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.