  A gravitational-wave (GW) transient was identified in data recorded by the
Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) detectors
on 2015 September 14. The event, initially designated G184098 and later given
the name GW150914, is described in detail elsewhere. By prior arrangement,
preliminary estimates of the time, significance, and sky location of the event
were shared with 63 teams of observers covering radio, optical, near-infrared,
X-ray, and gamma-ray wavelengths with ground- and space-based facilities. In
this Letter we describe the low-latency analysis of the GW data and present the
sky localization of the first observed compact binary merger. We summarize the
follow-up observations reported by 25 teams via private Gamma-ray Coordinates
Network circulars, giving an overview of the participating facilities, the GW
sky localization coverage, the timeline and depth of the observations. As this
event turned out to be a binary black hole merger, there is little expectation
of a detectable electromagnetic (EM) signature. Nevertheless, this first
broadband campaign to search for a counterpart of an Advanced LIGO source
represents a milestone and highlights the broad capabilities of the transient
astronomy community and the observing strategies that have been developed to
pursue neutron star binary merger events. Detailed investigations of the EM
data and results of the EM follow-up campaign are being disseminated in papers
by the individual teams.
