of 59
Multi-messenger Observations of a Binary Neutron Star Merger
LIGO Scienti
fi
c Collaboration and Virgo Collaboration, Fermi GBM, INT
EGRAL, IceCube Collaboration, AstroSat Cadmium Zinc
Telluride Imager Team, IPN Collaboration, The Insight-Hxmt Co
llaboration, ANTARES Collaboratio
n, The Swift Collaboration, AGILE
Team, The 1M2H Team, The Dark Energy Camera GW-EM Collabo
ration and the DES Collaborati
on, The DLT40 Collaboration,
GRAWITA: GRAvitatio
nal Wave Inaf TeAm, The Fermi Large Area Telescope Collaboration, ATCA: Australia Telescope Compact
Array, ASKAP: Australian SKA Path
fi
nder, Las Cumbres Observatory Group, OzGrav, DWF
(
Deeper, Wider, Faster Program
)
,AST3,
and CAASTRO Collaborations, The VINRO
UGE Collaboration, MASTER Collaboratio
n, J-GEM, GROWTH, JAGWAR, Caltech-
NRAO, TTU-NRAO, and NuSTAR Collaboratio
ns,Pan-STARRS,TheMAXITeam,TZACC
onsortium, KU Colla
boration, Nordic
Optical Telescope, ePESSTO, GROND, Texas T
ech University, SALT Group, TOROS: Trans
ient Robotic Observatory of the South
Collaboration, The BOOTES Collabo
ration, MWA: Murchison Wide
fi
eld Array, The CALET Collaboration, IKI-GW Follow-up
Collaboration, H.E.S.S. Collaboration, LOFAR Collaboration, LWA: Long Wavelength Array, HAWC Collaboration, The Pierre Auger
Collaboration, ALMA Collaboration, Euro VL
BI Team, Pi of the Sky Collaboration, The C
handra Team at McGill University, DFN:
Desert Fireball Network, ATLAS, High Time Resolution Universe Survey, RIMAS and RATIR, and SKA South Africa
/
MeerKAT
(
See the end matter for the full list of authors.
)
Received 2017 October 3; revised 2017 October 6; accepted 2017 October 6; published 2017 October 16
Abstract
On 2017 August 17 a binary neutron star coalescence candidate
(
later designated GW170817
)
with merger time
12:41:04 UTC was observed through gra
vitational waves by the Advanced LIG
O and Advanced Virgo detectors. The
Fermi
Gamma-ray Burst Monitor independently detected a gamma-ray burst
(
GRB 170817A
)
with a time delay of
~
1.7 s
with respect to the merger time. From the gravitational-
wave signal, the source was initially localized to a sky
region of 31 deg
2
at a luminosity distance of
-
+
4
0
8
8
Mpc and with component masses cons
istent with neutron stars. The
component masses were later measured to be in the range 0.86 to 2.26
M
. An extensive observing campaign was
launched across the electromagnetic spectrum leadi
ng to the discovery of a bright optical transient
(
SSS17a, now with
the IAU identi
fi
cation of AT 2017gfo
)
in NGC 4993
(
at
~
40 Mpc
)
less than 11 hours after the merger by the One-
Meter, Two Hemisphere
(
1M2H
)
team using the 1 m Swope Telescope. The o
ptical transient was independently
detected by multiple teams within an hour
. Subsequent observations targeted th
e object and its environment. Early
ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a
redward evolution over
10 days. Following early non-detections, X-ray and radio emission were discovered at
the transient
s position
~
9
and
~
16
days, respectively, after the merger. Both the X-ray and radio emission likely
arise from a physical process that is distinct from the one that generates the UV
/
optical
/
near-infrared emission. No
ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches.
These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in
NGC4993followedbyashortgamma-rayburst
(
GRB 170817A
)
and a kilonova
/
macronova powered by the
radioactive decay of
r
-process nuclei synthesized in the ejecta.
Key words:
gravitational waves
stars: neutron
1. Introduction
Over 80 years ago Baade & Zwicky
(
1934
)
proposed the idea
of neutron stars, and soon after, Oppenheimer & Volkoff
(
1939
)
carried out the
fi
rst calculations of neutron star models. Neutron
stars entered the realm of observational astronomy in the 1960s by
providing a physical interpretation of X-ray emission from
Scorpius
X-1
(
Giacconi et al.
1962
; Shklovsky
1967
)
and of
radio pulsars
(
Gold
1968
;Hewishetal.
1968
;Gold
1969
)
.
The discovery of a radio pulsar in a double neutron star
system by Hulse & Taylor
(
1975
)
led to a renewed interest in
binary stars and compact-object astrophysics, including the
development of a scenario for the formation of double neutron
stars and the
fi
rst population studies
(
Flannery & van den
Heuvel
1975
; Massevitch et al.
1976
; Clark
1979
; Clark et al.
1979
; Dewey & Cordes
1987
; Lipunov et al.
1987
; for reviews
see Kalogera et al.
2007
; Postnov & Yungelson
2014
)
. The
Hulse-Taylor pulsar provided the
fi
rst
fi
rm evidence
(
Taylor &
Weisberg
1982
)
of the existence of gravitational waves
(
Ein-
stein
1916
,
1918
)
and sparked a renaissance of observational
tests of general relativity
(
Damour & Taylor
1991
,
1992
;
Taylor et al.
1992
; Wex
2014
)
. Merging binary neutron stars
(
BNSs
)
were quickly recognized to be promising sources of
detectable gravitational waves, making them a primary target
for ground-based interferometric detectors
(
see Abadie et al.
2010
for an overview
)
. This motivated the development of
accurate models for the two-body, general-relativistic dynamics
(
Blanchet et al.
1995
; Buonanno & Damour
1999
; Pretorius
2005
; Baker et al.
2006
; Campanelli et al.
2006
; Blanchet
2014
)
that are critical for detecting and interpreting gravita-
tional waves
(
Abbott et al.
2016c
,
2016d
,
2016e
,
2017a
,
2017c
,
2017d
)
.
The Astrophysical Journal Letters,
848:L12
(
59pp
)
, 2017 October 20
https:
//
doi.org
/
10.3847
/
2041-8213
/
aa91c9
© 2017. The American Astronomical Society. All rights reserved.
Original content from this work may be used under the terms
of the
Creative Commons Attribution 3.0 licence
. Any further
distribution of this work must maintain attribution to the author
(
s
)
and the title
of the work, journal citation and DOI.
1
In the mid-1960s, gamma-ray bursts
(
GRBs
)
were discovered
by the Vela satellites, and their cosmic origin was
fi
rst established
by Klebesadel et al.
(
1973
)
. GRBs are classi
fi
ed as
long
or
short
,
based on their duration and spectral hardness
(
Dezalay et al.
1992
;
Kouveliotou et al.
1993
)
. Uncovering the progenitors of GRBs
has been one of the key challenges in high-energy astrophysics
ever since
(
Lee & Ramirez-Ruiz
2007
)
. It has long been
suggested that short GRBs might be related to neutron star
mergers
(
Goodman
1986
; Paczynski
1986
; Eichler et al.
1989
;
Narayan et al.
1992
)
.
In 2005, the
fi
eldofshortgamma-rayburst
(
sGRB
)
studies
experienced a breakthrough
(
for reviews see Nakar
2007
;Berger
2014
)
with the identi
fi
cation of the
fi
rsthostgalaxiesofsGRBs
and multi-wavelength observation
(
from X-ray to optical and
radio
)
of their afterglows
(
Berger et al.
2005
; Fox et al.
2005
;
Gehrels et al.
2005
;Hjorthetal.
2005b
; Villasenor et al.
2005
)
.
These observations provided strong hints that sGRBs might be
associated with mergers of neutron stars with other neutron stars
or with black holes. These hints included:
(
i
)
their association with
both elliptical and star-forming galaxies
(
Barthelmy et al.
2005
;
Prochaska et al.
2006
;Bergeretal.
2007
;Ofeketal.
2007
;Troja
et al.
2008
;D
Avanzo et al.
2009
; Fong et al.
2013
)
, due to a very
wide range of delay times, as predicted theoretically
(
Bagot et al.
1998
; Fryer et al.
1999
; Belczynski et al.
2002
)
;
(
ii
)
abroad
distribution of spatial offsets from host-galaxy centers
(
Berger
2010
; Fong & Berger
2013
; Tunnicliffe et al.
2014
)
,whichwas
predicted to arise from supernova kicks
(
Narayan et al.
1992
;
Bloom et al.
1999
)
;and
(
iii
)
the absence of associated
supernovae
(
Fox et al.
2005
;Hjorthetal.
2005c
,
2005a
;
Soderberg et al.
2006
;Kocevskietal.
2010
;Bergeretal.
2013a
)
. Despite these strong hints, proof that sGRBs were
powered by neutron star mergers remained elusive, and interest
intensi
fi
ed in following up gravitational-wave detections electro-
magnetically
(
Metzger & Berger
2012
; Nissanke et al.
2013
)
.
Evidence of beaming in som
e sGRBs was initially found by
Soderberg et al.
(
2006
)
and Burrows et al.
(
2006
)
and con
fi
rmed
by subsequent sGRB discoveries
(
see the compilation and
analysis by Fong et al.
2015
and also Troja et al.
2016
)
.Neutron
star binary mergers are also expected, however, to produce
isotropic electromagnetic signals, which include
(
i
)
early optical
and infrared emission, a so-called kilonova
/
macronova
(
hereafter
kilonova; Li & Paczy
ń
ski
1998
; Kulkarni
2005
; Rosswog
2005
;
Metzger et al.
2010
; Roberts et al.
2011
;Barnes&Kasen
2013
;
Kasen et al.
2013
; Tanaka & Hotokezaka
2013
;Grossmanetal.
2014
;Barnesetal.
2016
; Tanaka
2016
; Metzger
2017
)
due to
radioactive decay of rapid neutron-capture process
(
r
-process
)
nuclei
(
Lattimer & Schramm
1974
,
1976
)
synthesized in
dynamical and accretion-disk-wind ejecta during the merger;
and
(
ii
)
delayed radio emission from the interaction of the merger
ejecta with the ambient medium
(
Nakar & Piran
2011
; Piran et al.
2013
; Hotokezaka & Piran
2015
; Hotokezaka et al.
2016
)
.The
late-time infrared excess associated with GRB 130603B was
interpreted as the signature of
r
-process nucleosynthesis
(
Berger
et al.
2013b
;Tanviretal.
2013
)
, and more candidates were
identi
fi
ed later
(
for a compilation see Jin et al.
2016
)
.
Here, we report on the global effort
958
that led to the
fi
rst joint
detection of gravitational and electromagnetic radiation from a
single source. An
100 s long gravitational-wave signal
(
GW170817
)
was followed by an sGRB
(
GRB 170817A
)
and
an optical transient
(
SSS17a
/
AT 2017gfo
)
found in the host
galaxy NGC 4993. The source was detected across the
electromagnetic spectrum
in the X-ray, ultraviolet, optical,
infrared, and radio bands
over hours, days, and weeks. These
observations support the hypothesis that GW170817 was
produced by the merger of two neutron stars in NGC4993,
followed by an sGRB and a kilonova powered by the radioactive
decay of
r
-process nuclei synthesized in the ejecta.
Figure 1.
Localization of the gravitational-wave, gamma-ray, and optical signals. The left panel shows an orthographic projection of the 90% credible region
s from
LIGO
(
190 deg
2
; light green
)
, the initial LIGO-Virgo localization
(
31 deg
2
; dark green
)
, IPN triangulation from the time delay between
Fermi
and
INTEGRAL
(
light
blue
)
, and
Fermi
-GBM
(
dark blue
)
. The inset shows the location of the apparent host galaxy NGC 4993 in the Swope optical discovery image at 10.9 hr after the
merger
(
top right
)
and the DLT40 pre-discovery image from 20.5 days prior to merger
(
bottom right
)
. The reticle marks the position of the transient in both images.
958
A follow-up program established during initial LIGO-Virgo observations
(
Abadie et al.
2012
)
was greatly expanded in preparation for Advanced LIGO-
Virgo observations. Partners have followed up binary black hole detections,
starting with GW150914
(
Abbott et al.
2016a
)
, but have discovered no
fi
rm
electromagnetic counterparts to those events.
2
The Astrophysical Journal Letters,
848:L12
(
59pp
)
, 2017 October 20
Abbott et al.