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.

Several site‐testing programmes and observatories currently use combined Multi‐Aperture Scintillation Sensor (MASS)–Differential Image Motion Monitor (DIMM) instruments for monitoring parameters of optical turbulence. The instrument is described here. After a short recall of the measured quantities and operational principles, the optics and electronics of MASS–DIMM, interfacing to telescopes and detectors, and operation are covered in some detail. Particular attention is given to the correct measurement and control of instrumental parameters to ensure valid and well‐calibrated data, to the data quality and filtering. Examples of MASS–DIMM data are given, followed by the list of present and future applications.

The main goal of the MASTER-Net project is to produce a unique fast sky survey with all sky observed over a single night down to a limiting magnitude of 19-20mag. Such a survey will make it possible to address a number of fundamental problems: search for dark energy via the discovery and photometry of supernovas (including SNIa), search for exoplanets, microlensing effects, discovery of minor bodies in the Solar System and space-junk monitoring. All MASTER telescopes can be guided by alerts, and we plan to observe prompt optical emission from gamma-ray bursts synchronously in several filters and in several polarization planes.

An algorithm that permits one to measure atmospheric turbulence by statistical analysis of light flux fluctuations in four concentric‐ring apertures is described in detail. It consists of computing the scintillation indices for each aperture and pairwise aperture combination and in fitting the set of measured indices to a model with a small number of turbulent layers. The performance of this method is analysed by means of simulations and using the real data from a multi‐aperture scintillation sensor. It is shown that a turbulence profile with a vertical resolution of Δh/h∼ 0.5 can be reconstructed and that the errors of the measured intensities of turbulent layers are typically around 10 per cent of the integrated intensity. The integral parameters such as the seeing and the isoplanatic angle are measured with few per cent accuracy.

The Advanced LIGO observatory recently reported [1] the first direct detection of gravitational waves predicted by Einstein (1916) [10]. The detection of this event was predicted in 1997 on the basis of the Scenario Machine population synthesis calculations [38] Now we discuss the parameters of binary black holes and event rates predicted by different scenarios of binary evolution. We give a simple explanation of the big difference between detected black hole masses and the mean black hole masses observed in of X-ray Nova systems. The proximity of the masses of the components of GW150914 is in good agreement with the observed initial mass ratio distribution in massive binary systems, as is used in Scenario Machine calculations for massive binaries.