Effects of transients in LIGO suspensions on
searches for gravitational waves
M. Walker,
1
T. D. Abbott,
1
S. M. Aston,
2
G. Gonz ́alez,
1
D. M. Macleod,
1
J. McIver,
3
B. P. Abbott,
3
R. Abbott,
3
C. Adams,
2
R. X. Adhikari,
3
S. B. Anderson,
3
A. Ananyeva,
3
S. Appert,
3
K. Arai,
3
S. W. Ballmer,
4
D. Barker,
5
B. Barr,
6
L. Barsotti,
7
J. Bartlett,
5
I. Bartos,
8
J. C. Batch,
5
A. S. Bell,
6
J. Betzwieser,
2
G. Billingsley,
3
J. Birch,
2
S. Biscans,
3
,
7
C. Biwer,
4
C. D. Blair,
9
R. Bork,
3
A. F. Brooks,
3
G. Ciani,
10
F. Clara,
5
S. T. Countryman,
8
M. J. Cowart,
2
D. C. Coyne,
3
A. Cumming,
6
L. Cunningham,
6
K. Danzmann,
11
,
12
C. F. Da Silva Costa,
10
E. J. Daw,
13
D. DeBra,
14
R. T. DeRosa,
2
R. DeSalvo,
15
K. L. Dooley,
16
S. Doravari,
2
J. C. Driggers,
5
S. E. Dwyer,
5
A. Effler,
2
T. Etzel,
3
M. Evans,
7
T. M. Evans,
2
M. Factourovich,
8
H. Fair,
4
A. Fern ́andez Galiana,
7
R. P. Fisher,
4
P. Fritschel,
7
V. V. Frolov,
2
P. Fulda,
10
M. Fyffe,
2
J. A. Giaime,
1
,
2
K. D. Giardina,
2
E. Goetz,
12
R. Goetz,
10
S. Gras,
7
C. Gray,
5
H. Grote,
12
K. E. Gushwa,
3
E. K. Gustafson,
3
R. Gustafson,
17
E. D. Hall,
3
G. Hammond,
6
J. Hanks,
5
J. Hanson,
2
T. Hardwick,
1
G. M. Harry,
18
M. C. Heintze,
2
A. W. Heptonstall,
3
J. Hough,
6
K. Izumi,
5
R. Jones,
6
S. Kandhasamy,
16
S. Karki,
19
M. Kasprzack,
1
S. Kaufer,
11
K. Kawabe,
5
N. Kijbunchoo,
5
E. J. King,
20
P. J. King,
5
J. S. Kissel,
5
W. Z. Korth,
3
G. Kuehn,
12
M. Landry,
5
B. Lantz,
14
N. A. Lockerbie,
21
M. Lormand,
2
A. P. Lundgren,
12
M. MacInnis,
7
S. M ́arka,
8
Z. M ́arka,
8
A. S. Markosyan,
14
E. Maros,
3
I. W. Martin,
6
D. V. Martynov,
7
K. Mason,
7
T. J. Massinger,
4
F. Matichard,
3
,
7
N. Mavalvala,
7
R. McCarthy,
5
D. E. McClelland,
22
S. McCormick,
2
G. McIntyre,
3
G. Mendell,
5
E. L. Merilh,
5
P. M. Meyers,
23
J. Miller,
7
R. Mittleman,
7
G. Moreno,
5
G. Mueller,
10
A. Mullavey,
2
J. Munch,
20
L. K. Nuttall,
4
J. Oberling,
5
M. Oliver,
24
P. Oppermann,
12
Richard J. Oram,
2
B. O’Reilly,
2
D. J. Ottaway,
20
H. Overmier,
2
J. R. Palamos,
19
H. R. Paris,
14
W. Parker,
2
A. Pele,
2
S. Penn,
25
M. Phelps,
6
V. Pierro,
15
I. Pinto,
15
M. Principe,
15
L. G. Prokhorov,
26
O. Puncken,
12
V. Quetschke,
27
E. A. Quintero,
3
F. J. Raab,
5
H. Radkins,
5
arXiv:1702.04701v1 [astro-ph.IM] 15 Feb 2017
Effects of transients in LIGO suspensions on searches for gravitational waves
2
P. Raffai,
28
S. Reid,
29
D. H. Reitze,
3
,
10
N. A. Robertson,
3
,
6
J. G. Rollins,
3
V. J. Roma,
19
J. H. Romie,
2
S. Rowan,
6
K. Ryan,
5
T. Sadecki,
5
E. J. Sanchez,
3
V. Sandberg,
5
R. L. Savage,
5
R. M. S. Schofield,
19
D. Sellers,
2
D. A. Shaddock,
22
T. J. Share,
5
B. Shapiro,
14
P. Shawhan,
30
D. H. Shoemaker,
7
D. Sigg,
5
B. J. J. Slagmolen,
22
B. Smith,
2
J. R. Smith,
31
B. Sorazu,
6
A. Staley,
8
K. A. Strain,
6
D. B. Tanner,
10
R. Taylor,
3
M. Thomas,
2
P. Thomas,
5
K. A. Thorne,
2
E. Thrane,
32
C. I. Torrie,
3
G. Traylor,
2
D. Tuyenbayev,
27
G. Vajente,
3
G. Valdes,
27
A. A. van Veggel,
6
A. Vecchio,
33
P. J. Veitch,
20
K. Venkateswara,
34
T. Vo,
4
C. Vorvick,
5
R. L. Ward,
22
J. Warner,
5
B. Weaver,
5
R. Weiss,
7
P. Weßels,
12
B. Willke,
11
,
12
C. C. Wipf,
3
J. Worden,
5
G. Wu,
2
H. Yamamoto,
3
C. C. Yancey,
30
Hang Yu,
7
Haocun Yu,
7
L. Zhang,
3
M. E. Zucker,
3
,
7
and J. Zweizig
3
(LSC Instrument Authors )
1
Louisiana State University, Baton Rouge, LA 70803, USA
2
LIGO Livingston Observatory, Livingston, LA 70754, USA
3
LIGO, California Institute of Technology, Pasadena, CA 91125, USA
4
Syracuse University, Syracuse, NY 13244, USA
5
LIGO Hanford Observatory, Richland, WA 99352, USA
6
SUPA, University of Glasgow, Glasgow G12 8QQ, United Kingdom
7
LIGO, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
8
Columbia University, New York, NY 10027, USA
9
University of Western Australia, Crawley, Western Australia 6009, Australia
10
University of Florida, Gainesville, FL 32611, USA
11
Leibniz Universit ̈at Hannover, D-30167 Hannover, Germany
12
Albert-Einstein-Institut, Max-Planck-Institut f ̈ur Gravitationsphysik, D-30167
Hannover, Germany
13
The University of Sheffield, Sheffield S10 2TN, United Kingdom
14
Stanford University, Stanford, CA 94305, USA
15
University of Sannio at Benevento, I-82100 Benevento, Italy and INFN, Sezione di
Napoli, I-80100 Napoli, Italy
16
The University of Mississippi, University, MS 38677, USA
17
University of Michigan, Ann Arbor, MI 48109, USA
18
American University, Washington, D.C. 20016, USA
19
University of Oregon, Eugene, OR 97403, USA
20
University of Adelaide, Adelaide, South Australia 5005, Australia
21
SUPA, University of Strathclyde, Glasgow G1 1XQ, United Kingdom
22
Australian National University, Canberra, Australian Capital Territory 0200,
Australia
23
University of Minnesota, Minneapolis, MN 55455, USA
24
Universitat de les Illes Balears, IAC3—IEEC, E-07122 Palma de Mallorca, Spain
25
Hobart and William Smith Colleges, Geneva, NY 14456, USA
26
Faculty of Physics, Lomonosov Moscow State University, Moscow 119991, Russia
27
The University of Texas Rio Grande Valley, Brownsville, TX 78520, USA
28
MTA E ̈otv ̈os University, “Lendulet” Astrophysics Research Group, Budapest 1117,
Effects of transients in LIGO suspensions on searches for gravitational waves
3
Hungary
29
SUPA, University of the West of Scotland, Paisley PA1 2BE, United Kingdom
30
University of Maryland, College Park, MD 20742, USA
31
California State University Fullerton, Fullerton, CA 92831, USA
32
Monash University, Victoria 3800, Australia
33
University of Birmingham, Birmingham B15 2TT, United Kingdom
34
University of Washington, Seattle, WA 98195, USA
E-mail:
mwalk49@lsu.edu
Abstract.
This paper presents an analysis of the transient behavior of the Advanced LIGO
suspensions used to seismically isolate the optics. We have characterized the transients
in the longitudinal motion of the quadruple suspensions during Advanced LIGO’s first
observing run. Propagation of transients between stages is consistent with modelled
transfer functions, such that transient motion originating at the top of the suspension
chain is significantly reduced in amplitude at the test mass. We find that there are
transients seen by the longitudinal motion monitors of quadruple suspensions, but
they are not significantly correlated with transient motion above the noise floor in
the gravitational wave strain data, and therefore do not present a dominant source of
background noise in the searches for transient gravitational wave signals.
1. Introduction
The Laser Interferometer Gravitational-wave Observatory (LIGO) was designed to
detect gravitational waves from astrophysical sources. [1, 2] After six science runs over
the course of several years, the detectors underwent major upgrades starting in 2010.
In September 2015, the newly upgraded Advanced LIGO detectors began taking data
for their first observational period (
O1
). Figure 1 shows the basic optical configuration
of Advanced LIGO. With a sensitivity more than three times better than that of the
previous generation, [3] the detectors had the astrophysical reach to make the first direct
observation of a gravitational wave signal, GW150914, from the merger of two black
holes, [4] and later a second unambiguous detection, GW151226. [5] These observations
have opened a new field of gravitational-wave astronomy, which will continue to grow
brighter with further improvements to the detector network.
The main low frequency noise source for LIGO is seismic activity. The LIGO
detectors are affected by earthquakes from around the world, windy weather that shakes
the buildings housing the interferometer instrumentation, microseismic vibrations from
ocean waves crashing on the shores of the Pacific Ocean, Atlantic Ocean, and the Gulf of
Mexico, and local anthropogenic activity. [6,7] The study of the effects of seismic activity
was especially important for improving the data quality of transient gravitational wave
searches in the initial LIGO era. [8, 9] One of the key improvements from initial to
Advanced LIGO is the implementation of a much more sophisticated seismic isolation
system, which includes stages of active and passive isolation for all of the cavity optics.
Effects of transients in LIGO suspensions on searches for gravitational waves
4
Figure 1.
Advanced LIGO optical configuration. [2] The mirrors at the input and end
of both arms (labeled ETM and ITM) are suspended from quadruple-stage pendulums,
in addition to active seismic isolation systems.
It is important to check that this entirely new system provides the very high
isolation expected, and that it does not introduce any new types of transient noise
that could add to the noise background for searches of short duration gravitational
waves, such as black hole mergers and supernovae. Here we present an investigation of
the transient motion of the Livingston suspension systems as measured by local sensors
on each suspension, specifically looking at the displacement of the quadruple stage
pendulums in the longitudinal degree of freedom, which is the direction of the optical
path used to sense spacetime strain induced by passing gravitational waves.
2. Advanced LIGO Suspensions
In Advanced LIGO, all optical cavities use optics suspended from multi-stage
pendulums, in order to benefit from the lowpassing of seismic motion. The input and
end mirrors (the optics whose motion most directly contributes to the gravitational-
wave readout signal) are all hung from quadruple-stage suspensions, [10] with each
stage providing additional isolation at frequencies above the suspension resonances,
Effects of transients in LIGO suspensions on searches for gravitational waves
5
Figure 2.
Quadruple suspensions design. The left image shows the suspension systems
with the blades, fibers, and reaction chain. On the right the whole structure is shown,
with the four masses labeled. [10]
which range from 0.4 Hz to 14 Hz. The quadruple pendulum is suspended at the top
from maraging steel blade springs, with two further sets of springs incorporated into
the top two masses, thus providing three stages of enhanced vertical isolation. The two
lower masses of the quad are cylindrical silica masses connected by fused silica fibers
to reduce thermal noise. Another similar quadruple suspension is hung next to the
test mass suspension, so the actuation on lower stages can be done from a similarly
isolated reaction chain. Figure 2 gives an overview of the design of the Advanced LIGO
quadruple suspensions.
The local displacement of each stage of the suspensions is measured using Optical
Sensor and ElectroMagnetic actuators, or OSEMs, which are electromagnetic sensors
and actuators used for damping the suspensions’ resonances and controlling the mirrors
to keep cavities aligned and locked. [10, 11]
The OSEMs can sense suspension motion at low frequencies where the
displacements are relatively large. At frequencies above 5 Hz the suspension motion
has typically fallen below the sensitivity level of the OSEMs such that the resulting
spectra are dominated by electronics noise. Multiple OSEMs on each stage allow the
calculation of the mass’s motion in each degree of freedom using a linear combination of
the sensors’ signals. The sensed displacement of the top stage is used in a feedback loop
to actuate on that stage of the suspension in order to damp the mechanical resonances
of the suspension. The sensors at lower stages are only used as witnesses of the optics’
displacement, for the purposes of diagnosing problems in the suspensions. The actuators
on lower stages use interferometer and cavity signals to keep various degrees of the
interferometer precisely on resonance.
Effects of transients in LIGO suspensions on searches for gravitational waves
6
3. Motion transients in suspensions
It is important to understand the origins of non-astrophysical noise transients in
the gravitational wave data in order to eliminate false positives from the gravitational
wave searches. Each subsystem of the detector itself is therefore investigated in great
detail to fully study all potential noise sources. [7]
In this article, we characterize transients in the displacement of the suspensions’
stages as measured by the OSEMs, the propagation of transients between different
stages, and their effects on the gravitational wave strain data. Specifically, transients in
the longitudinal degree of freedom were studied in the top three stages of the quadruple
suspensions.
Motion transients seen by the local displacement sensors have a few potential
sources. For example, they could be caused by motion that is intrinsic to the suspension
systems themselves, from the crackling in the suspension wires or the steel blades. [12,13]
Transients could also come from excess seismic motion by propagating through each
stage of active seismic isolation and then down through the suspension stages. Above
the suspension’s resonance f
0
, the seismic transients should decrease in amplitude by
a factor of (f/f
0
)
2
at each stage and be less likely to appear above the sensor noise at
lower stages. Therefore any seismic transients that affect multiple stages should appear
mostly at low frequencies. Another source of transients seen in the local sensors is the
actuation on the suspensions from the feedback loops used to control the interferometer.
3.1. Suspension behavior in Advanced LIGO’s first observing run
The typical spectrum of the suspension motion monitors is characterized by several
peaks near the low frequency pendulum resonances between 0.4 to 5 Hz, and the flat
noise above 5 Hz due to the sensors’ electronics noise. The main resonances in the
longitudinal degree of freedom are modeled for the quadruple suspensions to be at 0.435
Hz, 0.997 Hz, 2.006 Hz, and 3.416, but coupling from other degrees of freedom and the
active seismic isolation system creates additional peaks in the spectrum. Figure 3 shows
a typical spectrum of the Y-arm input quadruple suspension (Input Test Mass Y, or
ITMY
) motion in O1 along with estimated sensor noise levels.
Ideally, the noise would be stationary and the average spectrum would statistically
characterize the noise level, but in actuality there are non-stationary disturbances at
different frequencies. To demonstrate this, Figure 4 shows the time series of several
minutes of data from the top stage of one suspension with two different bandpass filters
applied to select for 4-5 Hz and 10-11 Hz. Above 10 Hz, the sensor noise dominates
the signal and the resulting time series is Gaussian distributed, but the lower frequency
shows large non-Gaussian transients.
Rather than visually inspecting time series, the Omicron algorithm is used to find
transients in the data, producing
triggers
that indicate the time, frequency, amplitude,
and signal-to-noise ratio of the transient noise. [14, 15] Figure 5 shows distribution in
frequency and signal-to-noise ratio of Omicron triggers for the longitudinal degree of
Effects of transients in LIGO suspensions on searches for gravitational waves
7
Figure 3.
Typical amplitude spectral density (ASD) for the ITMY longitudinal
motion monitors from O1. Many of the low frequency features of the ASD correspond
to the pendulum resonances of the suspension. The flat portion of the ASD above 5 Hz
shows where the electronics noise of the OSEM dominates the spectrum. The noise in
the penultimate stage at higher frequencies is slightly higher because it has a different
kind of OSEM. [11]
Figure 4.
The panel on the left shows a five minute time series from the top
stage of ITMY, with bandpass filters applied between 4 to 5 Hz and 10 to 11 Hz.
The distribution of time series amplitude over the same time is shown on the right,
with dashed lines to indicate a Gaussian distribution. While the Gaussian-distributed
stationary sensor noise dominates the higher frequency band shown, the time series
from 4 to 5 Hz exhibits large excursions from the average noise level.
Effects of transients in LIGO suspensions on searches for gravitational waves
8
Figure 5.
Above, the Signal-to-Noise Ratio (SNR) and central frequencies of Omicron
triggers of ITMY suspension during one week of O1. Below are histograms showing the
SNR distributions of the penultimate stage triggers at three selected frequency ranges
(note the different SNR scales). While the distribution of higher frequency triggers fall
off much like Gaussian noise, the lower frequency ranges contain more outliers.
freedom, using data from the Y-arm input suspension over one week of the observing
run. While stationary noise would produce a background of low SNR triggers across all
frequencies, the actual data from the suspension monitors shows a varying structure in
different frequency bands. This suggests the presence of non-stationary noise sources.
3.2. Motion transient propagation
To characterize the effects of short duration disturbances in the upper stages of
the suspensions on the motion lower in the suspension chain, we need more than just
the frequency domain models usually used to characterize the suspensions, due to the
influence of the impulse response of the system. Simulink was used to model the response
of a simple pendulum to a sine-Gaussian input signal. It is important to note that the
input signal is not a pure sine wave at a single frequency, but rather a sine-Gaussian
characterized by a peak frequency while also containing broader frequency content.
Therefore, the pendulum response more strongly attenuates the higher frequencies of
Effects of transients in LIGO suspensions on searches for gravitational waves
9
Figure 6.
Modelled transient response of a simple pendulum to a sine-Gaussian
injection. Simulink was used to model a simple pendulum with a resonance at 2 Hz.
A one-second 4 Hz sine-Gaussian signal (top panel) was used as the input to show the
response of the system (middle panel) compared with the input signal multiplied by
the transfer function of the system at 4 Hz. Local maxima and minima of the time
series can be used to calculate the frequency for each half-cycle (bottom panel). This
simulation shows that even for this simple model, the transient response of the system
deviates from the steady state frequency response at 4 Hz.
the input signal, and the peak frequency of the resulting motion is a mixture of the
driving and the resonance frequencies, as demonstrated in Figure 6.
To examine the propagation of transients in Advanced LIGO suspensions and
compare with expected behavior, sine-Gaussian waveforms were physically injected in
the Y-end quadruple suspension (End Test Mass Y, or
ETMY
) in the longitudinal
direction using the top mass actuators, with central frequencies ranging from 2 to 10
Hz. The Omicron algorithm was used to characterize the resulting transients caused in
the top stage as well as in lower stages.
Figure 7 shows the ratio of the Omicron trigger amplitudes of the lower suspension
stages to the top stage for different frequencies, using the Omicron frequency estimate of
the top stage trigger. The solid lines show the frequency response of the suspensions as
predicted by the quadruple suspension models. One reason for apparent discrepancies
from the model is variation of the transient motion frequency between stages, as well as
the fact that the motion at each stage is not characterized by only a single frequency.
To analyze this effect, time series of the injections were examined individually to
characterize the frequencies and amplitudes of the signals at each stage, similar to the
process used in analysis of the Simulink model shown in Figure 6.
Using a bandpass filter with a 1 Hz window around the injection frequency and
Effects of transients in LIGO suspensions on searches for gravitational waves
10
Figure 7.
Ratio of amplitudes of injection Omicron triggers at lower stages to
the top stage, plotted against the peak frequency estimated by Omicron for the top
stage motion, compared with the modeled transfer function. At lower frequencies the
propagation of the transients is close to the model, but above a few Hz, the motion
at the lower stages is smaller than the sensor noise, and the amplitude ratio is not as
close to the model. There are fewer Omicron triggers at the penultimate stage, since
the motion at that stage is at a lower amplitude and is not great enough at higher
frequencies to be seen above the sensor noise.
finding the local maxima and minima of the resulting time series, the peak frequency of
the induced transient motion was estimated with each cycle. Similar to the simulation
performed in Simulink, the suspension’s response is not exactly at the peak sine-
Gaussian frequency, and when the injection is finished the suspension’s motion begins
to ring down with a frequency approaching the nearest resonance. As the motion
propagates downwards, the pendulum filter response attenuates the signal more in the
frequency range farther from the resonance, resulting in a slight frequency shift towards
the resonance at the lower stage. Figure 8 shows the period increasing in the bandpassed
time series from one of the injections.
The bandpass filter reduces the noise so that the time series cycles can be clearly
determined. The frequency of the resulting motion at each stage was then estimated by
taking the mean frequency, weighting each cycle by the amplitude of its maximum or
minimum. The amplitude of motion was calculated using Omega, a multi-resolution
technique for studying transients related to Omicron. [14] [15] Using the weighted
average frequency and the amplitudes calculated by Omega, ratio of motion transient
amplitudes between suspension stages for each frequency can be better compared to the
suspension model. Figure 9 displays this comparison for the propagation of the motion
from the top stage to the second and third stages. Since the transient amplitude is much
smaller at higher frequencies for each successive stage, the higher frequency injection
measurements are farther from the model, due to the sensor noise at the lower stages.
In both lower stages we see a shift in the frequency away from the frequency at the top
Effects of transients in LIGO suspensions on searches for gravitational waves
11
Figure 8.
Time series from one injection at 4.1 Hz, after application of a bandpass
filter with a window of 1 Hz around the injection frequency. Similar to the simple
pendulum model analysis, the frequency shifts throughout the time series. The period
of the cycles in the top stage lengthens slightly, from 0.25 seconds (4.0 Hz) at the peak
of the transient to 0.28 seconds (3.6 Hz) a few cycles later.
stage, generally closer to the nearest suspension resonance, a pitch mode at 2.7 Hz.
Having understood the propagation of short transients in the suspension stages,
we turn now to studying the effect of the actual suspension transients on the LIGO
gravitational wave strain data during the first Advanced LIGO observing run.
3.2.1. Correlations with gravitational wave strain data
Taking the data from the first
week of November (the same week shown in Figure 5), Omicron was used to identify
transients in the gravitational wave (GW) strain data in the same frequency range
as used to produce the suspension motion triggers (0.1 to 60 Hz), as well as at higher
frequencies to check for any nonlinear coupling. Figure 10 shows the correlations between
transients in the ITMY longitudinal displacement data and the gravitational wave strain
data in both frequency ranges. The figures shown are Receiver Operator Characteristics
(ROC) curves, which show the time coincidence rate between the two sets of triggers,
with various coincidence windows from 0.1 to 10 seconds. This rate is compared with
the number of time coincidences that would occur by chance (false alarm rate), using
a number of time shifts between the two data sets. In both cases, the small number
of coincidences between the sets of data are consistent with the number that would be
expected by random chance. The observed transients in the ITMY suspension motion
monitors did not show any significant correlation with GW strain noise transients, at
any frequency.
We can now place upper limits on the level of noise that would be caused in
GW strain from the observed transients in suspension monitors. The amplitude of the
Effects of transients in LIGO suspensions on searches for gravitational waves
12
Figure 9.
Amplitude ratios as calculated by the Omega algorithm, and frequency
estimated using the maxima and minima of bandpassed time series. Errors are greater
as sensor noise becomes dominant at higher frequencies. Red and yellow points
represent the same amplitude ratios between stages, but red points show the frequency
estimate at the top stage while yellow points show the frequency estimate at the lower
stages. Error bars shown are the standard deviation of the measurement among the
various injections of the same frequency, weighted by the amplitude of the injection at
the top stage.
Omicron triggers from each of the upper stages of ITMY is multiplied by the suspension
transfer function to estimate the amplitude of noise transients that would be caused in
the test mass by a physical displacement of that amplitude. Figure 11 shows the resulting
projections in equivalent GW strain amplitude, alongside the GW strain triggers from
the same time. The sensor noise at the lower stages is much higher than the expected
amplitude of motion at those stages, so the upper limit of motion at the lowest stage
is above most of the GW strain triggers. The noise level predicted by the top stage
triggers, however, is below most of the GW strain triggers up to 37 Hz, so if noise
originating in that stage caused high amplitude transients in the GW data, it would be
Effects of transients in LIGO suspensions on searches for gravitational waves
13
Figure 10.
Receiver Operator Characteristic (ROC) curves showing the correlation
between noise transients in the GW strain and ITMY suspension data from November
1 to 8, 2015. The lefthand plot shows the correlation with higher frequency GW strain
triggers (above 60 Hz), while the plot on the right shows the correlation with GW strain
triggers below 60 Hz. The y axis shows the fraction of triggers coincident between the
two sets of data for varying time windows. The x axis represents the number of
coincidences that would appear by chance, estimated by repeating the analysis at each
time window with different time shifts between the two data sets. For both sets of GW
strain triggers, the coincidence rate is approximately equal to the false alarm rate,
whereas a significant correlation would have a much greater efficiency than false alarm
rate.
expected to also be seen by the top stage sensors.
Since the top stage triggers are not statistically correlated with any of the GW
strain triggers, we can conclude that transient noise originating at the top stage of the
suspension is not a significant contribution to the transient noise in the interferometer.
We cannot, however, rule out the possibility of GW strain noise transients caused by
motion originating in the lower stages of the suspension, since there are a significant
portion of GW strain triggers that fall below the level of transient noise caused by the
local sensor noise.
4. Conclusions
Using short duration hardware injections in the top stage of the suspension, we have
studied the propagation of transient motion down the suspension chain. The difference
of transient amplitudes at different stages is consistent with the models, although slight
variations in frequency must be taken into account. The frequency of the transients shifts
because the injected waveform is not a pure sine wave but a sine-Gaussian, and after the
short duration injection, the suspension motion oscillates with a decreasing amplitude
and frequency that shifts toward the closest mechanical resonance frequency. Transients
at different stages of the suspension therefore show slightly different frequencies from
the same initial sine-Gaussian injection.
Effects of transients in LIGO suspensions on searches for gravitational waves
14
Figure 11.
Livingston ITMY triggers from a week in the first observing run, multiplied
by the transfer function to the lowest stage and divided by the arm length to convert
the displacement into equivalent strain amplitude. The strain calibration of the
gravitational wave data is only accurate above 10 Hz, at frequencies where the OSEM
signals are dominated by sensor noise. Therefore, this calculation can only give us
the upper limit of transient motion from each stage that could appear in the GW
strain data without also appearing in the local displacement sensor. Where there are
GW strain noise transients above one of these levels, we can rule out an origin in a
particular stage of the suspension chain. A large number of the GW strain triggers are
above the noise level from the top stage, eliminating the origin of the noise at the top
of the suspension chain. However, only the very loudest GW strain triggers are above
the level of the second stage, and no GW strain triggers are higher than the level of
the third stage.
Statistical comparisons of the times of transients in the OSEMs and in the GW
strain data during O1 show that transients seen by the local displacement sensors
of the suspensions are not a significant source of background transient noise in the
interferometer. However, this does not rule out transient suspension motion that is below
the local sensor noise as a possible source of background noise. Using the suspension
models to propagate the sensor noise into the motion at the test mass, upper limits
can be placed on the level of noise that could be caused in the GW strain data from
transients in suspension motion at each stage. Most GW strain triggers are above the
sensor noise level of the top stage of the suspension, but below the noise level of the
third stage. Transient noise that originates in the lower stage of the suspension could
therefore be a cause of noise in the GW data while not being loud enough to appear
above the local sensor noise. [2]
Effects of transients in LIGO suspensions on searches for gravitational waves
15
Acknowledgments
LSU authors acknowledge the support of the United States National Science
Foundation (NSF) with grants PHY-1505779, 1205882, and 1104371.
The authors gratefully acknowledge the support of the 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 also gratefully acknowledge the support of LSC related research by these
agencies as well as by the Council of Scientific and Industrial Research of India,
Department of Science and Technology, India, Science & Engineering Research Board
(SERB), India, Ministry of Human Resource Development, India, the Istituto Nazionale
di Fisica Nucleare of Italy, the Spanish Ministerio de Econom ́ıa y Competitividad,
the Vicepresid`encia i Conselleria d’Innovaci ́o, Recerca i Turisme and the Conselleria
d’Educaci ́o i Universitat del Govern de les Illes Balears, the European Union, the
Royal Society, the Scottish Funding Council, the Scottish Universities Physics Alliance,
the Hungarian Scientific Research Fund (OTKA), 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, Canadian Institute for Advanced Research, the Brazilian Ministry of
Science, Technology, and Innovation, International Center for Theoretical Physics South
American Institute for Fundamental Research (ICTP-SAIFR), Russian Foundation
for Basic Research, the Leverhulme Trust, the Research Corporation, Ministry of
Science and Technology (MOST), Taiwan and the Kavli Foundation. The authors
gratefully acknowledge the support of the NSF, STFC, MPS and the State of
Niedersachsen/Germany for provision of computational resources.
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