of 56
Swift
and
NuSTAR
observations of GW170817: detection of a
blue kilonova
P.A. Evans
1
, S.B. Cenko
2
,
3
, J.A. Kennea
4
, S.W.K. Emery
5
, N.P.M. Kuin
5
, O.
Korobkin
6
, R.T. Wollaeger
6
, C.L. Fryer
6
, K.K. Madsen
7
, F.A. Harrison
7
, Y.
Xu
7
, E. Nakar
8
, K. Hotokezaka
9
, A. Lien
10
,
11
, S. Campana
12
, S.R. Oates
13
, E.
Troja
2
,
14
, A.A. Breeveld
5
, F. E. Marshall
2
, S.D. Barthelmy
2
, A. P. Beardmore
1
,
D.N. Burrows
4
, G. Cusumano
15
, A. D’A
`
ı
15
, P. D’Avanzo
12
, V. D’Elia
16
,
17
, M.
de Pasquale
18
, W.P. Even
6
,
19
, C.J. Fontes
6
, K. Forster
7
, J. Garcia
7
, P. Giommi
17
,
B. Grefenstette
7
, C. Gronwall
4
,
20
, D.H. Hartmann
21
, M. Heida
7
, A.L. Hungerford
6
,
M.M. Kasliwal
22
, H.A. Krimm
23
,
24
, A.J. Levan
13
, D. Malesani
25
, A. Melandri
12
,
H. Miyasaka
7
, J.A. Nousek
4
, P.T. O’Brien
1
, J.P. Osborne
1
, C. Pagani
1
, K.L.
Page
1
, D.M. Palmer
26
, M. Perri
16
,
17
, S. Pike
7
, J.L. Racusin
2
, S. Rosswog
27
,
M.H. Siegel
4
, T. Sakamoto
28
, B. Sbarufatti
4
, G. Tagliaferri
12
, N.R. Tanvir
1
, A.
Tohuvavohu
4
1
University of Leicester, X-ray and Observational Astronomy Research Group, Leicester Institute for Space and
Earth Observation, Department of Physics & Astronomy, University Road, Leicester, LE1 7RH, UK
2
Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt MD, 20771 USA
3
Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
4
Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802,
USA
5
University College London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, RH5 6NT, U.K.
6
Center for Theoretical Astrophysics, Los Alamos National Laboratory, Los Alamos, NM 87545 USA
7
Cahill Center for Astronomy and Astrophysics, California Institute of Technology, 1200 East California Boule-
vard, Pasadena, CA 91125, U
8
The Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
9
Center for Computational Astrophysics, Simons Foundation, 162 5th Ave, New York, 10010, NY, USA
10
Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space
Flight Center, Greenbelt MD, 20771 USA
11
Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250,
USA
12
INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, Italy
13
Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
1
arXiv:1710.05437v1 [astro-ph.HE] 16 Oct 2017
14
Department of Physics and Astronomy, University of Maryland, College Park, MD 20742-4111, USA
15
INAF – IASF Palermo, via Ugo La Malfa 153, I-90146, Palermo, Italy
16
INAF-Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio Catone, Italy
17
Space Science Data Center (SSDC) - Agenzia Spaziale Italiana (ASI), I-00133 Roma, Italy
18
Department of Astronomy and Space Sciences, University of Istanbul, Beyzıt 34119, Istanbul, Turkey
19
Southern Utah University, Cedar City, UT 84720, USA
20
Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802
21
Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, USA
22
Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125,
USA
23
Universities Space Research Association, 7178 Columbia Gateway Dr, Columbia, MD 21046, USA
24
National Science Foundation, 2415 Eisenhower Avenue, Alexandria, VA 22314, USA
25
Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100
Copenhagen Ø, Denmark
26
Los Alamos National Laboratory, B244, Los Alamos, NM, 87545, USA
27
The Oskar Klein Centre, Department of Astronomy, AlbaNova, Stockholm University, SE-106 91 Stockholm,
Sweden
28
Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara, Kanagawa, 252-5258, Japan
To whom correspondence should be addressed; E-mail: pae9@leicester.ac.uk
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 obser-
vations by
Swift
and the
Nuclear Spectroscopic Telescope ARray
(
NuSTAR
) of the EM counterpart
of the binary neutron star merger GW 170817. The bright, rapidly fading ultraviolet emission
indicates a high mass (
0
.
03
solar masses) wind-driven outflow with moderate electron fraction
(
Y
e
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 ultra-relativistic, highly collimated ejecta (a gamma-ray
burst afterglow).
One-sentence summary
We report X-ray and UV observations of the first binary neutron star merger detected via gravitational waves.
Main Text
At 12:41:04.45 on 2017 August 17 (UT times are used throughout this work), the Laser Interferometric Gravitational-
Wave Observatory (LIGO) and Virgo Consortium (LVC) registered a strong gravitational wave (GW) signal (LVC
trigger G298048; (
1
)), later named GW 170817 (
2
). Unlike previous GW sources reported by LIGO, which in-
volved only black holes (
3
), the gravitational strain waveforms indicated a merger of two neutron stars. Binary
neutron star mergers have long been considered a promising candidate for the detection of an electromagnetic
counterpart associated with a gravitational wave source.
Two seconds later, the Gamma-Ray Burst Monitor (GBM) on the
Fermi
spacecraft triggered on a short (du-
ration
2
s) gamma-ray signal consistent with the GW localization, GRB 170817A (
4, 5
). The location of the
2