Context.A stellar occultation by a trans-Neptunian object (TNO) provides an opportunity to probe the size and shape of these distant solar system bodies. In the past seven years, several occultations by TNOs have been observed, but mostly from a single location. Only very few TNOs have been sampled simultaneously from multiple locations. Sufficient data that enable a robust estimation of shadow size through an ellipse fit could only be obtained for two objects. Aims. We present the first observation of an occultation by the TNO 2007 UK 126 on 15 November 2014, measured by three observers, one nearly on and two almost symmetrical to the shadow's centerline. This is the first multi-chord dataset obtained for a so-called detached object, a TNO subgroup with perihelion distances so large that the giant planets have likely not perturbed their orbits. We also revisit Herschel/PACS far-infrared data, applying a new reduction method to improve the accuracy of the measured fluxes. Combining both datasets allows us to comprehensively characterize 2007 UK 126 . Methods. We use error-in-variable regression to solve the non-linear problem of propagating timing errors into uncertainties of the ellipse parameters. Based on the shadow's size and a previously reported rotation period, we expect a shape of a Maclaurin spheroid and derive a geometrically plausible size range. To refine our size estimate of 2007 UK 126 , we model its thermal emission using a thermophysical model code. We conduct a parametric study to predict far-infrared fluxes and compare them to the Herschel/PACS measurements. Results. The favorable geometry of our occultation chords, combined with minimal dead-time imaging, and precise GPS time measurements, allow for an accurate estimation of the shadow size (best-fitting ellipse with axes 645.80 ± 5.68 km × 597.81 ± 12.74 km) and the visual geometric albedo (p V = 15.0 ± 1.6%). By combining our analyses of the occultation and the far-infrared data, we can constrain the effective diameter of 2007 UK 126 to d eff = 599−629 km. We conclude that subsolar surface temperatures are in the order of ≈50−55 K.

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Context. Irregular satellites of Jupiter with their highly eccentric, inclined and distant orbits suggest that their capture took place just before the giant planet migration.Aims. We aim to improve our understanding of the surface composition of irregular satellites of Jupiter to gain insight into a narrow time window when our Solar System was forming.Methods. We observed three Jovian irregular satellites, Himalia (JVI), Elara (JVII), and Carme (JXI), using a medium-resolution 0.8-5.5 µm spectrograph, SpeX on the National Aeronautics and Space Administration (NASA) Infrared Teselscope Facility (IRTF). Using a linear spectral unmixing model we have constrained the major mineral phases on the surface of these three bodies. Article number, page 1 of 17 arXiv:1710.06200v2 [astro-ph.EP] 27 Dec 2017 M. Bhatt et al.: Composition of Jupiter irregular satellites sheds light on their originResults. Our results confirm that the surface of Himalia (JVI), Elara (JVII), and Carme (JXI) are dominated by opaque materials such as those seen in carbonaceous chondrite meteorites. Our spectral modeling of NIR spectra of Himalia and Elara confirm that their surface composition is the same and magnetite is the dominant mineral. A comparison of the spectral shape of Himalia with the two large main C-type asteroids, Themis (D∼176 km) and Europa (D∼352 km), suggests surface composition similar to Europa. The NIR spectrum of Carme exhibits blue slope up to 1.5 µm and is spectrally distinct from those of Himalia and Elara. Our model suggests that it is compositionally similar to amorphous carbon.Conclusions. Himalia and Elara are compositionally similar but differ significantly from Carme.These results support the hypotheses that the Jupiter's irregular satellites are captured bodies that were subject to further breakup events and clustered as families based on their similar physical and surface compositions.