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

TL;DR

This paper reports ultraviolet and X-ray observations of the electromagnetic counterpart to the neutron star merger GW170817, revealing a blue kilonova with a high mass wind-driven outflow and providing insights into the viewing angle and ejecta composition.

Contribution

It presents the first combined UV and X-ray observations of GW170817's kilonova, offering new constraints on the ejecta properties and viewing geometry.

Findings

Detection of a bright, rapidly fading UV emission.

Estimation of a high mass wind-driven outflow (~0.03 solar masses).

Inference of a viewing angle around 30 degrees from the orbital axis.

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 ($\approx0.03$ solar masses) wind-driven outflow with moderate electron fraction ($Y_{e}\approx0.27$). 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).