Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Swift and NuSTAR observations of GW170817: detection of a blue kilonova   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).

31
The categories of this paper are: ao-sci
8
The categories of this paper are: hep-ex
20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th
16
The categories of this paper are: stat
29
The categories of this paper are: funct-an
0
The categories of this paper are: hep-ph
5
The categories of this paper are: gr-qc
11
The categories of this paper are: hep-lat
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The categories of this paper are: comp-gas
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The categories of this paper are: chem-ph
1
The categories of this paper are: cs
4
The categories of this paper are: cond-mat
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The categories of this paper are: quant-ph
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The categories of this paper are: math-ph
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The categories of this paper are: bayes-an