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.
This Supplement provides supporting material for Abbott et al. (2016a). We briefly summarize past electromagnetic (EM) follow-up efforts as well as the organization and policy of the current EM follow-up program. We compare the four probability sky maps produced for the gravitational-wave transient GW150914, and provide additional details of the EM follow-up observations that were performed in the different bands.
We use the statistics of caustic crossings induced by microlensing in the lens system Q 2237+0305 to study the lens galaxy peculiar velocity. We calculate the caustic crossing rates for a comprehensive family of stellar mass functions and find a dependence of the average number of caustic crossings with the effective transverse velocity and the average mass, n ∝ v ef f / m , equivalent to the theoretical prediction for the case of microlenses with identical masses. We explore the possibilities of the method to measure v ef f using the ∼12 years of OGLE monitoring of the four images of Q 2237+0305. To determine a lower limit for v ef f we count, conservatively, a single caustic crossing for each one of the 4 high magnification events identified in the literature (plus one additional proposed by us) obtaining v ef f 240 m /0.17M ⊙ km s −1 at 68% of confidence. From this value and the average F W HM of 4 high magnification events we obtain a lower limit of r s 1.4 m /0.17M ⊙ light-days for the radius of the source (r s = F W HM/2.35). Tentative identification of 3 additional caustic crossing events leads to estimates of v ef f ≃ (493 ± 246) m /0.17M ⊙ km s −1 for the effective transverse velocity and of r s ≃ (2.7 ± 1.3) m /0.17M ⊙ light-days for the source size. The estimated transverse peculiar velocity of the galaxy is v t ≃ (429 ± 246) m /0.17M ⊙ km s −1 .
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