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 blackholes was observed by the two Advanced Laser Interferometer Gravitational-Wave Observatory detectors with anetwork signal-to-noise ratio of 13. This system is the lightest black hole binary so far observed, with componentmasses of 12⁺⁷ <inf>-2</inf>M<inf>⊙</inf>7⁺² <inf>-2</inf> (90% credible intervals). These lie in the range of measured black hole masses inlow-mass X-ray binaries, thus allowing us to compare black holes detected through GWs with electromagneticobservations. The source's luminosity distance is 340⁺¹⁴⁰ <inf>-140</inf>corresponding to redshift -0.07^+0.030 <inf>03.</inf> We verify thatthe signal waveform is consistent with the predictions of general relativity.