Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger:
GW170817 and GRB 170817A
LIGO Scienti
fi
c Collaboration and Virgo Collaboration,
Fermi
Gamma-ray Burst Monitor, and INTEGRAL
(
See the end matter for the full list of authors.
)
Received 2017 October 6; revised 2017 October 9; accepted 2017 October 9; published 2017 October 16
Abstract
On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo
detectors, and the gamma-ray burst
(
GRB
)
GRB
170817A was observed independently by the
Fermi
Gamma-ray
Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the
International Gamma-Ray Astrophysics
Laboratory
. The probability of the near-simultaneous temporal and spatial observation of GRB
170817A and
GW170817 occurring by chance is
5
.0 10
8
́
-
. We therefore con
fi
rm binary neutron star mergers as a progenitor of
short GRBs. The association of GW170817 and GRB
170817A provides new insight into fundamental physics and
the origin of short GRBs. We use the observed time delay of
1.74 0.05 s
+
(
)
between GRB
170817A and
GW170817 to:
(
i
)
constrain the difference between the speed of gravity and the speed of light to be between
310
1
5
-
́
-
and
710
16
+ ́
-
times the speed of light,
(
ii
)
place new bounds on the violation of Lorentz invariance,
(
iii
)
present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and
electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region
emitting the gamma-rays. GRB
170817A is the closest short GRB with a known distance, but is between 2 and 6
orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors,
and subthreshold searches in existing detectors, will be essential to detect similar short bursts at greater distances.
Finally, we predict a joint detection rate for the
Fermi
Gamma-ray Burst Monitor and the Advanced LIGO and Virgo
detectors of 0.1
–
1.4 per year during the 2018
–
2019 observing run and 0.3
–
1.7 per year at design sensitivity.
Key words:
binaries: close
–
gamma-ray burst: general
–
gravitational waves
1. Introduction and Background
GW170817 and GRB
170817A mark the discovery of a
binary neutron star
(
BNS
)
merger detected both as a gravitational
wave
(
GW; LIGO Scienti
fi
c Collaboration & Virgo Collabora-
tion
2017a
)
and a short-duration gamma-ray burst
(
SGRB;
Goldstein et al.
2017
; Savchenko et al.
2017b
)
. Detecting GW
radiation from the coalescence of BNS and neutron star
(
NS
)
–
black hole
(
BH
)
binary systems has been a major goal
(
Abbott
et al.
2017a
)
of the LIGO
(
Aasi et al.
2015
)
and Virgo
(
Acernese
et al.
2015
)
experiments. This was at least partly motivated by
their promise of being the most likely sources of simultaneously
detectable GW and electromagnetic
(
EM
)
radiation from the
same source. This is important as joint detections enable a wealth
of science unavailable from either messenger alone
(
Abbott et al.
2017f
)
. BNS mergers are predicted to yield signatures across the
EM spectrum
(
Metzger & Berger
2012
; Piran et al.
2013
)
,
including SGRBs
(
Blinnikov et al.
1984
; Paczynski
1986
;Eichler
et al.
1989
; Paczynski
1991
; Narayan et al.
1992
)
, which produce
prompt emission in gamma-rays and longer-lived afterglows.
A major astrophysical implication of a joint detection of an
SGRB and of GWs from a BNS merger is the con
fi
rmation that
these binaries are indeed the pro
genitors of at least some SGRBs.
GRBs are classi
fi
ed as short or long depending on the duration of
their prompt gamma-ray emission
. This cut is based on spectral
differences in gamma-rays and t
he bimodality of the observed
distribution of these durations
(
Dezalay et al.
1992
; Kouveliotou
et al.
1993
)
. This empirical division was accompanied by
hypotheses that the two classes have different progenitors. Long
GRBs have been
fi
rmly connected to the collapse of massive stars
through the detection of associated Type Ibc core-collapse
supernovae
(
see Galama et al.
1998
,aswellasHjorth&Bloom
2012
and references therein
)
. Prior to the results reported here,
support for the connection between SGRBs and mergers of BNSs
(
or NS
–
BH binaries
)
came only from indirect observational
evidence
(
Nakar
2007
;Bergeretal.
2013
;Tanviretal.
2013
;
Berger
2014
)
and numerical simulations
(
e.g., Aloy et al.
2005
;
Rezzolla et al.
2011
;Kiuchietal.
2015
; Baiotti & Rezzolla
2017
;Kawamuraetal.
2016
;Ruizetal.
2016
)
. The unambiguous
joint detection of GW and EM radiation from the same event
con
fi
rms that BNS mergers are progenitors of
(
at least some
)
SGRBs.
In Section
2
we describe the independent observations of
GW170817 by the LIGO
–
Virgo and of GRB
170817A by the
Fermi
Gamma-ray Burst Monitor
(
GBM
)
and by the SPectro-
meter on board
INTEGRAL
Anti-Coincidence Shield
(
SPI-
ACS
)
. In Section
3
we establish the
fi
rm association between
GW170817 and GRB
170817A. In Section
4
we explore the
constraints on fundamental physics that can be obtained from
the time separation between the GW and EM signals. In
Section
5
we explore the implications of the joint detection of
GW170817 and GRB
170817A on the SGRB engine and the
NS equation of state
(
EOS
)
. In Section
6
we explore the
implications of the comparative dimness of GRB
170817A
relative to the known SGRB population and revise the
The Astrophysical Journal Letters,
848:L13
(
27pp
)
, 2017 October 20
https:
//
doi.org
/
10.3847
/
2041-8213
/
aa920c
© 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
expectation rates for joint BNS
–
SGRB detections in the light of
this discovery.
2. Observational Results
The observations of GW170817 and of GRB
170817A are
described in detail in Abbott et al.
(
2017e
)
, Goldstein et al.
(
2017
)
, and Savchenko et al.
(
2017b
)
. Here we summarize the
observations relevant to the results presented in this Letter and
report the results of two fully coherent searches for GWs from
the sky location of GRB
170817A. For convenience, all
measurements of time have been converted to their geocentric
equivalent.
2.1. LIGO
–
Virgo Observation of GW170817
GW170817 is a GW signal from the inspiral of two low-mass
compact objects and is the
fi
rst GW observation consistent with
a BNS coalescence
(
Abbott et al.
2017e
,
2017f
)
. GW170817
was
fi
rst observed by a low-latency search
(
Cannon et al.
2012
;
Messick et al.
2017
)
on 2017 August 17 at 12:41:04 UTC as a
single-detector trigger in the LIGO-Hanford detector
(
Abbott
et al.
2017e
; LIGO Scienti
fi
c Collaboration & Virgo Collabora-
tion
2017a
)
. The temporal proximity of GRB
170817A was
immediately identi
fi
ed by automatic comparison of the
Fermi
-
GBM Gamma-ray Coordinates Network notice to the GW
trigger
(
Urban
2016
)
. Rapid of
fl
ine re-analysis
(
Usman et al.
2016
;Nitzetal.
2017b
)
of data from the LIGO
/
Virgo network
con
fi
rmed the presence of a signi
fi
cant coincident signal in the
LIGO GW detectors with a combined signal-to-noise ratio
(
S
/
N
)
of 32.4. The combination of observations from the LIGO and
Virgo detectors allowed a precise sky position localization to an
area of
2
8deg
2
at 90% probability shown in green in Figure
1
(
Abbott et al.
2017e
; LIGO Scienti
fi
c Collaboration & Virgo
Collaboration
2017b
)
. A time-frequency representation of the
LIGO data containing GW170817 is shown in the bottom panel
of Figure
2
. The GPS time of the merger of GW170817 is
T
1187008882.430
0
GW
0.002
0.002
=
-
+
s
(
Abbott et al.
2017e
)
.Atthe
observed signal strength, the false alarm rate of the all-sky search
for compact-object mergers is less than 1 in 80,000 years
(
Abbott et al.
2017e
)
. The of
fl
ine searches target binaries with
(
detector frame
)
total mass 2
–
M
5
00
. Signals are required to be
coincident in time and mass in the LIGO detectors, but Virgo
data are not used in the signi
fi
cance estimates of the all-sky
of
fl
ine search
(
Abbott et al.
2017e
)
.
We present the results of two of
fl
ine targeted searches that
coherently combine the data from the LIGO and Virgo
detectors and restrict the signal offset time and sky-location
using information from the EM observation of GRB
170817A.
The onset of gamma-ray emission from a BNS merger
progenitor is predicted to be within a few seconds after the
merger, given that the central engine is expected to form within
a few seconds and that the jet propagation delays are at most of
the order of the SGRB duration
(
see, e.g., Finn et al.
1999
;
Abadie et al.
2012
and references therein
)
. The gravitational
and EM waves are expected to travel at the same speed.
The
fi
rst targeted search
(
Harry & Fairhurst
2011
; Williamson
et al.
2014
; Abbott et al.
2017b
;Nitzetal.
2017a
)
assumes that
the source is a BNS or NS
–
BH binary merger and is located at
the sky-position observed for the optical counterpart to
GW170817 and GRB
170817A
(
Coulter et al.
2017a
,
2017b
;
Abbott et al.
2017f
)
and that there is a
1, 5
-+
[
]
s time delay in
the arrival of gamma-rays
(
determined by the GBM trigger time
)
compared to the binary merger time
(
Abbott et al.
2017b
)
.Atthe
detection statistic value assigned to GW170817, this search has a
p
-value of
9.4 10
4.2
6
s
<
́>
-
(
)
,withthissigni
fi
cance estimate
limited by computational resources used to estimate the noise
background. The second coherent search does not assume any
particular GW morphology or GRB model
(
Sutton et al.
2010
;
Was et al.
2012
;Abbottetal.
2017b
)
and uses the GBM
localization of GRB
170817A to constrain the sky location of
the source. This search allows for a
60, 600
-+
[]
s coincidence
between the gamma-rays and the GWs in order to include
potentially larger delays in collapsar models of long GRBs. At
the detection-statistic value observed for GW170817, this search
has a
p
-value of
1.3104.2
5
s
́
-
()
.
Figure 1.
Final localizations. The 90% contour for the
fi
nal sky-localization map from LIGO
–
Virgo is shown in green
(
LIGO Scienti
fi
c Collaboration & Virgo
Collaboration
2017a
,
2017b
,
2017c
)
. The 90% GBM targeted search localization is overlaid in purple
(
Goldstein et al.
2017
)
. The 90% annulus determined with
Fermi
and
INTEGRAL
timing information is shaded in gray
(
Svinkin et al.
2017
)
. The zoomed inset also shows the position of the optical transient marked as a yellow star
(
Abbott et al.
2017f
; Coulter et al.
2017a
,
2017b
)
. The axes are R.A. and decl. in the Equatorial coordinate system.
2
The Astrophysical Journal Letters,
848:L13
(
27pp
)
, 2017 October 20
Abbott et al.