<i>Swift</i> and <i>NuSTAR</i> observations of GW170817: Detection of a blue kilonova.

Evans, P A; Cenko, S B; Kennea, J A; Emery, S W K; Kuin, N P M; Korobkin, O; Wollaeger, R T; Fryer, C L et al. · Science · 2017

basic_science · Level V

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Abstract

With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and x-ray observations by <i>Swift</i> and the <i>Nuclear Spectroscopic Telescope Array</i> of the EM counterpart of the binary neutron star merger GW170817. The bright, rapidly fading UV emission indicates a high mass (≈0.03 solar masses) wind-driven outflow with moderate electron fraction (<i>Y</i><sub>e</sub> ≈ 0.27). Combined with the x-ray limits, we favor an observer viewing angle of ≈30° away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultrarelativistic, highly collimated ejecta (a γ-ray burst afterglow).