SUBJECT: Fermi GBM candidate with ID 702818765: LIGO/Virgo/KAGRA identification of a low-significance GW compact binary merger counterpart, S3456
 **NOTE TO ADVOCATES: If our circular is the first one to mention the sub-threshold GRB, please use the subject line:
SUBJECT: LIGO/Virgo/KAGRA S3456: Identification of a GW compact binary merger candidate possibly associated with sub-threshold Fermi GBM candidate with ID 702818765**

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA
Collaboration along with the Fermi GBM Collaboration report:

We identified the compact binary merger candidate S3456 during real-time
processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston
Observatory (L1) at 2018-06-28 03:08:04.741 UTC (GPS time: 1214190502.741). The
candidate was found by the MBTA [1] analysis pipeline.

Based on the analysis of gravitational-wave data alone, this candidate does not
meet our criteria for a high-significance public alert. However, a search
performed by the RAVEN pipeline found a significant coincidence between this
candidate and 702818765. The event's properties can be found at this URL:

https://gracedb.invalid/superevents/S3456

The classification of the GW signal, in order of descending probability, is
NSBH (68%), Terrestrial (32%), BNS (<1%), or BBH (<1%).

Assuming the candidate is astrophysical in origin, the probability that at
least one of the compact objects is consistent with a neutron star mass (HasNS)
is >99%. [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 for maximum neutron star mass. The probability that either
of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

One GW-only sky map is available at this time and can be retrieved from the
GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR
[3], distributed via GCN and SCiMMA notices about 10 hours after the candidate
event time.

For the bayestar.multiorder.fits,0 sky map, the 90% credible region is 1079
deg2. Marginalized over the whole sky, the a posteriori luminosity distance
estimate is 158 +/- 44 Mpc (a posteriori mean +/- standard deviation).

A search performed by the RAVEN pipeline [4] found a temporal coincidence
between S3456 and a sub-threshold Fermi GBM candidate with ID 702818765 **CITE
ORIGINAL GCN FOR THE EXTERNAL CANDIDATE FROM https://gcn.nasa.gov/circulars,
e.g., (Bhalerao et al., GCN Circular XXXXX)**. The GRB candidate time is 0.6
seconds before the GW candidate event. The estimated joint false alarm rate for
the coincidence using just timing info before trials are applied is 3e-11 Hz,
or about one in 1e3 years. The GRB candidate was found during a joint targeted
search between the LIGO/Virgo/KAGRA collaboration and Fermi GBM, and has a
false alarm rate of 1.2e-06 Hz, or about one in 9 days.

A combined sky map is also available:
 * combined-ext.multiorder.fits,0, an initial localization, distributed via GCN
and SCiMMA notices about 10 hours after the candidate event time.

For the combined-ext.multiorder.fits,0 sky map, the 90% credible region is 28
deg2. Considering the overlap of the individual sky maps, the estimated joint
false alarm rate for the spatial and temporal coincidence before trials are
applied is 4e-12 Hz, or about one in 1e4 years.

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/.

 [1] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
 [4] Urban, A. L. 2016, Ph.D. Thesis https://dc.uwm.edu/etd/1218 and
Piotrzkowski, B. J. 2022, Ph.D. Thesis https://dc.uwm.edu/etd/3060