On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

We derive the characteristic nutational damping time Td for a linear, anelastic ellipsoid of revolution. Our calculation is based on the well‐known idea that energy loss within an isolated spinning body causes the axis of maximum inertia of the body to align with its angular momentum vector, leading to pure spin. Energy loss occurs within an anelastic material whenever internal stresses are time variable; thus even freely rotating bodies in space, if they are wobbling, lose energy because internal stresses are associated with the accelerations caused by nutation. We find that , where D(h) is a constant of the order of a few times 102 that depends on the shape of the body with h being the (aspect) ratio of the lengths of axes to one another, μ is the elastic modulus, Q is a quality factor that describes the anelasticity of the material, ρ is the density of the body, a is its radius and Ω is an angular velocity. This functional form of the damping time is consistent with previous results but is more soundly based. Coefficients in past expressions vary between various authors, leading to predicted damping times that can differ by factors of the order of 10. To estimate damping times for typical asteroids, we choose values for the various parameters in this expression. We conclude that the extent of energy dissipation was over, rather than underestimated, in previous treatments. None the less, we argue that asteroids will generally be found in pure rotation, unless objects are small, spinning slowly and recently excited.

Many asteroids are thought to be particle aggregates held together principally by self-gravity. Here we study-for static and dynamical situations-the equilibrium shapes of spinning asteroids that are permitted for rubble piles. As in the case of spinning fluid masses, not all shapes are compatible with a granular rheology. We take the asteroid to always be an ellipsoid with an interior modeled as a rigid-plastic, cohesion-less material with a Drucker-Prager yield criterion. Using an approximate volume-averaged procedure, based on the classical method of moments, we investigate the dynamical process by which such objects may achieve equilibrium. We first collapse our dynamical approach to its statical limit to derive regions in spinshape parameter space that allow equilibrium solutions to exist. At present, only a graphical illustration of these solutions for a prolate ellipsoid following the DruckerPrager failure law is available (Sharma et al. BAAS 37 [2005a]

We investigate planetary fly-bys of asteroids using an approximate volume-averaged method that offers a relatively simple, but very flexible, approach to study the rotational dynamics of ellipsoids. The asteroid is considered to be a deformable, prolate ellipsoid, with its interior being modelled as a rigid-granular material. Effects due to the asteroid's rotation, its self-gravity and gravitational interaction with the planet are included. Using a simplified approach allows us to explore in detail the mechanics of asteroid's deformations and disruptions during planetary encounters. We also compare our results with those obtained by Richardson et al. (1998) who used a large numerical code. We find that many of the features reported by them can indeed be captured by our rather simple methodology, and we discuss the reasons why some of our results differ from theirs.

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