On the Progenitor of Binary Neutron Star Merger GW170817
LIGO Scienti
fi
c Collaboration and Virgo Collaboration
(
See the end matter for the full list of authors.
)
Received 2017 October 12; revised 2017 October 16; accepted 2017 October 16; published 2017 December 1
Abstract
On 2017 August 17 the merger of two compact objects w
ith masses consistent with two neutron stars was
discovered through gravitational-wave
(
GW170817
)
, gamma-ray
(
GRB
170817A
)
,andoptical
(
SSS17a
/
AT
2017gfo
)
observations. The optical source was associated with the early-type galaxy NGC 4993 at a distance of
just
∼
40 Mpc, consistent with the gravitational-wave measurement, and the merger was localized to be at a
projected distance of
∼
2 kpc away from the galaxy
’
s center. We use this minimal set of facts and the mass
posteriors of the two neutron stars to derive the
fi
rst constraints on the progenitor of GW170817 at the time of
the second supernova
(
SN
)
. We generate simulated progenitor populations and follow the three-dimensional
kinematic evolution from binary neutron star
(
BNS
)
birth to the merger time, accounting for pre-SN galactic
motion, for considerably different input distributio
ns of the progenitor mass, pre-SN semimajor axis, and
SN-kick velocity. Though not considerably tight, we
fi
nd these constraints to be comparable to those for Galactic
BNS progenitors. The derived constraints are very strongly in
fl
uenced by the requirement of keeping the binary
bound after the second SN and having the merger occur relatively close to the center of the galaxy. These
constraints are insensitive to the galaxy
’
s star formation history, provided t
he stellar populations are older
than 1 Gyr.
Key words:
binaries: general
–
gravitational waves
–
stars: kinematics and dynamics
–
stars: neutron
1. Introduction
The era of observational gravitational-wave
(
GW
)
astronomy
was
fi
rmly marked by the detection of the
fi
rst binary black
hole coalescence GW150914
(
Abbott et al.
2016
)
by the
Advanced LIGO detectors
(
Aasi et al.
2015
)
. Discovery of a
GW source accompanied by coincident electromagnetic
(
EM
)
emission, however, remained elusive until now.
On 2017 August 17 the Advanced LIGO
(
Aasi et al.
2015
)
and Advanced Virgo
(
Acernese et al.
2015
)
interferometer
network recorded a transient GW signal consistent with the
coalescence of a binary neutron star
(
BNS
)
GW170817
(
Abbott
et al.
2017b
)
. Independently, a gamma-ray signal, classi
fi
ed as
a short gamma-ray burst
(
sGRB
)
, GRB
170817A, coincident in
time and sky location with GW170817 was detected by the
Fermi
-GBM instrument
(
Abbott et al.
2017a
,
2017b
)
. The
three-detector GW data analysis led to the smallest sky-
localization area ever achieved for a GW source:
;
31 deg
2
when initially shared with the astronomy LIGO
–
Virgo partners
(
LIGO Scienti
fi
c Collaboration & Virgo Collaboration
2017
)
and later improved to
;
28 deg
2
with a fully coherent data
analysis
(
Abbott et al.
2017b
)
.
Aided by the tight localization constraints of the three-
detector network and the proximity of the GW source, multiple
independent surveys across the EM spectrum were launched in
search of a counterpart beyond the sGRB
(
Abbott et al.
2017c
)
.
Such a counterpart, SSS17a
(
later IAU-designated
AT 2017gfo
)
, was
fi
rst discovered in the optical less than 11
hours after merger, associated with the galaxy NGC 4993
(
Coulter et al.
2017a
,
2017b
)
, a nearby early-type E
/
S0 galaxy
(
Lauberts
1982
)
. Five other teams made independent detections
of the same optical transient and host galaxy all within about
one hour and reported their results within about
fi
ve hours of
one another
(
Allam et al.
2017
; Arcavi et al.
2017a
,
2017b
;
Lipunov
2017b
; Tanvir & Levan
2017
; Yang et al.
2017
;
Soares-Santos et al.
2017
; Lipunov et al.
2017a
)
. The same
source was followed up and consistently localized at other
wavelengths
(
e.g., Corsi et al.
2017
; Deller et al.
2017a
,
2017b
,
2017c
; Goldstein et al.
2017
; Haggard et al.
2017a
,
2017b
; Mooley et al.
2017
; Savchenko et al.
2017
; Alexander
et al.
2017
; Haggard et al.
2017c
; Goldstein et al.
2017
;
Savchenko et al.
2017
)
. The source was reported to be offset
from the center of the galaxy by a projected distance of about
10
′′
(
e.g., Coulter et al.
2017a
,
2017b
; Haggard et al.
2017a
,
2017b
; Kasliwal et al.
2017
; Yang et al.
2017
; Yu et al.
2017
)
.
NGC 4993 has a Tully
–
Fisher distance of
∼
40 Mpc
(
Freedman
et al.
2001
; NASA
/
IPAC Extragalactic Database
164
)
, which is
consistent with the luminosity distance measurement from
gravitational waves
(
4
0
14
8
-
+
Mpc
)
. Using the Tully
–
Fisher
distance, the
∼
10
′′
offset corresponds to a physical offset of
;
2.0 kpc. This value is consistent with offset measurements of
sGRBs in other galaxies, though below the median value
of
∼
3
–
4 kpc
(
Fong et al.
2010
; Fong & Berger
2013
; Berger
2014
)
.
BNS systems were
fi
rst revealed with the discovery of PSR
B1913
+
16, the
fi
rst binary radio pulsar ever detected
(
Hulse &
Taylor
1975
)
. This immediately triggered new ideas for how
such close pairs of neutron stars can form in nature
(
De Loore
et al.
1975
; Flannery & van den Heuvel
1975
; Massevitch
et al.
1976
; Clark et al.
1979
)
, based on models for the
formation of high-mass X-ray binaries
(
van den Heuvel &
Heise
1972
; Tutukov & Yungelson
1973
)
and Wolf
–
Rayet
X-ray binaries, for which strong orbital shrinkage is needed
The Astrophysical Journal Letters,
850:L40
(
18pp
)
, 2017 December 1
https:
//
doi.org
/
10.3847
/
2041-8213
/
aa93fc
© 2017. The American Astronomical Society.
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.
164
The NASA
/
IPAC Extragalactic Database
(
NED
)
is operated by the Jet
Propulsion Laboratory, California Institute of Technology, under contract with
the National Aeronautics and Space Administration.
1
(
van den Heuvel & De Loore
1973
)
. With years of pulsar-
timing observations PSR B1913
+
16 provided the
fi
rst
fi
rm
evidence that GWs existed
(
Einstein
1916
,
1918
)
and were
emitted by close binary compact objects
(
Taylor & Weisberg
1982
)
. This discovery greatly motivated the efforts to directly
detect GWs with laser-interferometric detectors and made BNS
coalescence events key targets in GW searches
(
see Abadie
et al.
2010
for an overview
)
.
The formation of close binaries with two neutron stars that
will merge within a Hubble time is now understood to require
complex evolutionary sequences of massive binaries that
involve stable and unstable mass-transfer phases and two
core-collapse supernova
(
SN
)
explosions through which the
binary system survives
(
for reviews, see, e.g., Kalogera
et al.
2007
; Postnov & Yungelson
2014
; Tauris et al.
2017
)
.
In particular, the SN explosions that lead to the formation of
neutron stars are expected to develop asymmetries during the
collapse, either due to neutrino emission or an anisotropic
explosion
(
e.g., Kusenko and Segrè
1996
; Janka et al.
2007
;
Janka
2013
)
. This anisotropy imparts linear momentum on the
stellar remnant, known as an
SN kick
or
natal kick
.
Strong evidence for this process comes from observations of
Galactic pulsar proper motions, which indicate some neutron
stars are moving substantially faster than the inferred speed of
their progenitors and must receive a large SN kick of
∼
400
–
500 km s
−
1
at birth
(
Lyne & Lorimer
1994
; Kaspi
et al.
1996
; Arzoumanian et al.
2002
; Chatterjee et al.
2005
;
Hobbs et al.
2005
; Verbunt et al.
2017
)
. However, compre-
hensive studies of the known BNS systems in the Milky Way
have shown that some neutron stars, particularly those in binary
systems, might receive smaller kicks than their isolated
counterparts
(
Podsiadlowski et al.
2004
; van den Heuvel
2007
)
.
About a decade after the Hulse-Taylor discovery, mergers of
two neutron stars were proposed as a potential source of GRBs
(
Goodman
1986
; Paczynski
1986
; Eichler et al.
1989
; Narayan
et al.
1992
)
, especially those of short duration
(
Kouveliotou
et al.
1993
)
. Since the discovery of host galaxies for short
GRBs in 2005
(
Berger et al.
2005
; Fox et al.
2005
; Gehrels
et al.
2005
; Hjorth et al.
2005
; Villasenor et al.
2005
)
,
substantial evidence had accumulated in support of this
hypothesis. For example, many sGRBs have a signi
fi
cant
offset relative to the center of their host galaxy
(
see, e.g., Troja
et al.
2008
; Fong et al.
2010
; Church et al.
2011
; Behroozi
et al.
2014
)
: this suggests that the progenitors of these sources
have migrated from their birth sites to their eventual explosion
sites. Speci
fi
cally, the offset distribution, together with the
locations of sGRBs relative to the stellar light of their hosts, are
indicative of systemic kicks
(
see, e.g., Berger
2014
)
. To date,
GW170817 is the strongest observational evidence for an
extragalactic BNS system and the
fi
rst GW signal con
fi
dently
coincident with an sGRB
(
Abbott et al.
2017a
)
.
In this study, we focus on constraining the immediate
progenitor of GW170817 right before the second SN
(
SN2
)
that formed the BNS system. We use
(
i
)
SN-kick dynamics and
kinematic modeling within the host galaxy from SN2 to
merger, and
(
ii
)
the GW-measured neutron star masses, the
identi
fi
cation of the source host galaxy, and its projected
distance offset from the galactic center based on the early
optical detections
(
Section
2
)
. We emphasize that we develop
this analysis using the very limited knowledge about the galaxy
properties available in the literature prior to the announcement
of the GW170817 discovery, as at this time we do not have
access to the new analysis of galaxy characteristics and star
formation history. We present our main results for constraints
on the SN kicks, progenitor masses, pre-SN semimajor axes,
and galactic radii of SN2 in Section
3
, and we explore the
sensitivity of our results to all our input assumptions. We
fi
nd
that the constraints are
(
i
)
primarily dictated by the requirement
that the progenitor remains bound after SN2 and
(
ii
)
insensitive
to the star formation history of the host galaxy, provided stellar
ages are longer than
;
1 Gyr. In Section
4
, we use the GW BNS
merger rate to estimate a BNS formation ef
fi
ciency for
NGC 4993, comment on the role of NGC 4993
ʼ
s globular
cluster content in BNS formation, and conclude our analysis.
2. Analysis Methodology
To investigate the constraints that can be placed on the
progenitor of GW170817, we develop a modeling approach
comprised of the following elements:
(
i
)
assume a gravita-
tional-potential model for the host galaxy, described by a stellar
and dark-matter
(
DM
)
density pro
fi
le;
(
ii
)
place binary systems
in the galaxy according to the stellar pro
fi
le, and give them a
pre-SN orbit in the galaxy;
(
iii
)
sample the pre-SN binary
properties
(
pre-SN semimajor axis, progenitor mass of the
second neutron star, location of SN2 within the galaxy
)
and the
SN-kick velocity imparted on the binary following from SN2,
using multiple assumptions about the underlying distribution of
these parameters;
(
iv
)
sample the post-SN masses from GW
parameter-estimation posterior samples of GW170817;
(
v
)
determine if the binary remains bound after SN2 and calculate
the post-SN orbital properties, systemic velocity, and inspiral
time, assuming two-body orbital mechanics and an instanta-
neous SN explosion;
(
vi
)
evolve the system forward in time,
following the trajectory of the binary through the static galactic
potential until it merges;
(
vii
)
select the systems with a
projected offset at merger consistent with the GW170817
measurements, and label them as
GW170817-like
;
(
viii
)
impose
constraints based on the age at which the binary formed
(
thus,
its delay time between SN2 and merger
)
and the true
(
unprojected
)
distance from the galactic center, and investigate
how such constraints affect our inference on progenitor
properties;
(
ix
)
repeat the above steps for different input
assumptions of the progenitor properties to assess the
robustness of our results.
For each set of input assumptions, we evolve 50 million
binaries according to the above procedures, which is suf
fi
cient
to properly sample the distributions of GW170817-like
systems. This section provides the model details that are
adopted in our analysis.
2.1. Source Properties
The orbital-dynamics and kinematic analyses presented here
require both GW and EM information. The post-SN orbital
characteristics of a binary, such as the semimajor axis,
eccentricity, and systemic velocity, depend on the component
masses of the binary, which are measured in the GW inspiral.
The projected offset of the binary relative to NGC 4993
ʼ
s
center, measured by EM observations, allows us to select
GW170817-like systems in the model populations.
The best-measured property of a GW inspiral is a
combination of the component masses known as the
chirp
2
The Astrophysical Journal Letters,
850:L40
(
18pp
)
, 2017 December 1
Abbott et al.