Multi-messenger astrophysics is a fast-growing, interdisciplinary field that combines data, which vary in volume and speed of data processing, from many different instruments that probe the Universe using different cosmic messengers: electromagnetic waves, cosmic rays, gravitational waves and neutrinos. In this Expert Recommendation, we review the key challenges of real-time observations of gravitational wave sources and their electromagnetic and astroparticle counterparts, and make a number of recommendations to maximize their potential for scientific discovery. These recommendations refer to the design of scalable and computationally efficient machine learning algorithms; the cyber-infrastructure to numerically simulate astrophysical sources, and to process and interpret multi-messenger astrophysics data; the management of gravitational wave detections to trigger real-time alerts for electromagnetic and astroparticle follow-ups; a vision to harness future developments of machine learning and cyber-infrastructure resources to cope with the big-data requirements; and the need to build a community of experts to realize the goals of multi-messenger astrophysics.

Research on smart cities has illustrated the use of data analytics, open data, smart sensors and other data-intensive applications that have significant potential to transform urban environments. As the complexity and intensity of these projects has increased, there is a need to understand smart city data ecosystems as an integrated view of data applications by the various city entities that operate within an institutional environment. This paper examines how authorities involved in such ecosystems coordinate data initiatives from an orchestration perspective. A case study of London's city data initiatives highlights the challenges faced in complex city data environments and the importance of an integrated view. Three elements of orchestration in smart city data ecosystems -namely openness, diffusion and shared visionare identified as the main enablers of city data initiatives within London's local government authorities. The study contributes to our theoretical understanding of orchestration within data ecosystems, as well as the social and technological impacts of city data.

Gravitational wave observations of eccentric binary black hole mergers will provide unequivocal evidence for the formation of these systems through dynamical assembly in dense stellar environments. The study of these astrophysically motivated sources is timely in view of electromagnetic observations, consistent with the existence of stellar mass black holes in the globular cluster M22 and in the Galactic center, and the proven detection capabilities of ground-based gravitational wave detectors. In order to get insights into the physics of these objects in the dynamical, strong-field gravity regime, we present a catalog of 89 numerical relativity waveforms that describe binary systems of non-spinning black holes with mass-ratios 1 ≤ q ≤ 10, and initial eccentricities as high as e0 = 0.18 fifteen cycles before merger. We use this catalog to quantify the loss of energy and angular momentum through gravitational radiation, and the astrophysical properties of the black hole remnant, including its final mass and spin, and recoil velocity. We discuss the implications of these results for gravitational wave source modeling, and the design of algorithms to search for and identify eccentric binary black hole mergers in realistic detection scenarios.

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