All-sky search for long-duration gravitational-wave bursts in the third Advanced
LIGO and Advanced Virgo run
LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration
(compiled July 30, 2021)
After the detection of gravitational waves from compact binary coalescences, the search for tran-
sient gravitational-wave signals with less well-defined waveforms for which matched filtering is not
well-suited is one of the frontiers for gravitational-wave astronomy. Broadly classified into “short”
.
1 s and “long”
&
1 s duration signals, these signals are expected from a variety of astrophysi-
cal processes, including non-axisymmetric deformations in magnetars or eccentric binary black hole
coalescences. In this work, we present a search for long-duration gravitational-wave transients from
Advanced LIGO and Advanced Virgo’s third observing run from April 2019 to March 2020. For this
search, we use minimal assumptions for the sky location, event time, waveform morphology, and
duration of the source. The search covers the range of 2 – 500 s in duration and a frequency band
of 24
−
2048 Hz. We find no significant triggers within this parameter space; we report sensitivity
limits on the signal strength of gravitational waves characterized by the root-sum-square amplitude
h
rss
as a function of waveform morphology. These
h
rss
limits improve upon the results from the
second observing run by an average factor of 1.8.
I. INTRODUCTION
The third observing run of the Advanced LIGO [1]
and Advanced Virgo [2] detectors has revealed a large
number of new gravitational-wave signals from the col-
lision of compact objects. Many binary black hole sys-
tems [3] have been identified. These include GW190521
[4] with the largest progenitor masses discovered so far,
and GW190814, a merger containing an object in the
“mass-gap” between neutron stars and black holes [5]. A
second binary neutron star (BNS) system was also dis-
covered, GW190425 [6], following the first BNS system
GW170817 [7], which also produced GRB 170817A [8]
and an optical transient, AT 2017gfo [9]. In addition,
two neutron star-black hole (NSBH) binary coalescences
(GW200105
162426 and GW200115
042309) have been
finally detected [10].
Searches for “long”
&
1 s duration signals cover a vari-
ety of astrophysical phenomena [11]. While well-modeled
compact binary coalescences can have similar durations
in the sensitive band of the interferometers and the meth-
ods employed in this paper are also sensitive to them,
this search is not aimed at these systems as matched fil-
tering is much more sensitive. However, there are less
well-defined waveforms for which matched filtering is not
well-suited. Plausible processes include fallback accretion
onto a rapidly rotating black hole [12] or in newborn neu-
tron stars [13–15]. They also include non-axisymmetric
deformations in magnetars [16] or accretion disk instabil-
ities and fragmentation of material spiraling into a black
hole [17–19] and in the central engine of super-luminous
supernovae [20, 21]. Figure 1 shows several different re-
alizations of the corresponding waveform morphologies.
In this paper, we present the results of unmodeled
long-duration transient searches from the third observ-
ing run, updating the results from the first two observ-
ing runs [22, 23]. As in previous analyses [22–25], three
pipelines are used; their different assumptions and data
handling techniques yield complementary coverage of the
signal models.
The paper is organized as follows. The data used in
the analysis is described in Section II. The algorithms
used to analyze the data are outlined in Section III. The
results of the analysis and their implications are discussed
in Section IV.
II. DATA
The third observing run (O3) of Advanced LIGO and
Advanced Virgo spanned April 1, 2019 - March 27, 2020.
O3 was broken up into two segments, with O3a running
April 1, 2019 - Oct 1, 2019 and O3b running Novem-
ber 1, 2019 - March 27, 2020; together, these corre-
spond to 330 days. It is customary to assess detector
sensitivities in terms of a binary neutron star inspiral
range (BNS range), which is the average distance to
which these signals could be detected [26, 27]. Detec-
tor upgrades to the LIGO detectors in Hanford, WA
and Livingston, LA yielded binary neutron star ranges
of
∼
115 Mpc and 133 Mpc respectively, amounting to
improvements of
∼
50% with respect to O2. Similarly,
Advanced Virgo reached a binary neutron star range of
∼
50 Mpc, a
∼
100% improvement. In the following, the
algorithms employed require at least two detectors to be
available to process the data; therefore, only data where
both LIGO detectors are simultaneously available is used.
Due to the significant difference in detector alignment
and sensitivities, the Virgo data in the analysis would
not improve the coincidence selection when the other two
detectors are active, while the high rate of non-Gaussian
noise would increase the overall false-alarm rate. We plan
to include Virgo in the analysis of the next observing run.
A major challenge in searches for gravitational-wave
transients is non-Gaussian noise. Known sources of noise,
including non-linear sources such as time-varying spectral
lines, from, e.g., machinery on-site, side-bands from the
60 Hz power lines, and time-varying spectral lines, can
arXiv:2107.13796v1 [gr-qc] 29 Jul 2021
2
0 s
500 s
1000 s
1500 s
2000 s
2500 s
Time
0 Hz
250 Hz
500 Hz
750 Hz
1000 Hz
1250 Hz
1500 Hz
1750 Hz
2000 Hz
Frequency
ECBC
ISCOChirp
Magnetar
GRBplateau
ADI
WNB
SG
A
B
C
D
E
F
G
H
I
A
B
C
D
E
F
FIG. 1. Time-frequency spectrogram of the reference waveforms used in this search. The ISCOChirp waveforms have been
shifted up in frequency by 50 Hz for readability. Durations range from 6 s (ADI-B) to 470 s (GRBplateau).
be witnessed and subtracted using both linear Wiener
filters [28] and machine learning techniques [29, 30]. The
analyses that follow use data for which some of the identi-
fied sources of noise that couple in linearly to the detector
have been subtracted. Beyond spectral features, there
are transient noise triggers known as
glitches
, which have
a variety of origins [31]. Glitch rejection procedures rely
on correlations with auxiliary channels [32, 33]; yet, noise
transients not witnessed by auxiliary sensors remain and
reduce sensitivity of the searches [34, 35]. Each pipeline,
described in the next section, implements different strate-
gies to reduce the impact from glitches. Altogether, dur-
ing the third observing run, coincident data of sufficient
quality to be analyzed totaled 204.4 days. Since some
time segments are too short to be processed by search
pipelines, a small fraction (
<
2%) of this coincident data
is not analyzed.
III. SEARCHES
Long-duration unmodeled searches are now briefly re-
viewed, and we refer the reader to previous publications
for further detail [22, 23]. Most unmodeled searches
use time-frequency spectrograms with statistics derived
from Fourier transforms or wavelet analysis performed
on consecutive time segments. Pattern-recognition al-
gorithms then are employed to search for gravitational
waves in these spectrograms.
These algorithms can
be classified as: “seed-based” [36, 37], for which pix-
els above pre-determined thresholds are clustered, and
“seedless” [38, 39], for which sequences of pixels are de-
rived from generic models, such as B ́ezier curves [38–42].
Seedless clustering algorithms are sensitive to narrow-
band signals at the price of sensitivity to broadband
sources, while seed-based algorithms are generally more
sensitive to more generic waveform morphologies. These
algorithms identify candidate gravitational-wave events
known as
triggers
.
To estimate the background, all
pipelines use “time-slides,” [43, 44], where detector data
is shifted by non-physical time delays and reanalyzed;
this procedure is repeated a sufficient number of times
such that at least 50 years of coincident live time is ana-
lyzed, allowing for a false alarm rate of 1 per 50 years to
be estimated.
Three pipelines are deployed in the analysis: two differ-
ent versions of the Stochastic Transient Analysis Multi-
detector Pipeline - all sky (STAMP-AS) pipeline [11,
37, 42] and the long-duration configuration of coherent
WaveBurst (cWB) [45]. The cWB pipeline is seed-based
while the two STAMP-AS algorithms, Zebragard and
Lonetrack, use seed-based and seedless clustering algo-
rithms respectively. Altogether, the analyses are sensi-
tive to transients lasting 2 – 500 s and covering a fre-
quency band of 24 – 2048 Hz. Due to the short duration
of binary black hole signals and the weakness of the co-
alescences containing neutron stars observed during O3
[6], we are not sensitive to and therefore do not excise
any time around known compact binary coalescences. All
false alarm rates reported are per pipeline, with no com-
bination of searches made outside of reporting the most
sensitive limit across the parameter space below.