LIGO/Virgo/KAGRA S230731an

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

We identified the compact binary merger candidate S230731an during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-07-31 21:53:07.889 UTC (GPS
time: 1374875605.889). The candidate was found by the CWB [1], MBTA
[2], GstLAL [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S230731an is an event of interest because its false alarm rate, as
estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230731an

The classification of the GW signal, in order of descending
probability, is BBH (70%), NSBH (15%), Terrestrial (15%), or BNS
(<1%).

Assuming the candidate is astrophysical in origin, the probability
that the lighter compact object is consistent with a neutron star mass
(HasNS) is <1%. [6] Using the masses and spins inferred from the
signal, the probability of matter outside the final compact object
(HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the
support of several neutron star equations of state. The probability
that either of the binary components lies between 3 and 5 solar masses
(HasMassgap) is 5%.

Two sky maps are available at this time and can be retrieved from the
GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by
BAYESTAR [7], distributed via GCN notice about 30 seconds after the
candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by
BAYESTAR [7], distributed via GCN notice about 5 minutes after the
candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For
the bayestar.multiorder.fits,2 sky map, the 90% credible region is 646
deg2. Marginalized over the whole sky, the a posteriori luminosity
distance estimate is 1056 +/- 279 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] Klimenko et al. PRD 93, 042004 (2016)
 [2] Aubin et al. CQG 38, 095004 (2021)
 [3] Tsukada et al. arXiv:2305.06286
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  (2023) and Ewing et al.
arXiv:2305.05625
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  (2023)
 [4] Dal Canton et al. ApJ 923, 254 (2021)
 [5] Chu et al. PRD 105, 024023 (2022)
 [6] Chatterjee et al. ApJ 896, 54 (2020)
 [7] Singer & Price PRD 93, 024013 (2016)

M. Nakajima, H. Negoro,  (Nihon U.),
T. Mihara, N. Kawai (RIKEN),
S. Sugita, M. Serino, H. Hiramatsu, H. Nishikawa, Y. Kondo (AGU),
Y. Kawakubo (LSU)
report on behalf of the MAXI team:

We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after compact binary merger candidate S230731an at 2023-07-31 21:53:07
UTC (GCN 34303
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 ).

At the trigger time of S230731an, the high-voltage of MAXI/GSC was off,
and it was turned on at T0+628 sec (+10.5 min).
The first one-orbit (92 min) scan observation with GSC after the event
covered 95%
of the 90% credible region of the bayestar skymap from 22:03:35 to
23:07:43 UTC (T0+628 to T0+4476 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.

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

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S230731an (GCN Circular 34303
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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/S230731an

Based on posterior support from parameter estimation [1], under the assumption that the candidate S230731an is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [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 either of the binary components lies between 3 and 5 solar masses (HasMassgap) is <1%.

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 599 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1001 +/- 242 Mpc (a posteriori mean +/- standard deviation).

The GCN Circular 34303
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  quoted incorrect values for the classification of the GW signal. These differ from the correct values found in the Initial Notice. The correct values from the Initial Notice are: BBH (81%), NSBH (18%), Terrestrial (<1%), or BNS (0%).

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. ApJ 896, 54 (2020)

S. Dalessi (UAH) reports on behalf of the Fermi-GBM Team:

At the time of LVK S230731an, Fermi was passing through the South Atlantic Anomaly from 12.6 minutes prior to 5.7 minutes after the trigger time; therefore the GBM detectors were disabled.

G. Waratkar (IITB), V. Bhalerao (IITB), M. Dixit (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 S230731an (UTC 2023-07-31 21:53:07, GraceDB event). We use the Bilby.multiorder.fits,0 map (https://gracedb.ligo.org/api/superevents/S230731an/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 = 00:47:41.1, 85:07:31.0 (11.9212,85.1253), which is ~60 deg away from the maximum probability location. At the time of the merger event, the Earth-satellite-transient angle corresponding to the maximum probability location is ~87 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 is not occulted by Earth in the satellite's frame has a total probability of 0.93 (93%) of containing the source.

CZTI data were de-trended to remove orbit-wise background variation. We then searched data from the four independent, identical quadrants to look for coincident spikes in the count rates. Searches were undertaken by binning the data in 0.1s, 1s, and 10s respectively. Statistical fluctuations in background count rates were estimated by 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= 1.18e-05 ergs/cm^2/s; fluence limit = 1.18e-06 ergs/cm^2
1.0 s: flux limit= 2.30e-06 ergs/cm^2/s; fluence limit = 2.30e-06 ergs/cm^2
10.0 s: flux limit= 2.74e-07 ergs/cm^2/s; fluence limit = 2.74e-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 GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=emgw

Samuele Ronchini (PSU), Gayathri Raman (PSU), Aaron Tohuvavohu (U Toronto), James DeLaunay (U Alabama), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:

Swift/BAT was observing 64.47 % of the GW localization probability (bayestar.multiorder.fits) at merger time. A fraction 5.72 % of the GW localization posterior is contained inside the BAT coded FoV.

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.

We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395
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 ], and spectral shape from GRB170817A [arXiv:1710.05446
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 ]) and for four time bins. 
In units of 10^-7 erg/s/cm^2:

time_bin (s) soft normal hard GRB170817
-------------------------------
0.256        22.8 15.3   13.6 17.1  
1.024        11.6 7.8    6.9  8.7   
4.096        6.2  4.2    3.7  4.6   
16.384       3.8  2.6    2.3  2.9


The upper limits as function of sky position are plotted here, alongside the GW localization:
https://doi.org/10.5281/zenodo.8206758
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The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively.

The corresponding fits file can be found here:
https://doi.org/10.5281/zenodo.8206771
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GUANO is a fully autonomous, extremely low latency, spacecraft
commanding pipeline designed for targeted recovery of BAT event mode
data around the times of compelling astrophysical events to enable
more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be
found at: https://www.swift.psu.edu/guano/