  We report the observation of a gravitational-wave signal produced by the
coalescence of two stellar-mass black holes. The signal, GW151226, was observed
by the twin detectors of the Laser Interferometer Gravitational-Wave
Observatory (LIGO) on December 26, 2015 at 03:38:53 UTC. The signal was
initially identified within 70 s by an online matched-filter search targeting
binary coalescences. Subsequent off-line analyses recovered GW151226 with a
network signal-to-noise ratio of 13 and a significance greater than 5 $\sigma$.
The signal persisted in the LIGO frequency band for approximately 1 s,
increasing in frequency and amplitude over about 55 cycles from 35 to 450 Hz,
and reached a peak gravitational strain of $3.4_{-0.9}^{+0.7} \times 10^{-22}$.
The inferred source-frame initial black hole masses are $14.2_{-3.7}^{+8.3}
M_{\odot}$ and $7.5_{-2.3}^{+2.3} M_{\odot}$ and the final black hole mass is
$20.8_{-1.7}^{+6.1} M_{\odot}$. We find that at least one of the component
black holes has spin greater than 0.2. This source is located at a luminosity
distance of $440_{-190}^{+180}$ Mpc corresponding to a redshift
$0.09_{-0.04}^{+0.03}$. All uncertainties define a 90 % credible interval. This
second gravitational-wave observation provides improved constraints on stellar
populations and on deviations from general relativity.
