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New binary black hole mergers in the LIGO-Virgo O3b data
Ajit Kumar Mehta ,
1
,*
Seth Olsen ,
2
Digvijay Wadekar ,
3
Javier Roulet ,
4
Tejaswi Venumadhav ,
1,5
Jonathan Mushkin,
6
Barak Zackay ,
6
and Matias Zaldarriaga
3
1
Department of Physics,
University of California at Santa Barbara
, Santa Barbara, California 93106, USA
2
Department of Physics,
Princeton University
, Princeton, New Jersey 08540, USA
3
School of Natural Sciences,
Institute for Advanced Study
,
1 Einstein Drive, Princeton, New Jersey 08540, USA
4
TAPIR, Walter Burke Institute for Theoretical Physics,
California Institute of Technology
,
Pasadena, California 91125, USA
5
International Centre for Theoretical Sciences
,
Tata Institute of Fundamental Research, Bangalore 560089, India
6
Department of Particle Physics and Astrophysics,
Weizmann Institute of Science
, Rehovot 76100, Israel
(Received 4 May 2024; accepted 23 December 2024; published 21 January 2025)
We report the detection of six new candidate binary black hole (BBH) merger signals in the publicly
released data from the second half of the third observing run (O3b) of advanced LIGO and advanced Virgo.
The LIGO
Virgo
KAGRA (LVK) Collaboration reported 35 compact binary coalescences in their analysis
of the O3b data [
Phys. Rev. X
13
, 041039 (2023).
], with 30 BBH mergers having coincidence in the
Hanford and Livingston detectors. We confirm 17 of these for a total of 23 detections in our analysis of the
Hanford
Livingston coincident O3b data. We identify candidates using a search pipeline employing
aligned-spin quadrupole-only waveforms. Our pipeline is similar to the one used in our O3a coincident
analysis [
Phys. Rev. D
106
, 043009 (2022).
], except for a few improvements in the veto procedure and the
ranking statistic, and we continue to use an astrophysical probability of one half as our detection threshold,
following the approach of the LVK catalogs. Most of the new candidates reported in this work are placed in
the upper/lower-mass gap of the black hole mass distribution. We also identify a possible neutron star-black
hole merger. We expect these events to help inform the black hole mass and spin distributions inferred in a
full population analysis.
DOI:
10.1103/PhysRevD.111.024049
I. INTRODUCTION
The Advanced LIGO
[1]
and Advanced Virgo
[2]
detectors have now been joined by KAGRA
[3]
to form
the LIGO
Virgo
KAGRA (LVK) Collaboration, which
recently released data from the second half of the third
observing run (O3b) to the public
[4]
. The LVK detected
gravitational waves (GW) from 35 compact binary coa-
lescence (CBC) events including two probable neutron
star
black hole (NSBH) mergers
[5]
. Of these, 32 were
found to be coincident in the Hanford
Livingston (HL)
detectors, among which 30 were reported to be binary black
hole (BBH) signals. Nitz
et al.
[6]
independently produced
the 4-OGC catalog, which included an additional 7 BBH
mergers that were not in the LVK catalog. However, it also
missed eight events reported in GWTC-3 that have a
probability of astrophysical origin (
p
astro
) greater than
0.5. This emphasizes that different pipelines identify events
near the threshold with different significance.
In this work, we perform a search analysis on the O3b HL
coincident data (excluding Virgo) using a pipeline closely
resembling the one used in Olsen
et al.
[7]
(see Venumadhav
et al.
[8]
for an overview of the pipeline). We report
detections of a total of 23 compact binary mergers, six of
which are reported for the first time here. The search did not
recover 13 events reported in GWTC-3 and the additional
seven events reported in 4-OGC (more details follow later in
the Introduction, with a detailed description provided in
Sec.
IV
). We show these detections in Fig.
1
alongside the 90
CBC signals detected by LVK, additional events detected in
4-OGC, and the events previously detected by our pipeline in
O1-O3a with
p
astro
greater than one half
[7
15]
.
Understanding the mass and spin distributions of the
astrophysical BBH population is one of the major goals of
GW astronomy. As the number of events in the catalog
increases, statistically inferred measurements of the pop-
ulation parameters become more and more precise
[18,19]
.
These improved measurements help us to constrain BBH
formation channel rates, and they allow us to probe deeper
into the fundamental physics of black holes (BHs) and even
cosmology
[20
24]
.
*
Contact author: ajit_mehta@ucsb.edu
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One of the standard BBH formation channels consists of
binary stars that evolve in isolation. Stellar evolution
models predict that, if massive enough, stars can go through
pulsational pair-instability (PPISN) and pair-instability
supernovae (PISN). In particular, the latter can completely
destroy the stars, leaving no remnant at the end of the
evolution. The standard prediction is that this will lead to
an
upper mass gap
(UMG) in which the astrophysical
distribution drops sharply for BH masses between
45
M
and
135
M
[25
34]
. Local observations of BHs also hint
at the existence of an empirical
lower mass gap
(LMG)
between
2
M
and
5
M
corresponding to the region
between the maximum Neutron star (NS) mass
[35]
and the
lowest observed BH mass
[36
41]
(note that according to
Alsing
et al.
[35]
, the maximum mass of the NS can vary
between
½
2
;
2
.
6

M
). Detecting BHs in these gaps can have
several implications, e.g., detecting a BH in the UMG
suggests an alternative to the isolated stellar evolution
mechanism for its formation, such as hierarchical mergers
in dense stellar environments (the
dynamical formation
channel)
[42
52]
) and/or they can make us revisit the
problem of stellar collapse
[53
55]
. As we will see later,
among the new events presented here we do find sources
whose constituents lie in these gaps (see Table
I
). Including
such events in the population analysis could impact the
inferred BH mass distribution
[18,19]
.
In addition to the masses, the BH spins can also be
measured from GW signals. The best measured parameter,
however, is a particular combination of the masses and
spins called the effective spin:
χ
eff
¼
m
1
χ
1
;z
þ
m
2
χ
2
;z
m
1
þ
m
2
;
ð
1
Þ
where
m
i
are the masses and
χ
i;z
are the dimensionless
spins projected on the orbital angular momentum. Together
with the measurement of the masses, measuring
χ
eff
could
give us more information about the underlying formation
channels. For example, as the total mass of the binaries
increases, highly spinning systems with
χ
eff
<
0
(antia-
ligned) become increasingly difficult to form in the isolated
binary evolution channel even under a broad range of BH
natal kicks
[56]
. Similar to our previous work
[7]
,we
employ a spin prior which is uniform in
χ
eff
. This prior is
also useful if a reader wants to reweight these results to
another prior of their choice, e.g., a prior that is isotropic in
the individual spins such as the one used in the GWTC-3
[5]
and 4-OGC
[6]
catalogs. Though we use uniform in
χ
eff
as our default spin prior for our parameter estimation runs,
we perform an additional set of parameter estimation runs
to compare against the isotropic spin prior results.
In this work, we follow up on our previous study (Olsen
et al.
[7]
) which looked for new events in the HL data from
the first half of the third observing run (O3a), and analyze
the data from the second half of the third observing run
(O3b) to complete an O3 catalog with a similar pipeline
(see Sec.
II
for a description of the changes). We detect six
new BBH candidate events under the criterion that the
astrophysical probability
p
astro
is at least one half (which is
also the threshold used in the GWTC-3 and 4-OGC
catalogs). By simply summing the complements of the
p
astro
values assigned to these events, we expect that
roughly two of the six events may be noise transients
rather than astrophysical signals (note, however, that the
estimates of
p
astro
depend on multiple factors including the
astrophysical prior used, and that the dependence is
strongest in the case of near-threshold detections such
as these).
FIG. 1. The source-frame total mass and effective spin for the BBH events detected so far from all the search pipelines across the first
three observing runs of LVK. Posterior contours enclose 50% of the probability, and the median is represented by a dot. For parameter
inference, we use the
IMRP
henom
XPHM
waveform approximant
[16]
and a prior that is uniform in detector-frame constituent masses and
comoving volume time (redshifts based on Planck15 results
[17]
). Contours for the six new events are colored by their
p
astro
values, and
the events appearing in previous IAS, OGC, and LVK catalogs up to 3-OGC and GWTC-3
[5,7
10,13
15]
are transparent gray. The
additional (seven) events reported in the 4-OGC catalog
[6]
from O3b are shown with a solid gray color. Though our search only covers
events with Hanford-Livingston coincidence triggers, we include all BBH events declared in the LVK and OGC catalogs through O3b in
this figure. Left panel: results obtained with uniform prior on the effective spin (default prior used in this work). Right panel: results with
isotropic priors on individual spins, as used in the GWTC-3 and 4-OGC catalogs.
AJIT KUMAR MEHTA
et al.
PHYS. REV. D
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Our pipeline confirmed the detections of 17 out of the 30
HL coincident BBH mergers reported in the GWTC-3
catalog. When we investigated our pipeline
s performance
on the remaining events in the LVK catalog, we found that
six of them were marginal in our catalog, i.e., recovered
with a
p
astro
lower than one half, while six were vetoed for
failing one or more of our signal consistency and excess
power tests, and one event was missed. Most of the
marginally recovered events were detected by only one
of the LVK search pipelines, while three of the vetoed
events show signs of physical effects like spin precession
and higher harmonics that were not modeled in our
template bank. We also note that four of the vetoed events
were flagged as having data-quality issues in the GWTC-3
catalog paper. Finally, we also checked that four of the six
new candidates reported in this work were also present in
the list of GWTC-3 subthreshold candidates
[57]
.
The new GW sources reported here could come from a
range of astrophysical scenarios. Three events have their
primary (i.e., more massive) constituents confidently
placed in the upper mass gap. One event has its secondary
(i.e., less massive) constituent confidently placed in the
lower mass gap, i.e.,
2
M
m
src
2
5
M
. If we allow for
the possibility that the maximum mass of the NS can reach
up to
2
.
6
M
[35]
, this event could also be a NSBH signal
at 90% confidence (note however that we did not perform a
dedicated NSBH search). There is also a particular low-
mass event which, under uniform prior on effective spin,
seems to have strongly antialigned spins (
χ
eff
≤−
0
.
8
at
90% confidence; see the discussion in the text for caveats).
We find that for at least two of the new events, detailed
parameter estimation shows some evidence for effects such
as higher-order modes (HM) and spin precession, and also
some information from the VIRGO data, which suggests
that their significance could be potentially improved by
including these elements in future improved searches [our
current search of the Hanford-Livingston coincident data
uses aligned-spin templates that only have the fundamental
(
l
¼j
m
2
) harmonic].
As noted earlier, some loud events in the LVK catalog
that were vetoed by our pipeline also showed evidence for
these physical effects, which additionally motivates the
development of improved pipelines incorporating them. We
note that the LVK catalog was also produced using aligned-
spin templates limited to the
l
¼j
m
2
harmonic, as in
our case. However, our veto tests are comparatively more
stringent, making it crucial for us to search for signals
showing hints of HM or precession using templates that
explicitly include these effects.
The rest of the paper is organized as follows: in Sec.
II
we
review changes to the IAS pipeline between the O3a and O3b
analyses. In Sec.
III
we discuss the BBH mergers first
reported in this work (see Table
I
). In Sec.
IV
we report
our results for events already included in GWTC-3
[5]
and
4-OGC
[6]
, noting differences (see Table
II
). We summarize
the results in Sec.
V
and discuss the astrophysical implica-
tions of the new events. In Appendix
A
,weshowthe
computation of the astrophysical probability,
p
astro
and also
discuss certain details of our veto procedure.
Corner plots of posterior distributions for new events can
be found in Appendix
B
, with PE samples publicly available
at
[58]
.
II. CHANGES TO THE O3A ANALYSIS PIPELINE
Our analysis pipeline is very similar to the version used
in our recent analysis of the Hanford-Livingston coincident
O3a data
[7]
, but we make changes to two components of
TABLE I. New events with
p
astro
>
0
.
5
. The parameter values correspond to the median and 90% uncertainties, and ln
L
max
denotes
the maximum log likelihood from PE runs. While our search templates only include the aligned-spin (2,2) mode (based on the
IMRP
henom
D
approximant), PE was done with the
IMRP
henom
XPHM
waveform which additionally includes effects of precession and
higher harmonics: (2,1), (3,3), and (4,4) modes. The PE also uses Virgo data when available, unlike the search.
ρ
H
;
L
denote the SNR of
the triggers in the Hanford and Livingston detectors. The IFAR column shows the IFAR within the bank and the one combined across the
chirp-mass banks assuming an equal astrophysical rate density for each chirp-mass bank.
PE
IFAR (yr)
Event
Bank
m
src
1
ð
M
Þ
m
src
2
ð
M
Þ
χ
eff
z
ln
L
max
ρ
H
ρ
L
Within bank | | Overall
p
astro
GW200109_195634
a
(5, 4)
69
þ
24
19
48
þ
22
17
0
.
5
þ
0
.
3
0
.
8
1
.
0
þ
0
.
7
0
.
4
42.0 5.5 5.8
4.4
0.93
0.81
GW191228_085854
b
(1, 0)
9
.
7
þ
4
.
2
1
.
5
5
.
5
þ
0
.
9
1
.
4
0
.
89
þ
0
.
42
0
.
10
0
.
14
þ
0
.
05
0
.
04
43.2 5.2 7.3
1.8
0.25
0.67
GW200225_075134
(3, 1)
51
þ
17
11
37
þ
13
11
0
.
5
þ
0
.
9
0
.
4
0
.
6
þ
0
.
4
0
.
3
43.0 4.5 7.0
0.95
0.15
0.60
GW191117_023843
(5, 4)
62
þ
26
15
44
þ
17
15
0
.
4
þ
0
.
6
0
.
4
0
.
9
þ
0
.
5
0
.
4
33.3 6.5 4.7
0.52
0.12
0.56
GW200210_100022
(5, 2)
74
þ
15
17
17
þ
29
4
0
.
92
þ
0
.
07
0
.
42
1
.
3
þ
0
.
7
0
.
4
34.2 4.9 6.2
0.45
0.097 0.52
GW200316_235947
(0, 0)
6
.
4
þ
4
.
9
1
.
4
3
.
6
þ
1
.
0
1
.
3
0
.
11
þ
0
.
34
0
.
13
0
.
17
þ
0
.
05
0
.
05
38.9 6.1 5.7
0.59
0.092 0.52
a
This event
s significance could be potentially improved in future searches in which Virgo data are also included in the coincident
analysis.
b
This event has bimodal posterior distributions in the intrinsic parameters. The values quoted here correspond to the first mode.
See
III B
for further details.
NEW BINARY BLACK HOLE MERGERS IN THE LIGO-VIRGO
...
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