (90% credible intervals). These lie in the range of measured black hole masses in low-mass X-ray binaries, thus allowing us to compare black holes detected through GWs with electromagnetic observations. The source’s luminosity distance is {340}_-140⁺¹⁴⁰ {Mpc}, corresponding to redshift {0.07}_-0.03^+0.03. We verify that the signal waveform is consistent with the predictions of general relativity.