  On September 14, 2015, a gravitational wave signal from a coalescing black
hole binary system was observed by the Advanced LIGO detectors. This paper
describes the transient noise backgrounds used to determine the significance of
the event (designated GW150914) and presents the results of investigations into
potential correlated or uncorrelated sources of transient noise in the
detectors around the time of the event. The detectors were operating nominally
at the time of GW150914. We have ruled out environmental influences and
non-Gaussian instrument noise at either LIGO detector as the cause of the
observed gravitational wave signal.
