Localization and Broadband Follow-up of the Gravitational-wave Transient GW150914

A gravitational-wave (GW) transient was identified in data recorded bythe 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. Byprior arrangement, preliminary estimates of the time, significance, andsky location of the event were shared with 63 teams of observerscovering radio, optical, near-infrared, X-ray, and gamma-ray wavelengthswith ground- and space-based facilities. In this Letter we describe thelow-latency analysis of the GW data and present the sky localization ofthe first observed compact binary merger. We summarize the follow-upobservations reported by 25 teams via private Gamma-ray CoordinatesNetwork circulars, giving an overview of the participating facilities,the GW sky localization coverage, the timeline, and depth of theobservations. 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 acounterpart of an Advanced LIGO source represents a milestone andhighlights the broad capabilities of the transient astronomy communityand the observing strategies that have been developed to pursue neutronstar binary merger events. Detailed investigations of the EM data andresults of the EM follow-up campaign are being disseminated in papers bythe individual teams.