  We report the observation of gravitational waves from a binary-black-hole
coalescence during the first two weeks of LIGO's and Virgo's third observing
run. The signal was recorded on April 12, 2019 at 05:30:44 UTC with a network
signal-to-noise ratio of 19. The binary is different from observations during
the first two observing runs most notably due to its asymmetric masses: a ~30
solar mass black hole merged with a ~8 solar mass black hole companion. The
more massive black hole rotated with a dimensionless spin magnitude between
0.22 and 0.60 (90% probability). Asymmetric systems are predicted to emit
gravitational waves with stronger contributions from higher multipoles, and
indeed we find strong evidence for gravitational radiation beyond the leading
quadrupolar order in the observed signal. A suite of tests performed on
GW190412 indicates consistency with Einstein's general theory of relativity.
While the mass ratio of this system differs from all previous detections, we
show that it is consistent with the population model of stellar binary black
holes inferred from the first two observing runs.
