  We describe the observation of GW170104, a gravitational-wave signal produced
by the coalescence of a pair of stellar-mass black holes. The signal was
measured on January 4, 2017 at 10:11:58.6 UTC by the twin advanced detectors of
the Laser Interferometer Gravitational-Wave Observatory during their second
observing run, with a network signal-to-noise ratio of 13 and a false alarm
rate less than 1 in 70,000 years. The inferred component black hole masses are
$31.2^{+8.4}_{-6.0}\,M_\odot$ and $19.4^{+5.3}_{-5.9}\,M_\odot$ (at the 90%
credible level). The black hole spins are best constrained through measurement
of the effective inspiral spin parameter, a mass-weighted combination of the
spin components perpendicular to the orbital plane, $\chi_\mathrm{eff} =
-0.12^{+0.21}_{-0.30}.$ This result implies that spin configurations with both
component spins positively aligned with the orbital angular momentum are
disfavored. The source luminosity distance is $880^{+450}_{-390}~\mathrm{Mpc}$
corresponding to a redshift of $z = 0.18^{+0.08}_{-0.07}$. We constrain the
magnitude of modifications to the gravitational-wave dispersion relation and
perform null tests of general relativity. Assuming that gravitons are dispersed
in vacuum like massive particles, we bound the graviton mass to $m_g \le 7.7
\times 10^{-23}~\mathrm{eV}/c^2$. In all cases, we find that GW170104 is
consistent with general relativity.
