Swift and NuSTAR observations of GW170817: detection of a blue kilonova

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 Swift and the Nuclear Spectroscopic Telescope ARray (NuSTAR) of the EM counterpart of the binary neutron star merger GW170817. The bright, rapidly fading ultraviolet emission indicates a high mass ($\approx 0.03$ solar masses) wind-driven outflow with moderate electron fraction ($Y_e\approx 0.27$). Combined with the X-ray limits, we favor an observer viewing angle of $\approx 30^{\circ }$ away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultra-relativistic, highly collimated ejecta (a gamma-ray burst afterglow).

Science, in press; 56 pages, 12 figures

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