  On September 14, 2015, the Laser Interferometer Gravitational-wave
Observatory (LIGO) detected a gravitational-wave transient (GW150914); we
characterize the properties of the source and its parameters. The data around
the time of the event were analyzed coherently across the LIGO network using a
suite of accurate waveform models that describe gravitational waves from a
compact binary system in general relativity. GW150914 was produced by a nearly
equal mass binary black hole of $36^{+5}_{-4} M_\odot$ and $29^{+4}_{-4}
M_\odot$; for each parameter we report the median value and the range of the
90% credible interval. The dimensionless spin magnitude of the more massive
black hole is bound to be $<0.7$ (at 90% probability). The luminosity distance
to the source is $410^{+160}_{-180}$ Mpc, corresponding to a redshift
$0.09^{+0.03}_{-0.04}$ assuming standard cosmology. The source location is
constrained to an annulus section of $610$ deg$^2$, primarily in the southern
hemisphere. The binary merges into a black hole of $62^{+4}_{-4} M_\odot$ and
spin $0.67^{+0.05}_{-0.07}$. This black hole is significantly more massive than
any other inferred from electromagnetic observations in the stellar-mass
regime.
