LIGO/Virgo/KAGRA S230529ay

The LIGO Scientific Collaboration, the Virgo Collaboration, and the
KAGRA Collaboration report:

We identified the compact binary merger candidate S230529ay during
real-time processing of data from LIGO Livingston Observatory (L1) at
2023-05-29 18:15:00.746 UTC (GPS time: 1369419318.746). The candidate
was found by the GstLAL [1] and PyCBC Live [2] analysis pipelines.

S230529ay is an event of interest because its false alarm rate, as
estimated by the online analysis, is 2e-10 Hz, or about one in 1e2
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230529ay

The classification of the GW signal, in order of descending
probability is NSBH (62%), BNS (31%), Terrestrial (7%), or BBH (<1%).

Assuming the candidate is astrophysical in origin, the probability
that the lighter compact object is consistent with a neutron star mass
(HasNS) is >99%. [3] Using the masses and spins inferred from the
signal, the probability of matter outside the final compact object
(HasRemnant) is 12%. [3] Both HasNS and HasRemnant consider the
support of several neutron star equations of state. The probability
that any one of the binary components lie between 3 to 5 solar mass
(HasMassgap) is 98%.

Two sky maps are available at this time and can be retrieved from the
GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by
BAYESTAR [4], distributed via GCN notice about 25 seconds after the
candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by
BAYESTAR [4], distributed via GCN notice about 5 minutes after the
candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For
the bayestar.multiorder.fits,1 sky map, the 90% credible region is
well fit by an ellipse with an area of 31171 deg2 described by the
following DS9 region (right ascension, declination, semi-major axis,
semi-minor axis, position angle of the semi-minor axis):
   icrs; ellipse(00h00m, +00d00m, 163.70d, 97.20d, 147.56d)
Marginalized over the whole sky, the a posteriori luminosity distance
estimate is 217 +/- 71 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of
this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide
<https://emfollow.docs.ligo.org/userguide/>.

 [1] Tsukada et al. arXiv:2305.06286
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  (2023) and Ewing et al. arXiv:2305.05625
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  (2023)
 [2] Dal Canton et al. ApJ 923, 254 (2021)
 [3] Chatterjee et al. The Astrophysical Journal 896, 1 (2020)
 [4] Singer & Price PRD 93, 024013 (2016)

V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland)
J. Rodi (IAPS-Roma, Italy)
A. Coleiro (APC, France)
S. Mereghetti (INAF IASF-Milano, Italy)

on behalf of the INTEGRAL multi-messenger collaboration:
https://www.astro.unige.ch/cdci/integral-multimessenger-collaboration

Using INTEGRAL/SPI-ACS realtime data (following [1]) we have performed
a search for a prompt gamma-ray counterpart of S230529ay (GCN 33889
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 ).

At the time of the event (2023-05-29 18:15:00 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 122 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (2.8% of optimal) response of ISGRI, somewhat suppressed
(57% of optimal) response of IBIS/Veto, and somewhat suppressed (47%
of optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was somewhat
unstable (excess variance 1.8). 
In addition, we note that excess noise at near Hz frequency has been noted 
in SPI-ACS data in recent days, related to satellite environment conditions.

We have performed a search for any impulsive events in INTEGRAL SPI-
ACS (as described in [2]) data.

We do not detect any significant counterparts and estimate a 3-sigma
upper limit on the 75-2000 keV fluence of 3.4e-07 erg/cm^2 (within the
50% probability containment region of the source localization) for a
burst lasting less than 1 s with a characteristic short GRB spectrum
(an exponentially cut off power law with alpha=-0.5 and Ep=600 keV)
occurring at any time in the interval within 300 s around T0. For a
typical long GRB spectrum (Band function with alpha=-1, beta=-2.5, and
Ep=300 keV), the derived peak flux upper limit is ~3.2e-07 (8e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

For the mean reported distance 217.0 Mpc this corresponds to the limit
on the total isotropic  equivalent energy in 1 s of 1.9e+48 erg for
the short GRB spectrum and for a long GRB spectrum isotropic
equivalent luminosity in 1 s (8 s) of 9.4e+47 erg/s (4.5e+47 erg/s)

We report for completeness and in order of FAP, all excesses
identified in the search region. We find: 4 likely background
excesses:

T-T0     | scale   | S/N | luminosity ( x 1e+48 erg/s) | FAP    
262      | 1.7     | 4.7 |     33.8 +/- 8.3    +/- 30.4   | 0.276 
-12.1    | 0.45    | 3.1 |     3.87 +/- 1.62   +/- 3.48   | 0.498 
-6.59    | 0.1     | 3.3 |     8.42 +/- 3.46   +/- 7.58   | 0.728 
-91.3    | 0.95    | 3.5 |     3.07 +/- 1.11   +/- 2.76   | 0.756 

Note that FAP estimates (especially at timescales above 2s) may be
possibly further affected by enhanced non-stationary local background
noise. This list excludes any excesses for which FAP is close to
unity.



All results quoted are preliminary.

This circular is an official product of the INTEGRAL Multi-Messenger
team.

Note that we send GCNs Circulars only when one of the following
conditions is met: merger contains at least one neutron star, a
significant counterpart is reported.

[1] Savchenko et al. 2017, A&A 603, A46 
[2] Savchenko et al. 2012, A&A 541A, 122S

The LIGO Scientific Collaboration, the Virgo Collaboration, and the
KAGRA Collaboration report:

We have conducted further analysis of the LIGO Livingston Observatory
(L1) data around the time of the compact binary merger (CBC) candidate
S230529ay (GCN Circular 33889
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 ).
Parameter estimation has been performed using Bilby [1] and a new sky
map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available
for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S230529ay

Based on posterior support from parameter estimation [1], under the
assumption that the candidate S230529ay is astrophysical in origin,the
probability that the lighter compact object is consistent with a
neutron star mass (HasNS) is 98%. [2] Using the masses and spins
inferred from the signal, the probability of matter outside the final
compact object (HasRemnant) is 1%. [2] Both HasNS and HasRemnant
consider the support of several neutron star equations of state. The
probability that any one of the binary components lie between 3 to 5
solar mass (HasMassgap) is 69%.

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is
25623 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 201 +/- 63 Mpc (a posteriori mean +/-
standard deviation).

For further information about analysis methodology and the contents of
this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide
<https://emfollow.docs.ligo.org/userguide/>.


 [1] Ashton et al. ApJS 241, 27 (2019)
 [2] Chatterjee et al. The Astrophysical Journal 896, 1 (2020)

S. Lesage (UAH) reports on behalf of the Fermi-GBM Team and the GBM-LIGO/Virgo/KAGRA group

For S230529ay (GCN 33889
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  and GCN 33891
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 ) and using the Bilby skymap, Fermi-GBM was observing 60.7% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo/KAGRA (LVK) detection of GW trigger S230529ay. An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates.

Part of the LVK localization region is behind the Earth for Fermi, located at an RA=31.5, Dec=23.9 with a radius of 67.6 degrees. We therefore set upper limits on impulsive gamma-ray emission for the GW localization region visible to Fermi at merger time. Using the representative soft, normal, and hard GRB-like templates described in arXiv:1612.02395
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 , we set the following 3 sigma flux upper limits over 10-1000 keV, weighted by GW localization probability (in units of 10^-7 erg/s/cm^2):

Timescale  Soft   Normal   Hard
------------------------------------
0.128 s:     1.2       1.9       3.5
1.024 s:     0.5       0.7       1.2
8.192 s:     0.2       0.2       0.4

Assuming the median luminosity distance of 200.6 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^50 erg/s):

Timescale  Soft    Normal   Hard
------------------------------------
0.128s:    0.009     0.013     0.039
1.024s:    0.003     0.005     0.013
8.192s:    0.001     0.002     0.005

S. Sugita, M. Serino (AGU),
H. Negoro (Nihon U.),
N. Kawai (RIKEN), 
M. Nakajima, K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.),
T. Mihara, T. Kawamuro, S. Yamada, T. Tamagawa, M. Matsuoka (RIKEN),
T. Sakamoto, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU),
Y. Tsuboi, S. Urabe, S. Nawa, N. Nemoto (Chuo U.),
M. Shidatsu (Ehime U.),
I. Takahashi, M. Niwano, S. Sato, N. Higuchi, Y. Yatsu (Tokyo Tech),
S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, T. Kurihara (JAXA),
Y. Ueda, K. Setoguchi, T. Yoshitake, Y. Nakatani (Kyoto U.),
M. Yamauchi, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.),
K. Yamaoka (Nagoya U.),
Y. Kawakubo (LSU),
M. Sugizaki (NAOC),
W. Iwakiri (Chiba U.) 
report on behalf of the MAXI team:

We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after the compact binary merger candidate S230529ay at 2023-05-29 18:15:00.746 UTC (GCN 33889
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 ).

At the trigger time of S230529ay, the high-voltage of MAXI/GSC was off
The first one-orbit (92 min) scan observation with GSC after the event covered 91%
of the 90% credible region of the Bilby skymap (GCN 33891
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 ) from 18:31:41 to 19:40:39 UTC (T0+1001 to T0+5139 sec).

No significant new source was found in the region in the one-orbit scan observation.
A typical 1-sigma averaged upper limit obtained in one scan observation
is 20 mCrab at 2-20 keV.

If you require information about X-ray flux by MAXI/GSC at specific coordinates,
please contact the submitter of this circular by email.

F. Longo (Univ. Trieste, and INFN Trieste),  M. Tavani (INAF/IAPS,
and Univ. Roma Tor Vergata), F. Verrecchia, C. Pittori (SSDC, and
INAF/OAR), M. Cardillo, C. Casentini, L. Foffano, G. Piano (INAF/IAPS),
A. Ursi (ASI and INAF/IAPS), F. Lucarelli (SSDC, and INAF/OAR),
L. Baroncelli, A. Bulgarelli, A. Ciabattoni, A. Di Piano, V.
Fioretti, G. Panebianco, N. Parmiggiani (INAF/OAS-Bologna), M.
Pilia (INAF/OA-Cagliari), report on behalf of the AGILE Team:

In response to the LIGO/Virgo/KAGRA GW event S230529y at
T0 = 2023-05-29 18:15:00.89 (UT), a preliminary analysis of the
AGILE minicalorimeter (MCAL) triggered data found no event candidates
within a time interval covering -/+ 15 sec from the LIGO/Virgo/KAGRA T0.

At the T0, about 60% of the S230529y 90% c.l. localization region
was accessible to the AGILE/MCAL.

Three-sigma upper limits (ULs) are obtained for a 1 s integration
time at different celestial positions within the accessible S230529y
localization region in the energy range 0.4-1 MeV, from a minimum
of 1.5E-06 erg cm^-2 to a maximum of 3.1E-06 erg cm^-2 (assuming as
spectral model a single power-law with photon index 1.5).

An independent procedure based on photon counting statistics provides
UL fluences in the same band, from a minimum of 1.1E-08 erg cm^-2
to a maximum of 2.1E-08 erg cm^-2.

The AGILE/MCAL detector is a CsI detector with a 4 pi FoV, sensitive
in the energy range 0.4-100 MeV. Additional analysis of AGILE data
is in progress.

G. Waratkar (IITB), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

We have carried out a search for X-ray candidates in AstroSat CZTI data in a 100-sec window around the trigger time of the event S230529ay (UTC 2023-05-29 18:15:00, GraceDB event). We use the Bilby.multiorder.fits,0 map (https://gracedb.ligo.org/api/superevents/S230529ay/files/Bilby.multiorder.fits,0) for our analysis. CZTI is a coded aperture mask instrument that has a considerable effective area for about 29% of the entire sky but is also sensitive to brighter transients from the entire sky. At the time of the merger, AstroSat's nominal pointing is RA, DEC = 12:26:03.3, 33:32:18.1 (186.5138,33.5384), which is ~103 deg away from the maximum probability location, which severely reduces the effective area of CZTI. At the time of the merger event, the Earth-satellite-transient angle corresponding to the maximum probability location is ~81 deg and hence is not occulted by Earth in the satellite's frame. In a time interval of 100 sec around the event, the region of the localization map which Earth does not occult in the satellite's frame has a total probability of 0.76 (76%).

CZTI data were de-trended to remove orbit-wise background variation. We then searched data from the four independent, identical quadrants for coincident spikes in the count rates. Searches were undertaken by binning the data in 0.1s, 1s, and 10s respectively. Statistical background count rate fluctuations were estimated using data from 5 preceding orbits. We selected confidence levels such that the probability of a false trigger in a 100-sec window is 10^-4. We do not find any evidence for any hard X-ray transient in this window, in the CZTI energy range of 20-200 keV.

We use a detailed mass model of the satellite to calculate the direction-dependent instrument response for points in the visible sky. We then assume the source is modeled as a power law with photon index alpha = -1, and convert our count rate upper limits to direction-dependent flux limits. We obtain the following upper limits for source flux in the 20-200 keV band by taking a probability-weighted mean over the visible sky:

0.1 s: flux limit= 1.31e-05 ergs/cm^2/s; fluence limit = 1.31e-06 ergs/cm^2
1.0 s: flux limit= 2.42e-06 ergs/cm^2/s; fluence limit = 2.42e-06 ergs/cm^2
10.0 s: flux limit= 3.22e-07 ergs/cm^2/s; fluence limit = 3.22e-06 ergs/cm^2

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI EMGW detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=emgw

S. Sugita, A. Yoshida, T. Sakamoto (AGU), Y. Kawakubo (LSU),
K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR),
S. Torii, Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu, 
T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC),
M. L. Cherry (LSU), S. Ricciarini (U of Florence),
P. S. Marrocchesi (U of Siena),
and the CALET collaboration:

At the trigger time of the compact binary merger candidate 
S230529ay T0 = 2023-05-29 18:15:00 UT (The LIGO Scientific 
Collaboration, the Virgo Collaboration, the KAGRA Collaboration, 
GCN Circ. 33889
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 , 33891
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 ) the CALET Gamma-ray Burst Monitor
(CGBM) high voltages were off (from T0-2 min to T0+3 min).

The CALET Calorimeter (CAL) was operating in the high energy trigger
mode at the trigger time of S230529ay. Using the CAL data, we searched 
for gamma-ray events in the 10-100 GeV band from -60 sec to +60 sec from 
the GW trigger time and found no candidates in the overwrap region with the
LVK high probability localization region.  The 90% upper limit of CAL is 
1.2 x 10^-5 erg/cm^2/s (10-100 GeV) when the summed LVK probability reaches
20%. The CAL FOV was centered at RA = 118 deg, DEC = +47 deg at T0.

The preliminary CGBM analysis for O4 events can be found here:
http://cgbm.calet.jp/cgbm_trigger/O4

Viraj Karambelkar (CIT), Tomas Ahumada (CIT),  Robert Stein (CIT), Akash
Anumarlapudi (UWM), Gaurav Waratkar (IITB), Harsh Kumar (IITB), Anirudh
Salgundi (IITB), Vishwajeet Swain (IITB), Theophile du Laz (CIT), Igor
Andreoni (UMD), Michael Coughlin (UMN), Mansi Kasliwal (CIT), Varun
Bhalerao (IITB), Simeon Reusch (DESY), Jannis Necker (DESY), Shreya Anand
(CIT), Eric Bellm (UW), Brian Healy (UMN), S. B. Cenko (UMD), D. Kaplan
(UWM), D. Perley (LJMU) report on behalf of the ZTF and GROWTH
collaborations:


We observed the localization region of the LVC trigger S230529ay as part of
routine Zwicky Transient Facility (ZTF; Graham et al., 2019; Bellm et al.,
2019) survey operations. We obtained images in the r, i, and g bands
beginning at 2023-05-30T04:13:34.003 UT (10 hours after the LVC trigger
time), covering ~7% of the probability enclosed in the localization region.


We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through
Fritz (Coughlin et al. 2023) and emgwcave (Karambelkar et al. in prep),
AMPEL (Nordin et al. 2019), and ZTFReST (Andreoni & Coughlin et al., 2021).
We required at least 2 detections separated by at least 15 minutes to
select against moving objects. Furthermore, we cross-match our candidates
with the Minor Planet Center to flag known asteroids, reject stellar
sources (Tachibana and Miller 2018), and apply machine learning algorithms
(Mahabal et al. 2019). We require that no spatially coincident ZTF alerts
were issued before the detection time of the LVC trigger. We also run
forced photometry on ZTF images (Masci et al. 2019) and ATLAS images (Tonry
et al. 2018, Smith et al. 2020) and require no detections before the LVC
trigger.


One source passed our criteria and is inside the 95% error region:

|-----------------------------------------------------------------------------------------------------
| id           | alias                |       ra |     dec |        mjd  |
mag±err (ab)   | filter   |
|-----------------------------------------------------------------------------------------------------
| ZTF23aamnpce | AT2023jtt
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             | 235.9839 | 15.2248 | 60094.23830 |
20.49±0.23     |  r       |
|-----------------------------------------------------------------------------------------------------


AT2023jtt
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  is 0.5" from a galaxy that has a photometric redshift of
photoz=0.22±0.06 from the Legacy Survey DR8 (Duncan, 2022), suggesting that
it is probably not associated with the LVC trigger.

Further follow-up of this localization region will continue as part of
regular survey operations.


ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC,
USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY,
Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan;
IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia.
ZTF acknowledges the generous support of the NSF under AST MSIP Grant No
1440341. GROWTH acknowledges generous support of the NSF under PIRE Grant
No 1545949. Alert distribution service provided by DIRAC@UW (Patterson et
al. 2019). Alert database searches are done by AMPEL (Nordin et al. 2019)
and Kowalski (Duev et al. 2019). GROWTH India telescope is located at the
Indian Astronomical Observatory (Hanle), operated by the Indian Institute
of Astrophysics (IIA). GROWTH-India project is supported by SERB and
administered by IUSSTF, under grant number IUSSTF/PIRE
Program/GROWTH/2015-16 and IUCAA.

IceCube Collaboration (http://icecube.wisc.edu/) reports:

IceCube has performed an additional search [1] for track-like muon neutrino events consistent with the sky localization of gravitational-wave candidate S230529ay in a time range of -0.1 day, +14 days from the alert event time (2023-05-29 15:51:00.03 UTC to 2023-06-12 18:15:00.03 UTC). 

During this time period IceCube was collecting good quality data. In this case, we report a p-value of 0.38, consistent with no significant excess of track events. IceCube's sensitivity assuming an E^-2 spectrum (E^2 dN/dE) to neutrino point sources within the locations spanned by the 90% spatial containment of the 4-Update map ranges from 0.028 to 1.219 GeV cm^-2 in this time window.

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu. 

[1] Abbasi et al. Astrophys.J. 944 (2023) 1, 80

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: 

We have conducted further offline analysis of the LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S230529ay (GCN Circular 33889
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 ). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S230529ay 

Based on posterior support from parameter estimation [1], under the assumption that the candidate S230529ay is astrophysical in origin,the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is 98%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is 7%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that any one of the binary components lie between 3 to 5 solar mass (HasMassgap) is 73%. 

The upward revision in the HasRemnant value relative to that quoted in circular number 33891 is primarily due to an issue that was identified in the em-bright code rather than the updated choice of offline analysis settings. This issue has been fixed and does not affect any values from posterior samples reported in circulars for this event or other events in O4.

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 24534 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 197 +/- 62 Mpc (a posteriori mean +/- standard deviation). 

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide <https://emfollow.docs.ligo.org/userguide/>. 


[1] Ashton et al. ApJS 241, 27 (2019)
[2] Chatterjee et al. The Astrophysical Journal 896, 1 (2020)