of 11
GW170608: Observation of a 19 Solar-mass Binary Black Hole Coalescence
LIGO Scienti
fi
c Collaboration and Virgo Collaboration
(
See the end matter for the full list of authors.
)
Received 2017 November 14; revised 2017 November 30; accepted 2017 December 2; published 2017 December 18
Abstract
On 2017 June 8 at 02:01:16.49 UTC, a gravitational-wave
(
GW
)
signal from the merger of two stellar-mass black
holes was observed by the two Advanced Laser Interferometer Gravitational-Wave Observatory detectors with a
network signal-to-noise ratio of
13. This system is the lightest black hole binary so far observed, with component
masses of
-
+
M
12
2
7
and
-
+
M
7
2
2
(
90% credible intervals
)
. These lie in the range of measured black hole masses in
low-mass X-ray binaries, thus allowing us to compare black holes detected through GWs with electromagnetic
observations. The source
s luminosity distance is
-
+
3
40 Mpc
140
140
, corresponding to redshift
-
+
0
.07
0.03
0.03
. We verify that
the signal waveform is consistent with the predictions of general relativity.
Key words:
binaries: general
gravitational waves
stars: black holes
1. Introduction
The
fi
rst detections of binary black hole mergers were made by
the Advanced Laser Interferometer Gravitational-Wave Observa-
tory
(
LIGO;Aasietal.
2015
; Abbott et al.
2016a
)
during its
fi
rst
observing run
(
O1
)
in 2015
(
Abbott et al.
2016b
,
2016c
,
2016d
)
.
Following a commissioning break, LIGO undertook a second
observing run
(
O2
)
from 2016 November 30 to 2017 August 25,
with the Advanced Virgo detector
(
Acernese et al.
2015
)
joining
the run on 2017 August 1. Two binary black hole mergers
(
Abbott
et al.
2017a
,
2017b
)
and one binary neutron star merger
(
Abbott
et al.
2017c
)
have been reported in O2 data. Here, we describe
GW170608, a binary black hole merger with likely the lowest
mass of any so far observed by LIGO.
GW170608 was
fi
rst identi
fi
ed in data from the LIGO
Livingston Observatory
(
LLO
)
, which was in normal observing
mode. The LIGO Hanford Observatory
(
LHO
)
was operating
stably with a sensitivity typical for O2, but its data were not
analyzed automatically as the de
tector was undergoing a routine
angular control procedure
(
Section
2
and the
Appendix
)
. Matched-
fi
lter analysis of a segment of data around this time revealed a
candidate with source paramete
rs consistent between both LIGO
detectors; further of
fl
ine analyses of a longer period of data
con
fi
rmed the presence of a gravitational-wave
(
GW
)
signal from
the coalescence of a binary black hole system, with high statistical
signi
fi
cance
(
Section
3
)
.
The source
s parameters were estimated via coherent Bayesian
analysis
(
Veitch et al.
2015
;Abbottetal.
2016e
)
. A degeneracy
between the component masses
m
1
,
m
2
prevents precise
determination of their indivi
dual values, but the chirp mass
=+
-
()(
)
mm m m
12
35
12
15
is well measured and is the
smallest so far observed for a mer
ging black hole binary system,
with the total mass
=+
Mm m
12
also likely the lowest so far
observed
(
Section
4
)
. Individual black hole spins are poorly
constrained; however, we
fi
nd a slight preference for a small
positive net component of spin in the
direction of the binary orbital
angular momentum.
Similarly to GW151226
(
Abbott et al.
2016c
)
, this system
s
black hole component masses are comparable to those of black
holes found in X-ray binaries
(
Section
5
)
and below those seen
in other LIGO
Virgo black hole binaries.
We also test the consistency of the observed GW signal with
the predictions of general relativity
(
GR
)
;we
fi
nd no deviations
from those predictions.
2. Detector Operation
The LIGO detectors measure GW strain using dual-recycled
Michelson interferometers with Fabry
Perot arm cavities
(
Aasi
et al.
2015
;Abbottetal.
2016a
)
. During O2, the horizon distance
for systems with component masses similar to GW170608
the
distance at which a binary merger optimally oriented with respect
to a detector has an expected signal-to-noise ratio
(
S
/
N
)
of 8
(
Allen et al.
2012
;Chenetal.
2017
)
peaked at
1 Gpc for LLO
and at
750 Mpc for LHO.
At the time of GW170608, LLO was observing with a
sensitivity close to its peak. LHO was operating in a stable
con
fi
guration with a sensitivity of
650 Mpc; a routine procedure
to minimize angular noise coupling to the strain measurement was
being performed
(
Kasprzack & Yu
2016
)
. Although such times
are in general not included in searches, it was determined that
LHO strain data were unaffected by the procedure at frequencies
above 30 Hz, and may thus be used to identify a GW source and
measure its properties. More details on LHO data are given in the
Appendix
.
Similar procedures to those used in verifying previous GW
detections
(
Abbott et al.
2017b
)
were followed and indicate that
no disturbance registered by LIGO instrumental or environ-
mental sensors
(
Ef
fl
er et al.
2015
)
was strong enough to have
caused the GW170608 signal.
Calibration of the LIGO detec
tors is performed by inducing
test-mass motion using photon pressure from modulated auxiliary
lasers
(
Karkietal.
2016
;Abbottetal.
2017d
; Cahillane
et al.
2017
)
. The maximum 1
σ
calibration uncertainties for strain
data used in this analysis are 5% in amplitude and 3
°
in phase
over the frequency range 20
1024 Hz.
The Advanced Virgo detector was, at the time of the event, in
observation mode with a horizon distance for signals comparable
to GW170608 of 60
70 Mpc. However, this was during an early
commissioning phase with still limited sensitivity; therefore,
Virgo data are not included in the analyses presented here.
The Astrophysical Journal Letters,
851:L35
(
11pp
)
, 2017 December 20
https:
//
doi.org
/
10.3847
/
2041-8213
/
aa9f0c
© 2017. The American Astronomical Society. All rights reserved.
1
3. Search for Binary Merger Signals
3.1. Low-latency Identi
fi
cation of a Candidate Event
GW170608 was
fi
rst identi
fi
ed as a loud
(
S
/
N
9
)
event in
LLO data, via visual inspection o
f single-detector events from a
low-latency compact binary matched-
fi
lter
(
template
)
analysi-
s
(
Usman et al.
2016
;Nitzetal.
2017a
,
2017b
)
. Such events are
displayed automatically to diagnose changes in detector operation
and in populations of non-Gaussian transient noise artifacts
(
glitches; Abbott et al.
2016f
)
. Low-latency templated searches
(
Cannon et al.
2015
; Adams et al.
2016
; Messick et al.
2017
; Nitz
et al.
2017b
)
did not detect the event with high signi
fi
cance
because LHO data were not analy
zed automatically. An initial
investigation of the LLO event did n
ot indicate that it was likely to
be caused by an instrumental or environmental artifact
(
Abbott
et al.
2016f
; Zevin et al.
2017b
)
. The morphology of the LLO
event is consistent with a compact binary merger signal, as shown
in Figure
1
(
lower panel
)
, but a noise origin could not be ruled out
using LLO data alone.
Consequently, LHO data were investigated and were deter-
mined to be stable at frequencies above 30 Hz
(
see the
Appendix
)
.
A segment of LHO data around the event time was then searched
with a
fi
lter starting frequency of 30 Hz, using templates
approximating the waveforms from compact binary systems with
component spins aligned with the orbital angular momentum
(
Pürrer
2016
;Bohéetal.
2017
)
. The fraction of S
/
N expected to
be lost due to imposing the 30 Hz cutoff, as compared to the lower
starting frequencies typically used in O2 data
(
Dal Canton &
Harry
2017
)
,is
1% or less. An event was found having
consistent template binary masses and spins, times of arrival, and
S
/
Ns in LHO and LLO. Based on this two-detector coincident
event an alert was issued to electromagnetic observing partners
13.5 hr after the event time, with a sky localization
(
Singer &
Price
2016
)
covering
860 deg
2
(
90% credible region
)
.GRB
Coordinates Network Circulars related to this event are archived
at
https:
//
gcn.gsfc.nasa.gov
/
other
/
G288732.gcn3
.
3.2. Of
fl
ine Search
To establish the signi
fi
cance of this coincident event, a period
between 2017 June 7 and 9 was identi
fi
ed for analysis during
whichbothLIGOinterferomete
rs were operating in the same
con
fi
guration as at the event time. Times at which commissioning
activities at LHO produced severe
or broadband disturbances in
the strain data were excluded from the analysis. Standard of
fl
ine
data quality vetoes for known environmental or instrumental
artifacts were also applied, re
sulting overall in 1.2 days of
coincident LHO
LLO data searched.
Two matched-
fi
lter pipelines identi
fi
ed GW170608, with a
network S
/
Nof
13. A candidate event is assigned a ranking
statistic value, in each pipeline, that represents its relative likelihood
of originating from a GW signal
versus
from noise. One pipeline
estimates the noise background using time-shifted data
(
Usman
et al.
2016
)
and limits the rate of occurrence of noise events ranked
higher than GW170608 to less than 1 in 3000 years. This limit
arises from the maximum background analysis time available from
time shifts separated by 0.1 s and is expected to be conservative as
indicated by previous studies
(
Was et al.
2010
; Abbott et al.
2016g
;
Capano et al.
2017
)
. The other pipeline uses different methods
for ranking candidate events and for estimating the background
(
Cannon et al.
2015
; Messick et al.
2017
)
and assigns the event a
false-alarm rate of 1 in 160,000 years.
A search for transient GW signals coherent between LHO and
LLO with frequency increasing o
ver time, without using wave-
form templates
(
Klimenko et al.
2016
)
,alsoidenti
fi
ed GW170608
with a false-alarm rate of 1 in
~
30
years; the lower signi
fi
cance is
expected as this analysis is typi
cally less sensitive to lower-mass
compact binary signals than matched-
fi
lter searches.
4. Source Properties
4.1. Binary Parameters
The parameters of the GW source are inferred from a coherent
Bayesian analysis
(
Veitch et al.
2015
;Abbottetal.
2016e
)
using
noise-subtracted data from the two LIGO observatories. Several
continuously present sources of noise in the detectors
GW strain
channel are independently measured, and are then subtracted via
Wiener
fi
ltering
(
Abbott et al.
2017b
and references therein
)
.This
step increases the expected S
/
N of compact binary signals in
LHO data typically by 25%
(
Driggers et al.
2017
)
. The likelihood
integration is performed starting at 30 Hz in LHO and 20 Hz in
LLO, includes marginalization ove
r strain calibration uncertainties
(
Farr et al.
2015
)
, and uses the noise power spectral densities
(
Littenberg & Cornish
2015
)
at the time of the event.
Two different GW signal models calibrated to numerical
relativity simulations of general r
elativistic binary black hole
mergers
(
Mroué et al.
2013
;Chuetal.
2016
;Husaetal.
2016
)
,
building on the breakthrough reported in Pretorius
(
2005
)
, Baker
et al.
(
2006
)
, and Campanelli et al.
(
2006
)
,areused.One
waveform family models the inspi
ral-merger-ringdown signal of
precessing binary black holes
(
Hannam et al.
2014
)
,which
includes spin-induced orbital pre
cession through a transformation
of the aligned-spin waveform model of Husa et al.
(
2016
)
and
Khan et al.
(
2016
)
; we refer to this model as the
effective
precession
model. The other waveform model describes binaries
with spin angular momenta aligned with the orbital angular
momentum
(
Pürrer
2016
;Bohéetal.
2017
)
, henceforth referred to
Figure 1.
Power maps of LIGO strain data at the time of GW170608 in a constant
Q sine-Gaussian basis
(
Chatterjietal.
2004
)
. The characteristic upward-chirping
morphology of a binary inspiral driven by GW emission is visible in both
detectors, with a higher si
gnal amplitude in LHO. This
fi
gure, and all others in this
Letter, were produced from noise-subtracted data
(
Section
4
)
.
2
The Astrophysical Journal Letters,
851:L35
(
11pp
)
, 2017 December 20
Abbott et al.
as
non-precessing
. For their common parameters, both waveform
models yield consistent parameter ranges.
A selection of inferred source parameters for GW170608 is
giveninTable
1
; unless otherwise noted, we report median values
and symmetric 90% credible intervals. The quoted parameter
uncertainties include statistical and systematic errors from
averaging posterior probability
samples over the two waveform
models. As in Abbott et al.
(
2017a
)
, our estimates of the mass and
spin of the
fi
nal black hole, the total energy radiated in GWs, as
well as the peak luminosity are computed from
fi
ts to numerical
relativity simulations
(
Hofmann et al.
2016
;Healy&Lousto
2017
;
Jiménez-Forteza et al.
2017
;Keiteletal.
2017
)
.
The posterior probability distributions for the source-frame
mass parameters of GW170608 are shown in Figure
2
, together
with those for GW151226
(
Abbott et al.
2016c
)
. The initial
binary of GW170608 had source-frame component masses
=
-
+
mM
12
12
7
and
=
-
+
mM
7
22
2
. As with previously reported
binary merger GW signals, GW170608
ʼ
s data are consistent
with an equal-mass binary; the mass ratio is loosely constrained
to
>
mm
0.3
3
21
. Since neutron stars are expected to have
masses below
~
M
3
(
Lattimer & Prakash
2016
)
, both objects
are most likely black holes.
Notably, we
fi
nd this binary black hole system to be the least
massive yet observed through GWs. The next lightest,
GW151226
(
Abbott et al.
2016c
)
, has a chirp mass
=
-
+
8.9
0.3
0.3
and a total mass
=
-
+
M
21.8
1.7
5.9
, compared to
values of
=
-
+
M
7.9
0.2
0.2
and
=
-
+
MM
19
1
5
for GW170608.
The probability that GW170608
ʼ
s total mass is smaller than
GW151226
ʼ
s is 0.89.
While the chirp mass is tightly co
nstrained, spins have a more
subtle effect on the GW signal. The effective inspiral spin
c
eff
,a
mass-weighted combinatio
n of the spin components
(
anti-
)
aligned
with the orbital angular momentum
(
Racine
2008
; Ajith
et al.
2011
)
, predominantly affects the inspiral rate of the binary
but also in
fl
uences the merger. We infer that
c
=
-
+
0.07
eff
0.09
0.23
,
disfavoring large, anti-aligned spins on both black holes.
An independent parameter estimation method comparing
LIGO strain data to hybridized numerical relativity simulations
of binary black hole systems with non-precessing spins
(
Abbott
et al.
2016h
)
yields estimates of component masses and
c
eff
consistent with our model-waveform analysis.
Spin components orthogonal to the orbital angular momentum
are the source of precession
(
Apostolatos et al.
1994
;Kidder
1995
)
and may be parameterized by a single effective precession spin
c
p
(
Schmidt et al.
2015
)
. For precessing binaries, component spin
orientations evolve over time; we report results evolved to a
reference GW frequency of 20 Hz. The spin prior assumed in this
analysis is uniform in dimensionless spin magnitudes
c
º
∣∣ (
)
S
cGm
i
ii
2
with
i
=
1, 2 between 0 and 0.89 and isotropic
in their orientation; this prior on component spins maps to priors
for the effective parameters
c
eff
and
c
p
. The top panel of Figure
3
shows the prior and posterior probability distributions of
c
eff
and
c
p
obtained for the effective precession waveform model. While
we gain some information about
c
eff
,the
c
p
posterior is
dominated by its prior, thus we cannot draw any strong conclusion
on the size of spin components in the orbital plane. Previous GW
events also yielded little in
formation on in-plane spins
(
Abbott
et al.
2016b
,
2016c
,
2017a
)
; possible effects of prior choice
on this inference were investigated in Vitale et al.
(
2017a
)
.The
inferred component spin magnitudes and orientations are shown
in the bottom panel of Figure
3
.We
fi
nd the dimensionless
spin magnitude of the primary black hole,
c
1
,tobelessthan0.75
Table 1
Source Properties for GW170608
Chirp mass
-
+
M
7
.9
0.2
0.2
Total mass
M
-
+
M
1
9
1
5
Primary black hole mass
m
1
-
+
M
1
2
2
7
Secondary black hole mass
m
2
-
+
M
7
2
2
Lower bound on mass ratio
mm
21
0.3
3
Effective inspiral spin parameter
c
ef
f
-
+
0.07
0.09
0.23
Final black hole mass
M
f
-
+
M
1
8.0
0.9
4.8
Final black hole spin
a
f
-
+
0.69
0.05
0.0
4
Radiated energy
E
rad
-
+
Mc
0.85
0.17
0.07
2
Peak luminosity
peak
́
-
+-
3
.4
10 erg s
1.6
0.5
56
1
Luminosity distance
D
L
-
+
3
40 Mpc
140
140
Source redshift
z
-
+
0.07
0.03
0.03
Note.
We quote median values with 90% credible intervals
(
90% bound on
mass ratio
)
. Source-frame masses are quoted; to convert to detector frame,
multiply by
+
(
)
z
1
(
Krolak & Schutz
1987
)
. The redshift assumes a
fl
at
cosmology with Hubble parameter
=
--
H
67.9 km s Mpc
0
11
and matter
density parameter
W=
0.306
5
m
(
Ade et al.
2016
)
.
Figure 2.
Posterior probabilit
y densities for binary component masses
(
m
1
,
m
2
)
,
total mass
(
M
)
, and chirp mass
(
)
in the source frame. One-dimensional
component mass distr
ibutions include posteriors f
or the effective precession
(
blue
)
and the non-precessing
(
red
)
waveform model, as well as their average
(
black
)
.
The dashed lines demarcate the 90% credible intervals for the average posterior.
The two-dimensional plot shows contours of the 50% and 90% credible regions
overlaid on a color-coded posterior den
sity function. For comparison, we show
both one- and two-dimensional distributions of averaged component mass
posterior samples for GW151226
(
orange; Abbott et al.
2016c
)
.Inthetoppanel,
we further compare GW170608 and GW151226
ʼ
s source-frame total mass
(
left
)
and source-frame chirp mass
(
right
)
. All other known binary black holes lie at
higher chirp masses than GW170608 and GW151226.
3
The Astrophysical Journal Letters,
851:L35
(
11pp
)
, 2017 December 20
Abbott et al.
(
90% credible limit
)
; this limit is robust to extending the prior
range of spin magnitudes and to using different waveform models.
The measurability of precession depends on the intrinsic source
properties as well as the angle of the binary orbital angular
momentum to the line of sight
(
i.e., inclination
)
. The inclination of
GW170608
ʼ
s orbit is likely close to either 0
°
or 180
°
, due to a
selection effect: the distance inside which a given binary merger
would be detectable at a
fi
xed S
/
N threshold is largest for these
inclination values
(
Schutz
2011
)
. For such values, the waveform
carries little information on precession.
The distance of GW170608 is extracted from the observed
signal amplitude given the binary
s inclination
(
Abbott
et al.
2016e
)
. With the network of two nearly co-aligned LIGO
detectors, the uncertainty on inclination translates into a large
distance uncertainty: we infer a luminosity distance of
=
-
+
D
340 Mpc
L
140
140
, corresponding to a redshift of
=
z
-
+
0
.07
0.03
0.03
assuming a
fl
at
L
CDM
cosmology
(
Ade et al.
2016
)
.
GW170608 is localized to a sky area of
~
520 deg
2
in the
northern hemisphere
(
90% credible region
)
, determined largely
by the signal
s measured arrival time at LLO
7
ms
later than
at LHO. This reduction in area relative to the low-latency map
is partly attributable to the use of noise-subtracted data with
of
fl
ine calibration
(
Abbott et al.
2017b
)
.
4.2. Consistency with General Relativity
To test whether GW170608 is consistent with the predictions of
GR, we consider possible deviations of coef
fi
cients describing the
binary inspiral part of the signal waveform from the values
expected in GR, as was done for previous detections
(
Abbott et al.
2016d
,
2016i
,
2017a
)
. Tests involving parameters describing the
merger and ringdown do not yield informative results, since the
merger happens at relatively high frequency where the LIGO
detectors are less sensitive. As in Abbott et al.
(
2017b
)
,wealso
allow a sub-leading phase contribution at effective
1PN order,
i.e., with a frequency dependence of
-
f
73
, which is absent in GR.
The GR predicted value is conta
ined within the 90% credible
interval of the posterior distribution for all parameters tested.
Assuming that gravitons are dispersed in vacuum similarly to
massive particles, we also obtain
ed an upper bound on the mass of
the graviton comparable to the constraints previously obtained
(
Abbott et al.
2016b
,
2016i
,
2017a
)
. Possible violations of local
Lorentz invariance, manifested via modi
fi
cations to the GW
dispersion relation, were investigated
(
Abbott et al.
2017a
)
,again
fi
nding upper bounds comparable to previous results.
5. Astrophysical Implications
The low mass of GW170608
ʼ
s source binary, in comparison to
other binary black hole systems
observed by LIGO and Virgo, has
potential implicati
ons for the binary
s progenitor environment.
High-metallicity progenitors are e
xpected to experience substantial
mass loss through strong stellar winds, while less mass loss is
exhibited for low-metallicity progenitors
(
Belczynski et al.
2010
;
Spera et al.
2015
)
. Thus, unlike more massive black hole binaries,
GW170608
ʼ
s low component masses do not necessarily require
formation at low metallicity. Further discussion of the relationship
between black hole masses and metallicity can be found in Abbott
et al.
(
2016j
)
.
We may compare GW170608
s relatively low-mass black hole
binary components to black holes found in X-ray binaries. X-ray
binary systems contain either a b
lack hole or neutron star that
accretes matter from a companion donor star. Low-mass X-ray
binaries
(
LMXBs
)
are X-ray binaries with a low-mass donor star
that transfer mass through Roche lobe over
fl
ow
(
Charles &
Coe
2003
)
. The inferred component masses of GW170608 are
consistent with dynamically measured masses of black holes
found in LMXBs, typically less than
M
10
(
Özel et al.
2010
;Farr
et al.
2011
; Corral-Santana et al.
2016
)
.
Binary black holes may form through many different channels,
including, but not limited to, dynamical interaction
(
Mapelli
2016
;
O
Leary et al.
2016
; Rodriguez et al.
2016
)
and isolated binary
evolution
(
Belczynski et al.
2016
;Eldridge&Stanway
2016
;
Lipunov et al.
2017
; Stevenson et al.
2017b
)
. While the inferred
masses and tilt measurements of GW170608 are not suf
fi
ciently
constrained to favor a formation channel, future measurements of
binary black hole systems may hint at the formation histories of
such systems
(
Abbott et al.
2017a
,
2016j
and references therein
)
.It
may be possible to determine the relative proportion of binaries
originating in each canonical formation channel following
()
100
Figure 3.
Top panel: marginalized one-dimensi
onal posterior den
sity functions for
the spin parameters
c
p
and
c
ef
f
(
blue
)
in comparison to their prior distributions
(
pink
)
as obtained from the effective precession model. The dashed lines indicate
the 90% credible interval
. The two-dimensional plot shows the 50% and 90%
credible regions plotted over the poste
rior density function. Bottom panel:
posterior probabilities for the di
mensionless component spins
c
i
with
i
=
1, 2
relative to the Newtonian o
rbital angular momentum
ˆ
L
, i.e., the normal of the
orbital plane. The tilt angles are 0
°
for spins parallel to
ˆ
L
and 180
°
for spins anti-
parallel to
ˆ
L
. The posterior density functions are marginalized over the azimuthal
angles. Each pixel has a
prior probability of
~ ́
-
1.8 10 ;
3
they are spaced
linearly in spin magnitudes and the cosine of the tilt angles.
4
The Astrophysical Journal Letters,
851:L35
(
11pp
)
, 2017 December 20
Abbott et al.
binary black hole detections
(
Farr et al.
2017a
;Farretal.
2017b
;
Stevenson et al.
2017a
;Talbot&Thrane
2017
; Vitale et al.
2017b
;
Zevin et al.
2017a
)
.
The detection of GW170608 is consistent with the merger
populations considered in Abbott et al.
(
2016k
,
2016d
)
,forwhicha
rate of
--
12 213 Gpc yr
31
was estimated in Abbott et al.
(
2017a
)
.
6. Outlook
LIGO
s detection of GW170608 extends the mass range of
known stellar-mass binary black hole systems and hints at
connections with other known astrophysical systems containing
black holes. The O2 run ended on 2017 August 25; a full catalog
of binary merger GW events for this run is in preparation,
including candidate signals with lower signi
fi
cance and systems
other than stellar-mass black hole binaries
(
Abbottetal.
2017c
)
.
Estimates of the merger rate and mass distribution for the
emerging compact binary population will also be updated.
With expected increases in detector sensitivity in the third
advanced detector network observing run, projected for late 2018
(
Abbott et al.
2016l
)
, detection of black hole binaries will be a
routine occurrence; studying this population will eventually answer
many questions about these systems
origins and evolution.
The authors gratefully acknowledge the support of the United
States National Science Foundation
(
NSF
)
for the construction and
operation of the LIGO Laboratory and Advanced LIGO as well as
the Science and Technology Facilities Council
(
STFC
)
of the
United Kingdom, the Max-Planck-Society
(
MPS
)
, and the State of
Niedersachsen
/
Germany for support of the construction of
Advanced LIGO and construction and operation of the GEO600
detector. Additional support for Advanced LIGO was provided by
the Australian Research Council. The authors gratefully acknowl-
edge the Italian Istituto Nazionale di Fisica Nucleare
(
INFN
)
,the
French Centre National de la Recherche Scienti
fi
que
(
CNRS
)
and
the Foundation for Fundamental Research on Matter supported by
the Netherlands Organisation for Scienti
fi
c Research, for the
construction and operation of the Virgo detector and the creation
and support of the EGO consortium. The authors also gratefully
acknowledge research support from these agencies as well as by
the Council of Scienti
fi
c and Industrial Research of India, the
Department of Science and Technology, India, the Science &
Engineering Research Board
(
SERB
)
, India, the Ministry of
Human Resource Development, India, the Spanish Agencia Estatal
de Investigación, the Vicepresidència i Conselleria d
Innovació
RecercaiTurismeandtheConselleriad
Educació i Universitat del
Govern de les Illes Balears, the Conselleria d
Educació Investi-
gació Cultura i Esport de la Generalitat Valenciana, the National
Science Centre of Poland, the Swiss National Science Foundation
(
SNSF
)
, the Russian Foundation for Basic Research, the Russian
Science Foundation, the European Commission, the European
Regional Development Funds
(
ERDF
)
, the Royal Society, the
Scottish Funding Council, the Scottish Universities Physics
Alliance, the Hungarian Scienti
fi
c Research Fund
(
OTKA
)
,the
Lyon Institute of Origins
(
LIO
)
, the National Research, Develop-
ment and Innovation Of
fi
ce Hungary
(
NKFI
)
, the National
Research Foundation of Korea, Industry Canada and the Province
of Ontario through the Ministry of Economic Development
and Innovation, the Natural Science and Engineering Research
Council Canada, the Canadian Institute for Advanced Research,
the Brazilian Ministry of Scienc
e, Technology, Innovations,
and Communications, the Interna
tional Center for Theoretical
Physics South American Institute for Fundamental Research
(
ICTP-SAIFR
)
, the Research Grants Council of Hong Kong, the
National Natural Science Foundation of China
(
NSFC
)
,the
Leverhulme Trust, the Research Corporation, the Ministry of
Science and Technology
(
MOST
)
, Taiwan and the Kavli
Foundation. The authors gratefully acknowledge the support of
the NSF, STFC, MPS, INFN, CNRS and the State of
Niedersachsen
/
Germany for provision of co
mputational resources.
Appendix
Angular Coupling Minimization
GW170608 was observed during a routine instrumental
procedure at LHO that minimizes the coupling of angular
control of the test masses to noise in the GW strain
measurement. To maintain resonant power in the arms, the
pitch and yaw angular degrees of freedom of the four
suspended cavity test masses at each detector
(
Abbott
et al.
2016a
)
must be controlled. This is achieved by actuating
on the second stage of the LIGO quadruple suspensions. A
feed-forward control is employed in order to leave the beam
position of the main laser on the test mass unchanged while this
actuation is applied. However, if this position differs from the
actuation point, the angular control can affect the differential
arm length, thus introducing additional noise in the strain
measurement
(
Kasprzack & Yu
2016
)
. As the beam position
can drift over periods of hours or days, the angular feed-
forward control must be periodically adjusted in order to
minimize the coupling to strain.
During this procedure, high amplitude pitch and yaw
excitations are applied to the te
st masses via actuation of the
suspensions. Each of the 8 angular degrees of freedom is excited
at a distinct frequency; the resultin
g length signals are observed
via demodulation at each excitation frequency, revealing how
strongly the corresponding degree of freedom couples to
differential arm length. The feed-f
orward gain settings are stepped
at intervals of approximately 45 s and the global minimum of
angular control coupling to strain is determined from the resulting
measurements. The frequencies of angular excitations are equally
spaced between
~
19 Hz
and
~
23 Hz
, generating excess power in
the differential arm motion, and thus in the measured strain around
these frequencies. This procedure covers from
2 minutes before
to
14 minutes after GW170608, shown in Figure
4
(
left
)
.During
the period from
2 to 2 minutes, substantial excess noise is visible
at frequencies around 20 Hz. To characterize this noise we show
amplitude spectral densities derived from 240 s of data both before
the onset of the angular excitations and during the excitations
around the event time in Figure
4
(
right
)
. No effect on the
spectrum is visible above 30 Hz.
During the procedure, angular control gain settings are
stepped abruptly; inspection of all such transition times shows
no evidence for transient excess noise in the strain data outside
the 19
23 Hz excitation band. The closest transition to the
event time was 10 s before the binary merger; thus, any
transient noise associated with this transition could not have
affected the matched-
fi
lter output at the event time
(
template
waveforms for GW170608-like signals have a duration
between 2 and 3 s
)
. Furthermore, the output of a matched-
fi
lter search analyzing LHO data from periods when this
procedure was performed shows a distribution of S
/
Ns similar
to that obtained from other times. Thus, we
fi
nd no evidence
that the angular coupling minimization affected the recorded
strain data at LHO around the event time at frequencies
above 30 Hz.
5
The Astrophysical Journal Letters,
851:L35
(
11pp
)
, 2017 December 20
Abbott et al.
References
Aasi, J., Abadie, J., Abbott, B. P., et al. 2015,
CQGra
,
32, 074001
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016a,
PhRvL
,
116, 131103
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016b,
PhRvL
,
116, 061102
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016c,
PhRvL
,
116, 241103
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016d,
PhRvX
,
6, 041015
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016e,
PhRvL
,
116, 241102
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016f,
CQGra
,
33, 134001
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016g,
PhRvD
,
93, 122003
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016h,
PhRvD
,
94, 064035
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016i,
PhRvL
,
116, 221101
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016j,
ApJL
,
818, L22
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016k,
ApJL
,
833, L1
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016l,
LRR
,
19, 1
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017a,
PhRvL
,
118, 221101
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017b,
PhRvL
,
119, 141101
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017c,
PhRvL
,
119, 161101
Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017d,
PhRvD
,
95, 062003
Acernese, F., Agathos, M., Agatsuma, K., et al. 2015,
CQGra
,
32, 024001
Adams, T., Buskulic, D., Germain, V., et al. 2016,
CQGra
,
33, 175012
Ade, P. A. R., Aghanim, N., Arnaud, M., et al. 2016,
A&A
,
594, A13
Ajith, P., Hannam, M., Husa, S., et al. 2011,
PhRvL
,
106, 241101
Allen, B., Anderson, W. G., Brady, P. R., Brown, D. A., & Creighton, J. D. E.
2012,
PhRvD
,
85, 122006
Apostolatos, T. A., Cutler, C., Sussman, G. J., & Thorne, K. S. 1994,
PhRvD
,
49, 6274
Baker, J. G., Centrella, J., Choi, D.-I., Koppitz, M., & van Meter, J. 2006,
PhRvL
,
96, 111102
Belczynski, K., Bulik, T., Fryer, C. L., et al. 2010,
ApJ
,
714, 1217
Belczynski, K., Holz, D. E., Bulik, T., & O
Shaughnessy, R. 2016,
Natur
,
534, 512
Bohé, A., Shao, L., Taracchini, A., et al. 2017,
PhRvD
,
95, 044028
Cahillane, C., Betzwieser, J., Brown, D. A., et al. 2017,
PhRvD
,
96, 102001
Campanelli, M., Lousto, C. O., Marronetti, P., & Zlochower, Y. 2006,
PhRvL
,
96, 111101
Cannon, K., Hanna, C., & Peoples, J. 2015, arXiv:
1504.04632
Capano, C., Dent, T., Hanna, C., et al. 2017,
PhRvD
,
96, 082002
Charles, P. A., & Coe, M. J. 2003, arXiv:
astro-ph
/
0308020
Chatterji, S., Blackburn, L., Martin, G., & Katsavounidis, E. 2004,
CQGra
,
21,
S1809
Chen, H.-Y., Holz, D. E., Miller, J., et al. 2017, arXiv:
1709.08079
Chu, T., Fong, H., Kumar, P., et al. 2016,
CQGra
,
33, 165001
Corral-Santana, J. M., Casares, J., Munoz-Darias, T., et al. 2016,
A&A
,
587, A61
Dal Canton, T., & Harry, I. 2017, arXiv:
1705.01845
Driggers, J. C., Dwyer, S., Ef
fl
er, A., et al. 2017, Improving Astrophysical
Parameter Estimation via Of
fl
ine Noise Subtraction for Advanced LIGO,
Tech. Rep. LIGO-P1700260,
https:
//
dcc.ligo.org
/
LIGO-P1700260
/
public
Ef
fl
er, A., Scho
fi
eld, R. M. S., Frolov, V. V., et al. 2015,
CQGra
,
32
035017
Eldridge, J. J., & Stanway, E. R. 2016,
MNRAS
,
462, 3302
Farr, B., Holz, D. E., & Farr, W. M. 2017a, arXiv:
1709.07896
Farr, W. M., Farr, B., Littenberg, T. & LIGO Scienti
fi
c Collaboration and Virgo
Collaboration 2015, Modelling Calibration Errors in CBC Waveforms, Tech.
Rep. LIGO-T1400682,
https:
//
dcc.ligo.org
/
P1500262
/
public
Farr, W. M., Sravan, N., Cantrell, A., et al. 2011,
ApJ
,
741, 103
Farr, W. M., Stevenson, S., Miller, M. C., et al. 2017b,
Natur
,
548, 426
Hannam, M., Schmidt, P., Bohé, A., et al. 2014,
PhRvL
,
113, 151101
Healy, J., & Lousto, C. O. 2017,
PhRvD
,
95, 024037
Hofmann, F., Barausse, E., & Rezzolla, L. 2016,
ApJL
,
825, L19
Husa, S., Khan, S., Hannam, M., et al. 2016,
PhRvD
,
93, 044006
Jiménez-Forteza, X., Keitel, D., Husa, S., et al. 2017,
PhRvD
,
95, 064024
Karki, S., Tuyenbayev, D., Kandhasamy, S., et al. 2016,
RScI
,
87, 114503
Kasprzack, M., & Yu, H. 2016, Beam Position from Angle to Length
minimization, Tech. Rep. LIGO-T1600397,
https:
//
dcc.ligo.org
/
LIGO-
T1600397
/
public
Keitel, D., Jiménez Forteza, X., Husa, S., et al. 2017,
PhRvD
,
96, 024006
Khan, S., Husa, S., Hannam, M., et al. 2016,
PhRvD
,
93, 044007
Kidder, L. E. 1995,
PhRvD
,
52, 821
Klimenko, S., Vedovato, G., Drago, M., et al. 2016,
PhRvD
,
93, 042004
Krolak, A., & Schutz, B. F. 1987,
GReGr
,
19, 1163
Lattimer, J. M., & Prakash, M. 2016,
PhR
,
621, 127
Lipunov, V. M., Kornilov, V., Gorbovskoy, E., et al. 2017,
NewA
,
51, 122
Littenberg, T. B., & Cornish, N. J. 2015,
PhRvD
,
91, 084034
Mapelli, M. 2016,
MNRAS
,
459, 3432
Messick, C., Blackburn, K., Brady, P., et al. 2017,
PhRvD
,
95, 042001
Mroué, A. H., Scheel, M. A., Szilágyi, B., et al. 2013,
PhRvL
,
111, 241104
Nitz, A., Harry, I., Brown, D., et al. 2017a, PyCBC software, v1.7.11, Zenodo,
doi:
10.5281
/
zenodo.883086
,
https:
//
ligo-cbc.github.io
/
Nitz, A. H., Dent, T., Dal Canton, T., Fairhurst, S., & Brown, D. A. 2017b,
ApJ
,
849, 118
O
Leary, R. M., Meiron, Y., & Kocsis, B. 2016,
ApJL
,
824, L12
Özel, F., Psaltis, D., Narayan, R., & McClintock, J. E. 2010,
ApJ
,
725, 1918
Pretorius, F. 2005,
PhRvL
,
95, 121101
Pürrer, M. 2016,
PhRvD
,
93, 064041
Figure 4.
Left: spectrogram of strain data from LHO around the time of GW170608. This plot shows variations in the noise spectrum of the detector over periods on
the scale of minutes; unlike Figure
1
, it is not designed to show short-duration transient events. The strain amplitude is normalized to the interval between
6 and
2
minutes relative to the event time. See the
Appendix
for a discussion of the feature around 20 Hz due to an angular control procedure. Right: amplitude spectral density
of strain data at both LIGO observatories for 240 s around the event time,
(
2, 2
)
minutes in the left panel, and for data before the start of the angular coupling
minimization at LHO,
(
6,
2
)
minutes. Excess noise is clearly visible around 20 Hz but data above 30 Hz are unaffected.
6
The Astrophysical Journal Letters,
851:L35
(
11pp
)
, 2017 December 20
Abbott et al.