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

On June 8, 2017 at 02:01:16.49 UTC, a gravitational-wave signal from the merger of two stellar-mass black holes was observed by the two Advanced LIGO detectors with a network signal-to-noise ratio of 13. This system is the lightest black hole binary so far observed, with component masses $12_{-2}^{+7}{M}_{\odot }$ and $7_{-2}^{+2}{M}_{\odot }$ (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 gravitational waves 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.