LIGO/Virgo/KAGRA S230825k

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

We identified the compact binary merger candidate S230825k during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-08-25 04:13:34.546 UTC (GPS
time: 1376972032.546). The candidate was found by the CWB [1], GstLAL
[2], and MBTA [3] analysis pipelines.

S230825k is an event of interest because its false alarm rate, as
estimated by the online analysis, is 2.4e-09 Hz, or about one in 13
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230825k

The classification of the GW signal, in order of descending
probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), 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%. [4] Using the masses and spins inferred from the
signal, the probability of matter outside the final compact object
(HasRemnant) is <1%. [4] 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 6%.

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 [5], distributed via GCN notice about 27 seconds after the
candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by
BAYESTAR [5], 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
2936 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 7075 +/- 2287 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] Tsukada et al. arXiv:2305.06286
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  (2023) and Ewing et al.
arXiv:2305.05625
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  (2023)
 [3] Aubin et al. CQG 38, 095004 (2021)
 [4] Chatterjee et al. ApJ 896, 54 (2020)
 [5] Singer & Price PRD 93, 024013 (2016)

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 S230825k (GCN Circular 34546
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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/S230825k

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 3012 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 5283 +/- 2117 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)

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

Swift/BAT was observing 86.41% of the GW localization probability (Bilby.offline0.multiorder.fits) at merger time. A fraction 27.82 % 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        11.0  7.4	  6.7	8.2
1.024        5.6   3.8	  3.4	4.2
4.096        3.0   2.0	  1.8	2.3
16.384       1.9   1.3	  1.1	1.4 


The upper limits as function of sky position are plotted here, alongside the GW localization:
https://doi.org/10.5281/zenodo.8285069
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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.8285073
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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/